chapter 1 rocks and minerals section i. mineralsrocks and minerals 1-2 cleavage cleavage is the...

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CHAPTER 1

R o c k s a n d M i n e r a l s

The crust of the earth is made up of rock;rock, in turn, is composed of minerals. Thegeologist classifies rocks by determining theirmodes of formation and their mineral contentin addition to examining certain chemical andphysical properties. Military engineers use asimpler diagnostic method that is discussedbelow. Rock classification is necessary be-cause particular rock types have beenrecognized as having certain properties or asbehaving in somewhat predictable ways. Therock type implies information on manyproperties that serves as a guide in determin-ing the geological and engineeringcharacteristics of a site. This implied infor-mation includes—

A range of rock strength.Possible or expected fracture systems.The probability of encountering bed-ding planes.Weak zones.Other discontinuities.Ease or difficulty of rock excavation.Permeability.Value as a construction material.Trafficability.

This chapter describes procedures for fieldidentification and classification of rocks andminerals. It also explains some of the proces-ses by which rocks are formed. The primaryobjective of identifying rock materials andevaluating their physical properties is to beable to recommend the most appropriate ag-gregate type for a given military constructionmission.

Section I. Minerals

PHYSICAL PROPERTIESRocks are aggregates of minerals. To un-

derstand the physical properties of rocks, it isnecessary to understand what minerals are.A mineral is a naturally occurring, inorgani-cally formed substance having an orderedinternal arrangement of atoms. It is a com-pound and can be expressed by a chemicalformula. If the mineral’s internal frameworkof atoms is expressed externally, it forms acrystal. A mineral’s characteristic physicalproperties are controlled by its compositionand atomic structure, and these propertiesare valuable aids in rapid field identification.Properties that can be determined by simplefield tests are introduced here to aid in theidentification of mineralsthe identification of rocks.are—

Hardness.Crystal form.Cleavage.Fracture.Luster and color.Streak.

and indirectly inThese properties

Specific gravity.

HardnessThe hardness of a mineral is a measure of

its ability to resist abrasion or scratching byother minerals or by an object of known hard-ness. A simple scale based on empirical testshas been developed and is called the Mohs

Rocks and Minerals 1-1

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Hardness Scale. The scale consists of 10minerals arranged in increasing hardnesswith 1 being the softest. The 10 mineralsselected to form the scale of comparison arelisted in Table 1-1. Hardness kits containingmost of the reference minerals are available,but equivalent objects can be substituted forexpediency. Objects with higher values onMobs’ scale are capable of scratching objectswith lower values. For example, a rockspecimen that can be scratched by a coppercoin but not by the fingernail is said to have ahardness of about 3. Military engineersdescribe a rock as either hard or soft. A rockspecimen with a hardness of 5 or more is con-sidered hard. The hardness test should beperformed on a fresh (unweathered) surfaceof the specimen.

Crystal FormMost, but not all, minerals form crystals.

The form, or habit, of the crystals can be diag-nostic of the mineral and can help to identifyit. The minerals galena (a lead ore) and halite(rock salt) commonly crystallize as cubes.Crystals of garnet (a silicate mineral) com-monly have 12 or 24 equidimensional faces.Some minerals typically display long needle-like crystals. Minerals showing no crystalform are said to be amorphous. Figure 1-1 il-lustrates two of the many crystal forms.


CleavageCleavage is the tendency of a mineral to

split or separate along preferred planes whenbroken. It is fairly consistent from sample tosample for a given mineral and is a valuableaid in the mineral’s identification. Cleavageis described by noting the direction, the de-gree of perfection, and (for two or morecleavage directions) the angle of intersectionof cleavage planes. Some minerals have onecleavage direction; others have two or moredirections with varying degrees of perfection.Figure 1-2 illustrates a mineral with onecleavage direction (mica) and one with threedirections (calcite). Some minerals, such asquartz, form crystals but do not cleave.

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FractureFracture is the way in which a mineral

breaks when it does not cleave alongcleavage planes. It can be helpful in fieldidentification. Figure 1-3 illustrates the com-mon kinds of fracture. They are—

Conchoidal. This fracture surface ex-hibits concentric, bowl-shaped struc-tures like the inside of a clam shell(for example, chert or obsidian).Fibrous or splintery. This fracture sur-face shows fibers or splinters (forexample, some serpentine).Hackly. This fracture surface hassharp, jagged edges (for example,shist).Uneven. This fracture surface isrough and irregular (for example, ba-salt).

Luster and ColorThe appearance of a mineral specimen in

reflected light is called its luster. Luster iseither metallic or nonmetallic. Common non-metallic lusters are—

Vitreous (having the appearance ofglass).Adamantine (having the brilliant ap-pearance of diamonds).Pearly (having the iridescence ofpearls).Silky (having a fibrous, silklike lus-ter).

Resinous (having the appearance ofresin).

For some minerals, especially the metallicminerals, color is diagnostic. Galena (leadsulphide) is steel gray, pyrite (iron sulphide)is brass yellow, and magnetite (an iron ore) isblack. However, many nonmetallic mineralsdisplay a variety of colors. The use of color inmineral identification must be madecautiously since it is a subjective determina-tion.

StreakThe color of a powdered or a crushed mineral

is called the streak. The streak is obtained byrubbing the rnineral on a piece of unglazed por-celain, called a streak plate. The streak ismuch more consistent in a mineral than thecolor of the intact specimen. For example, anintact specimen of the mineral hematite (aniron ore) may appear black, brown, or red, butthe streak will always be dark red. The streakis most useful for the identification of dark-colored minerals such as metallic sulfides andoxides. Minerals with hardness 6.5 will not ex-hibit a streak, because they are harder than apiece of unglazed porcelain.

Specific GravityThe specific gravity of a substance is the

ratio of its weight (or mass) to the weight (ormass) of an equal volume of water. In fieldidentification of minerals, the heft, or ap-parent weight, of the specimen is an aid to itsidentification. Specific gravity and heft arecontrolled by the kinds of atoms making upthe mineral and the packing density of theatoms. For example, ores of lead always haverelatively high specific gravity and feel heavy.

COMMON ROCK-FORMING MINERALSThere are approximately 2,000 known

varieties of minerals. Only about 200 arecommon enough to be of geologic andeconomic importance. Some of the more im-portant minerals to military engineers are—

Quartz.Feldspars.Micas.


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Amphiboles.Pyroxenes.Olivine.Chlorite.Calcite.Dolomite.Limonite.Clay.

QuartzQuartz (silicon dioxide) is an extremely

hard, transparent to translucent mineralwith a glassy or waxy luster. Colorless towhite or smoky-gray varieties are most com-mon, but impurities may produce many othercolors. Like man-made glass, quartz has aconchoidal (shell-like) fracture, often imper-fectly developed. It forms pointed, six-sidedprismatic crystals on occasion but occursmost often as irregular grains intergrownwith other minerals in igneous and metamor-phic rocks; as rounded or angular g-rains insedimentary rocks (particularly sandstones);and as a microcrystalline sedimentary rock orcementing agent. Veins of milky whitequartz, often quite large, fill cracks in manyigneous and metamorphic rocks. Unlikenearly all other minerals, quartz is virtuallyunaffected by chemical weathering.

FeldsparsFeldspars form very hard, blocky, opaque

crystals with a pearly or porcelainlike lusterand a nearly rectangular cross section. Crys-tals tend to cleave in two directions along flat,shiny, nearly perpendicular surfaces.Plagioclase varieties often have fine parallelgrooves (striations) on one cleavage surface.Orthoclase varieties are usually pink, red-dish, ivory, or pale gray. Where more thanone variety is present, color differences arenormally distinct. Crystalline feldspars aremajor components of most igneous rocks,gneisses, and schists. In the presence of airand water, the feldspars weather to clayminerals, soluble salts, and colloidal silica.

MicasMicas form soft, extremely thin, trans-

parent to translucent, elastic sheets andflakes with a bright glassy or pearly luster.

“Books” of easily separated sheets frequentlyoccur. The biotite variety is usually brown orblack, while muscovite is yellowish, white, orsilvery gray. Micas are very common ingranitic rocks, gneisses, and schists. Micasweather slowly to clay minerals.

AmphibolesAmphiboles (chiefly hornblende) are hard,

dense, glassy to silky minerals found chieflyin intermediate to dark igneous rocks andgneisses and schists. They generally occur asshort to long prismatic crystals with a nearlydiamond-shaped cross section. Dark green toblack varieties are most common, althoughlight gray or greenish types occur in somemarbles and schists. Amphiboles weatherrapidly to form chlorite and, ultimately, clayminerals, iron oxides, and soluble carbonates.

PyroxenesPyroxenes (chiefly augite) are hard, dense,

glassy to resinous minerals found chiefly indark igneous rocks and, less often, in darkgneisses and schists. They usually occur aswell- formed, short, stout, columnar crystalsthat appear almost square in cross section.Granular crystals are common in some verydark gabbroic rocks. Masses of nearly purepyroxene form a rock called pyroxenite.Colors of green to black or brown are mostcommon, but pale green or gray varietiessometimes occur in marbles or schists.Pyroxenes weather much like the am-phiboles.

OlivineOlivine is a very hard, dense mineral that

forms yellowish-green to dark olive-green orbrown, glassy grains or granular masses invery dark, iron-rich rocks, particularly gab-bro and basalt. Masses of almost pure olivineform a rare rock called peridotite. Olivineweathers rapidly to iron oxides and solublesilica.

ChloriteChlorite is a very soft, grayish-green to

dark green mineral with a pearly luster. Itoccurs most often as crusts, masses, or thin


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sheets or flakes in metamorphic rocks, par-ticularly schists and greenstone. Chloriteforms from amphiboles and pyroxenes byweathering or metamorphism and, in turn,weathers to clay minerals and iron oxides.

CalciteCalcite is a soft, usually colorless to white

mineral distinguished by a rapid bubblingor fizzing reaction when it comes in contactwith dilute hydrochloric acid (HC1). Calciteis the major component of sea shells andcoral skeletons and often occurs as well-formed, glassy to dull, blocky crystals. As arock-forming mineral, it usually occurs asfine to coarse crystals in marble, loose tocompacted granules in ordinary limestone,and as a cementing agent in many sedimen-tary rocks.

Calcite veins, or crack fillings, are commonin igneous and other rocks. Calcite weatherschiefly by solution in acidic waters or watercontaining dissolved carbon dioxide.

DolomiteDolomite is similar to calcite in appearance

and occurrence but is slightly harder andmore resistant to solutioning. It is distin-guished by a slow bubbling or fizzing reactionwhen it comes in contact with dilute HC1.Usually the reaction can be observed only ifthe mineral is first powdered (as by scrapingit with a knife). Coarse dolomite crystalsoften have curved sides and a pinkish color.Calcite and dolomite frequently occurtogether, often in intimate mixtures.

LimoniteLimonite occurs most often as soft,

yellowish-brown to reddish-brown, fine-grained, earthy masses or compact lumps orpellets. It is a common and durable cement-ing agent in sedimentary rocks and them majorcomponent of laterite. Most weathered rockscontain some limonite as a result of thedecomposition of iron-bearing minerals.

ClayClay minerals form soft microscopic flakes

that are usually mixed with impurities of

various types (particularly quartz, limonite,and calcite). When barely moistened, as bythe breath or tongue, clays give off a charac-teristic somewhat musty “clay” odor. Claysform a major part of most soils and of suchrocks as shale and slate. They are a commonimpurity in all types of sedimentary rocks.

Section II. Rocks

FORMATION PROCESSESA rock may be made of many kinds of

minerals (for example, granite containsquartz, mica, feldspar, and usuallyhornblende) or may consist essentially of onemineral (such as a limestone, which is com-posed of the mineral calcite). To the engineer,rock is a firm, hardened substance that, incontrast to soil, cannot be excavated by stand-ard earthmoving equipment. In reality, thereis a transitional zone separating rock and soilso that not all “rock” deposits require blast-ing. Some “rock” can be broken usingpowerful and properly designed rippingequipment. The geologist places less restric-tion on the definition of rock.

Rocks can be grouped into three broad clas-ses, depending on their origin. They are—

Igneous.Sedimentary.Metamorphic.

Igneous rocks are solidified products ofmolten material from within the earth’smantle. The term igneous is from a wordmeaning “formed by fire. ” Igneous rocks un-derlie all other types of rock in the earth’scrust and may be said to form the basement ofthe continents on which sedimentary rocksare laid down. Most sedimentary rocks areformed by the deposition of particles of olderrocks that have been broken down andtransported from their original positions bythe agents of wind, water, ice, or gravity.Metamorphic rocks are rocks that have beenaltered in appearance and physical propertiesby heat, pressure, or permeation by gases orfluids. All classes of rocks (sedimentary,igneous, and metamorphic) can bemetamorphosed. Igneous, sedimentary, and


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metamorphic rocks often occur in close as-sociation in mountainous areas, in areas onceoccupied by mountains but which have sinceeroded, and in broad, flat continental regionsknown as shields. Flat-lying sedimentaryrocks form much of the plains of the con-tinents and may occupy broad valleysoverlain by recent or active deposits of sedi-ments. The sediments being deposited intoday’s oceans, lakes, streams, floodplains,and deserts will be the sedimentary rocks oftomorrow. The rock-forming processes con-tinually interact in a scheme called the rockcycle, illustrated in Figure 1-4.

IgneousIgneous rocks are solidified from hot mol-

ten rock material that originated deep withinthe earth. This occurred either from magmain the subsurface or from lava extruded ontothe earth’s surface during volcanic eruptions.Igneous rocks owe their variations in physicaland chemical characteristics to differences inchemical composition of the original magmaand to the physical conditions under whichthe lava solidified.

The groups forming the subdivisions fromwhich all igneous rocks are classified are—

Intrusive igneous rocks (cooled frommagma beneath the earth’s surface).Extrusive igneous rocks (cooled frommagma on the earth’s surface).

Figure 1-5 is a block diagram illustratingthe major kinds of intrusive and extrusiverock bodies formed from the crystallization ofigneous rocks. Dikes and sills are tabular ig-neous intrusions that are thin relative totheir lengths and widths. Dikes are discor-dant; they cut across the bedding of the stratapenetrated. Sills are concordant; they in-trude parallel to and usually along beddingplanes or contacts of the surrounding strata.Dikes and sills may be of any geologic age andmay intrude young and old sediments.Batholiths are large, irregular masses of in-trusive igneous rock of at least 40 squaremiles in area. A stock is similar to a batholithbut covers less than 40 square miles in out-crop. Stocks and batholiths generallyincrease in volume (spread out) with depth

and originate so deep that their base usuallycannot be detected. Magma that reachesthe earth’s surface while still molten isejected onto the ground or into the air (orsea) to form extrusive igneous rock. Themolten rock may be ejected as a viscous liq-uid that flows out of a volcanic vent or fromfissures along the flanks of the volcano. Theflowing viscous mass is called lava and thelava flow may extend many miles from thecrater vent. Lava that is charged with gasesand ejected violently into the air formspyroclastic debris consisting of broken andpulverized rock and molten material. Thepyroclastics solidify and settle to the groundwhere they form deposits of ash and larger-sized material that harden into layered rock(tuff). Igneous rocks are usually durableand resistant and form ridges, caps, hills,and mountains while surrounding rockmaterial is worn away.

The chemical composition and thus themineral content of intrusive and extrusive ig-neous rocks can be similar. The differences inappearance between intrusive and extrusiverocks are largely due to the size and arrange-ment of the mineral grains or crystals. Asmolten material cools, minerals crystallizeand separate from solution. Silica-richmagma or lava solidifies into rocks high insilicon dioxide (quartz) and forms thegenerally light-colored igneous rocks. Moltenmaterial rich in ferromagnesian (iron-magnesium) compounds form the darker-colored igneous rocks, which are deficientin the mineral quartz. If the magma coolsslowly, large crystals have time to grow. Ifthe magma (or lava) cools quickly, large crys-tals do not have the chance to develop.Intrusive rocks are normally coarse-grainedand extrusive igneous rocks are fine-grainedfor this reason. If lava cools too quickly forcrystals to grow at all, then natural glassforms. Figure 1-6, page 1-8, illustrates thedifference between intrusive and extrusiveigneous rock crystals.

SedimentarySedimentary rocks, also called stratified

rocks, are composed of chemical precipitates,biological accumulations, or elastic particles.Chemical precipitates are derived from the


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decomposition of existing igneous, sedimen-tary, and metamorphic rock masses.Dissolved salts are then transported from theoriginal position and eventually become in-soluble, forming “precipitates”; or, throughevaporation of the water medium, they be-come deposits of “evaporites.” A relativelysmall proportion of the sedimentary rockmass is organic sediment contributed by theactivities of plants and animals. Clastic sedi-ments are derived from the disintegration ofexisting rock masses. The disintegratedrock is transported from its original posi-tion as solid particles. Rock particlesdropped from suspension in air, water, orice produce deposits of “elastic” sediments.Volcanically ejected material that istransported by wind or water and thendeposited forms another class of layered rockscalled “pyroclastics.” Most pyroclasticdeposits occur in the vicinity of a volcanicregion, but fine particles can be transportedby the wind and deposited thousands of milesfrom the source. Inorganic elastic sedimentsconstitute about three-fourths of thesedimentary rocks of the earth’s crust. Loosesediments are converted to rock by severalprocesses collectively known as lithification.These are—

Compaction.Cementation.Recrystallization.

The weight of overlying sediments thathave accumulated over a longtime producesgreat pressure in the underlying sediments.The pressure expels the water in the sedi-ments by the process of compaction and


forces the rock particles closer together.Compaction by the weight of overlying sedi-ments is most effective in fine-grainedsediments like clay and silt and in organicsediments like peat. Cementation occurswhen precipitates of mineral-rich waters, cir-culating through the pores of sediments, fillthe pores and bind the grains together. Themost common cementing materials arequartz, calcite (calcium carbonate), and ironoxides (limonite and hematite). Recrystal-lization and crystal growth of calciumcarbonate dissolved in saturated lime sedi-ments develop rocks (crystalline limestone anddolomite) with interlocking, crystalline fabrics.

Sedimentary rocks are normally depositedin distinct parallel layers separated byabrupt, fairly even contact surfaces calledbedding planes. Each layer represents a suc-cessive deposit of material. Bedding planesare of great significance as they are planes ofstructural weakness. Masses of sedimentaryrock can move along bedding planes duringrock slides. Figure 1-7 represents the layer-cake appearance of sedimentary rock beds.Sedimentary rocks cover about 75 percent ofthe earth’s surface. Over 95 percent of thetotal volume of sediments consists of a varietyof shales, sandstones, and limestones.

MetamorphicThe alteration of existing rocks to

metamorphic rocks may involve the forma-tion within the rock of new structures,textures, and minerals. The major agents inmetamorphism are—

Temperature.Pressure.Chemically active fluids and gases.

Heat increases the solvent action of fluidsand helps to dissociate and alter chemicalcompounds. Temperatures high enough toalter rocks commonly result from the in-trusion of magma into the parent rock in theform of dikes, sills, and stocks. The zone of al-tered rock formed near the intrusion is calledthe contact metamorphism zone (see Figure1-8). The alteration zone may be inches tomiles in width or length and may grade

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laterally from the unaltered parent rock to thehighly metamorphosed derivative rock. Pres-sures accompanying the compressive forcesresponsible for mountain building in the upperearth’s crust produce regional metamorphicrocks characterized by flattened, elongated,and aligned grains or crystals that give the rocka distinctive texture or appearance called folia-tion (see Figure 1-9, page 1-10). Hot fluids,especially water and gases, are powerfulmetamorphic agents. Water under heat and

pressure acts as a solvent, promotes recrys-tallization, and enters into the chemicalcomposition of some of the altered minerals.

CLASSIFICATIONIgneous, metamorphic, and sedimentary

rocks require different identification andclassification procedures. The fabric, tex-ture, and bonding strength imparted to a rockby its formation process determine the proce-dures that must be used to classify it.


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IgneousIgneous rocks are classified primarily on

the basis of—Texture.Color (or mineral content).

Texture is the relative size and arrange-ment of the mineral grains making up therock. It is influenced by the rate of cooling ofthe molten material as it solidifies into rock.Intruded magmas cool relatively slowly andform large crystals if the intrusion is deep andsmaller crystals if the intrusion is shallow.Extrusive lava is exposed abruptly to the airor to water and cools quickly, forming smallcrystals or no crystals at all. Therefore, refer-ring to Table 1-2, igneous rocks may havetextures that are coarse-grained (mineralgrains and crystals that can be differen-tiated by the unaided eye), veryfine-grained (mineral crystals too small tobe differentiated by the unaided eye), or ofcontrasting grain sizes (large crystals, or


“phenocrysts, “ in a fine-grained “groundmass”). The intrusive igneous rocks generallyhave a distinctive texture of coarse interlock-ing crystals of different minerals. Undercertain conditions, deep-seated intrusionsform “pegmatites” (rocks with very large crys-tals). The extrusive (volcanic) igneous rocks,however, show great variation in texture.Very fine-grained rocks maybe classified ashaving stony, glassy, scoriaceous, or fragmen-tal texture. A rock with a stony textureconsists of granular particles. Fine-grainedrocks with a shiny smooth texture showing aconchoidal fracture are said to be “glassy.”An example is obsidian, a black volcanicglass. Gases trapped in the extrusive lavamay escape upon cooling, forming bubblecavities, or vesicles, in the rock. The result is arock with scoriaceous texture. Fragmentalrocks are those compsed of lithified, pyroclas-tic material. The pyroclastic (volcanoclastic)deposits are made up of volcanic rock particles ofvarious sizes that have drifted and accumulatedby the action of wind and water after ejection

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from the volcanic vent. Fine-grained (smallerthan 32 millimeters (mm)) ejects (called lapil-li, or ash) form deposits that become volcanictuff. Lava flowing out of the volcanic vent orfissure forms a flow with a ropelike texture ifthe lava is very fluid. More viscous lava formsa blocky flow. Upon cooling, basaltic lavaflows, sills, and volcanic necks sometimescrack. They often acquire columnar joint-ing characterized by near-verticalcolumns with hexagonal cross sections(see Figure 1-10, page 1-12).

Igneous rocks are further grouped by theiroverall color, which is generally a result oftheir mineral content (see Table 1-2). The

light-colored igneous rocks are silica-rich,and the dark-colored igneous rocks aresilica-poor, with high ferromagnesian con-tent. The intermediate rocks show gradationsfrom light to dark, reflecting their mixed orgradational mineral content. An example il-lustrating the use of Table 1-2 is as follows:lava and other ejects charged with gases mayform scoria, a dark-colored, highly vesicularbasaltic lava or pumice, a frothy, light-coloredfelsite lava so porous that it floats on water.

The common igneous rocks are—Granite.Felsite.Gabbro and diorite.


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Basalt.Obsidian.Pumice.Scoria.

Granite. This is a coarsely crystalline, hard,massive, light-colored rock composed mainlyof potassium feldspar and quartz, usuallywith mica and/or hornblende. Commoncolors include white, gray, and shades of pinkto brownish red. Granite makes up most ofthe large intrusive masses of igneous rockand is frequently associated with (and maygrade into) gneisses and schists. In general,it is a reasonably hard, tough, and durablerock that provides good foundations, buildingstones, and aggregates for all types of con-struction. Relatively fine-grained varietiesare normally much tougher and more durablethan coarse-grained types, many of which dis-integrate rather rapidly under temperatureextremes or frost action. Very coarse-grainedand quartz-rich granites often bond poorlywith cementing materials, particularly as-phaltic cements. Antistripping agentsshould be employed when granite is used inbituminous pavements.

Felsite. This is a very fine-grained, usuallyextrusive equivalent of granite. Colors com-monly range from light or medium gray topink, red, buff, purplish, or light brownishgray. Felsites often contain scattered largecrystals of quartz or feldspar, Isolated gasbubbles and streaklike flow structures arecommon i n felsitic lavas. As a rule, felsitesare about as hard and dense as granites,but they are generally tougher and tend to


splinter and flake when crushed. Most fel-sites contain a form of silica, which producesalkali-aggregate reactions with portland ce-ments. Barring these considerations, felsitescan provide good general-purpose aggregatesfor construction.

Gabbro and Diorite. They form a series ofdense, coarsely crystalline, hard, dark-colored intrusive rocks composed mainly ofone or more dark minerals along withplagioclase feldspar. Since gabbro anddiorite have similar properties and maybedifficult to distinguish in the field, they areoften grouped under the name gabbro-diorite.They are gray, green, brown, or black.Gabbro-diorites are common in smaller in-trusive masses, particularly dikes andsills. As a group, they make strong foun-dations and excellent aggregates for alltypes of construction. However, theirgreat toughness and high density makeexcavation and crushing costs very high,particularly in finer-grained varieties.

Basalt. This is a very fine-grained, hard,dense, dark-colored extrusive rock that oc-curs widely in lava flows around the world.Colors are usually dark gray to black,greenish black, or brown. Scattered coarsecrystals of olivine, augite, or plagioclase arecommon, as are gas bubbles that may or maynot be mineral-filled. With increasing grainsize, basalt often grades into diabase, an ex-tremely tough variety of gabbro. Both basaltand diabase make aggregates of the highestquality despite a tendency to crush into chipsor flakes in sizes smaller than 2 to 4 cen-timeters.

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Obsidian. This is a hard, shiny, usuallyblack, brown, or reddish volcanic glass thatmay contain scattered gas bubbles or visiblecrystals. Like man-made glass, it breaksreadily into sharp-edged flakes. Obsidian ischemically unstable, weak, and valueless as aconstruction material of any type.

Pumice. This is a very frothy or foamy,light-colored rock that forms over glassy orfelsitic lava flows and in blocks blown fromerupting volcanoes. Innumerable closelyspaced gas bubbles make pumice lightenough to float on water and also impart goodinsulating properties. Although highlyabrasive, pumice is very weak and canusually be excavated with ordinary handtools. It is used in the manufacture of low-strength, lightweight concrete and concreteblocks. Most varieties are chemically un-stable and require the use of low-alkaliportland cements.

Scoria. It looks very much like a coarse,somewhat cindery slag. In addition to itsfrothy texture, scoria may also exhibit stonyor glassy textures or a combination of both.The color of scoria ranges from reddish brownto dark gray or black. Scoria is somewhatdenser and tougher than pumice, and the gasbubbles that give it its spongy or frothy ap-pearance are generally larger and morewidely spaced than those in pumice. Scoriais very common in volcanic regions andgenerally forms over basaltic lava flows. It iswidely used as a lightweight aggregate in con-crete and concrete blocks. Like pumice, itmay require the use of special low-alkali ce-ments.

SedimentarySedimentary rocks are classified primarily

by—Grain size.Composition.

They can be described as either elastic or non-elastic (see Table 1-3, page 1-14). The elasticrocks are composed of discrete particles, orgrains. The nonclastic rocks are composed ofinterlocking crystals or are in earthy masses.

Clastic sedimentary rocks are further clas-sified as coarse-grained or fine-grained. Thecoarse-grained rocks have individual grainsvisible to the naked eye and includesandstones, conglomerates (rounded grains),and breccias (angular grains). These are therock equivalents of sands and gravels.The fine-grained rocks have individualgrains that can only be seen with the aidof a hand lens or microscope and includesilts tones, shales, clays tones, andmudstones.

Shales, claystones, and mudstones arecomposed of similar minerals and may besimilar in overall appearance; however, ashale is visibly laminated (composed of thintabular layers) and often exhibits “fissility,”that is, it can be split easily into thin sheets.Claystone and mudstone are not fissile.Mudstone is primarily a field term used totemporarily identify fine-grained sedimen-tary rocks of unknown mineral content.

The nonelastic sedimentary rocks can befurther described as inorganic (or chemical)or organic. Dolomite is an inorganic calcium-magnesium carbonate. Chert, a widespread,hard, durable sedimentary rock, composed ofmicrocrystalline quartz, precipitates fromsilica-rich waters and is often found in or withlimestones. Limestone is a calcium carbonatethat can be precipitated both organically andinorganically. A diagnostic feature of lime-stone is its effervescence in dilute HC1. Coalis an accumulation and conversion of the or-ganic remains of plants and animals undercertain environments.

Important features of the exposed orsampled portion of a deposit includestratification, the thickness of strata, theuniformity or nonuniformity of strata lat-erally, and the attitude (strike and dip) of thebedding planes. Special sedimentary bed-ding features are—

Cross bedding (individual layerswithin a bed lie at an angle to thelayers of adjacent beds (see Figure1-11, page 1-14), typical of sand duneand delta front deposits).


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Mud cracks (polygonal cracks in thesurface of dried-out mud flats).Ripple marks (parallel ridges in somesediments that indicate the directionof wind or water movement duringdeposition).

Some typical sedimentary rocks are—Conglomerate and breccia.

Chert.

Conglomerate and Breccia. They resem-ble man-made concrete in that they consist ofgravel-sized or larger rock fragments in afiner-grained matrix. Different varieties aregenerally distinguished by the size or com-position of the rock fragments (such aslimestone breccia, boulder conglomerate, or

Sandstone. quartz pebble conglomerate). Wide varia-Shale. tions in composition, degree of cementation,Tuff. and degree of weathering of component par-Limestone. titles make these rocks highly unpredictable,Dolomite. even within the same deposit. Generally,


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they exhibit poor engineering propertiesand are avoided in construction. Some veryweakly cemented types may be crushed foruse as fill or subbase material in road or air-field construction.

Sandstone. This is a medium- to coarse-grained, hard, gritty, elastic rock composedmainly of sand-sized (1/16 mm to 2 mm)quartz grains, often with feldspar, calcite, orclay. Sandstone varies widely in propertiesaccording to composition and cementation.Clean, compact, quartz-rich varieties, well-cemented by silica or iron oxides, generallyprovide good material for construction of alltypes. Low-density, poorly cemented, andclayey varieties lack toughness anddurability and should be avoided as sources ofconstruction material; however, some clay-free types may be finely crushed to providesand.

Shale. This is a soft to moderately hardsedimentary rock composed of very fine-grained silt and clay-sized particles as well asclay minerals. Silica, iron oxide, or calcite ce-ments may be present, but many shales lackcement and readily disintegrate or slakewhen soaked in water. Characteristically,shales form in very thin layers, break intothin platy pieces or flakes, and give off amusty odor when barely moistened. Oc-casionally, massive shales (called mudstones)occur, which break into bulky fragments.Shales are frequently interbedded withsandstones and limestones and, with increas-ing amounts of sand or calcite, may grade intothese rock types. Most shales can be ex-cavated without the need for blasting.Because of their weakness and lack ofdurability, shales make very poor construc-tion material.

Tuff. This is a low-density, soft to moder-ately hard pyroclastic rock composed mainlyof fine-grained volcanic ash. Colors rangefrom white through yellow, gray, pink, andlight brown to a rather dark grayish brown.When barely moistened, some tuffs give off aweak “clay” odor. Very compact varietiesoften resemble felsite but can usually be dis-tinguished by their softness and the presence

of glass or pumice fragments. Loose, chalkytypes usually feel rough and produce a grittydust, unlike the smooth particles of a truechalk or clay. Tuff is a weak, easily excavatedrock of low durability. When finely ground, ithas weak cementing properties. It is oftenused as an “extender” for portland cementand as a pozzolan to improve workability andneutralize alkali-aggregate reactions. It canalso be used as a fill and base course material.

Limestone. This is a soft to moderately hardrock composed mainly of calcite in the form ofshells, crystals, grains, or cementingmaterial. All varieties are distinguished by arapid bubbling or fizzing reaction when theycome in contact with dilute HC1. Colors nor-mally range from white through variousshades of gray to black; other colors mayresult from impurities. Ordinary limestone isa compact, moderately tough, very fine-grained or coarsely crystalline rock thatmakes a quality material for all constructionneeds. Hardness, toughness, and durabilitywill normally increase with greater amountsof silica cement. However, more than about30 percent silica may produce bondingproblems or alkali-aggregate reactions.Clayey varieties usually lack durability andtoughness and should be avoided. Weak, low-density limestones (including limerock andcoral) are weakly recemented when crushed,wetted, and compacted. They are widely usedas fill and base course material. In mildclimates, some may prove suitable for use inlow-strength portland cement concrete.

Dolomite. It is similar to limestone exceptthat the mineral dolomite occurs in lieu of cal-cite. It is distinguished by a slow bubbling orfizzing reaction when it comes into contactwith dilute HC1. Often the reaction cannot beseen unless the rock is first powdered (as byscraping with a knife). Limestone anddolomite exhibit similar properties, and oftenone grades into the other within a singledeposit.

Chert. This is a very hard, very fine-grainedrock composed of microcrystalline silicaprecipitated from seawater or groundwater.It occurs mainly as irregular layers or


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nodules in limestones and dolomites and aspebbles in gravel deposits or conglomerates.Most cherts are white to shades of gray. Verydark, often black, cherts are called flint,while reddish-brown varieties are calledjasper. Pure, unweathered cherts breakalong smooth, conchoidal (shell-like) surfaceswith a waxy luster; weathered or impureforms may seem dull and chalky-looking. Al-though cherts are very hard and tough, theyvary widely in chemical stability anddurability. Many produce alkali-aggregatereactions with portland cement, and most re-quire the use of antistripping agents withbituminous cements. Low-density chertsmay swell slightly when soaked and break upreadily under frost action. Despite theseproblems, cherts are used in road construc-tion in many areas where better materials arenot available.

MetamorphicMetamorphic rocks are classified primariIy

by—Mineral content.Fabric imparted by the agents ofmetamorphism.

They are readily divided into two descriptivegroups (see Table 1-4) known as—

Foliated.Nonfoliated.

Foliated metamorphic rocks display apronounced banded structure as a result ofthe reformational pressures to which theyhave been subjected. The nonfoliated, ormassive, metamorphic rocks exhibit no direc-tional structural features.

The common foliated or banded metamor-phic rocks include—

Slate.Schist.Gneiss.

Slate. This is a very fine-grained, compactmetamorphic rock that forms from shale. Un-like shale, slates have no “clay” odor. Theysplit into thin, parallel, sharp-edged sheets(or plates) usually at some angle to any ob-servable bedding. Colors are normally darkred, green, purple, or gray to black. Slates arewidely used as tiles and flagstones, but theirpoor crushed shape and low resistance tosplitting makes them unsuitable for ag-gregates or building stones.

Schist. This is a fine- to coarse-grained, well-foliated rock composed of discontinuous, thinlayers of parallel mica, chlorite, hornblende,or other crystals. Schists split readily alongthe structural layers into thin slabs or flakes.This characteristic makes schists un-desirable for construction use and hazardousto excavate. However, varieties intermediateto gneiss maybe suitable for fills, base cour-ses, or portland cement concrete.

Gneiss (pronounced “nice”). This is a roughlyfoliated, medium- to coarse-grained rock thatconsists of alternating streaks or bands. Thebanding is caused by segregation of light-colored layers of quartz and feldsparalternating with dark layers of ferromag-nesian minerals. These streaks may bestraight, wavy, or crumpled and of uniform or


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variable thickness. Gneisses normally breakinto irregular, bulky pieces and resemble thegranitic rocks in properties and uses. Withincreasing amounts of mica or more perfectlayering, gneisses grade into schists.

The common nonfoliated metamorphicrocks include—

Quartzite.Marble.

Quartzite. This is an extremely hard, fine-to coarse-grained, massive rock that formsfrom sandstone. It is distinguished fromsandstone by differences in fracture.Quartzite fractures through its componentgrains rather than around them as insandstone, because the cement and sandgrains of quartzite have been fused or weldedtogether during metamorphism. Therefore,broken surfaces are not gritty and often havea splintery or sugary appearance like that of abroken sugar cube or hard candy. Quartziteis one of the hardest, toughest, and mostdurable rocks known. It makes excellent con-struction material, but excavation andcrushing costs are usually very high. Be-cause of its high quartz content, antistrippingagents are normally required withbituminous cements, Even so, bonds may bepoor with very fine-grained types.

Marble. This is a soft, fine- to coarselycrystalline, massive metamorphic rock thatforms from limestone or dolomite. It is distin-guished by its softness, acid reaction, lack offossils, and sugary appearance on freshlybroken surfaces. Marble is similar to ordi-nary compact or crystalline limestones in itsengineering properties and uses. However,because of its softness, it is usually avoided asan aggregate for pavements on highways andairfields. White calcite or pinkish dolomiteveins and subtle swirls or blotches of trace im-purities give marble its typical veined or“marbled” appearance.

Metamorphic rocks have been derived fromexisting sedimentary, igneous, or metamor-phic rocks as depicted in Figure 1-4, page 1-7and Figure 1-12, page 1-18.

IDENTIFICATIONMilitary engineers must frequently select

the best rock for use in different types of con-struction and evaluate foundation orexcavation conditions.

Field IdentificationA simple method of identification of rock

types that can be applied in the field will as-sist in identifying most rocks likely to beencountered during military construction.This method is presented in simple terms forthe benefit of the field engineer who is notfamiliar with expressions normally used intechnical rock descriptions.

The identification method is based on acombination of simple physical and chemicaldeterminations. In some cases, the grainscomposing a rock may be seen, and the rockmay be identified from a knowledge of its com-ponents. In other cases, the rock maybe sofine-grained that the identification must bebased on its general appearance and theresults of a few easy tests.

The equipment required consists only of agood steel knife blade or a nail and a bottle ofdilute (10 percent solution) HC1, preferablywith a dropper. A small 6- to 10-power mag-nifying glass may also be helpful. HC1(muriatic acid) is available at most hardwarestores and through the military supply sys-tem. Military hospitals are a typical sourcefor HC1.

Samples for identification should be clean,freshly broken, and large enough to clearlyshow the structure of the rock. In a smallsample, key characteristics (such as anyalignment of the minerals composing therock) may not be observed as readily as in alarger one. Pieces about 3 inches by 4 inchesby 2 inches thick are usually suitable.

General CategoriesThe identification system of geologic

materials is given in flow chart form (seeTable 1-5, page 1-19). In this method, all con-siderations are based on the appearance or


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character of a clean, freshly broken, un-weathered rock surface. Weathered surfacesmay exhibit properties that may not be trueindicators of the actual rock type.

For most rocks the identification process isdirect and uncomplicated. If a positive iden-tification cannot be made, the more detailedrock descriptions (in the preceding para-graphs) should be consulted after first usingthe flow chart to eliminate all clearly inap-propriate rock types. By the use of des-criptive adjectives, a basic rock classificationcan be modified to buildup a “word picture” ofthe rock (for example, “a pale brown, fine-grained, thin-bedded, compact, clayey,silica-cemented sandstone”).

To use the flow chart, the sample must beplaced into one of three generalized groups


based on physical appearance. These groupsare—

Foliated.Very fine-grained.Coarse-grained.

Foliated. Foliated rocks are those metamor-phic rocks that exhibit planar orientation oftheir mineral components. They may ex-hibit slaty cleavage (like slate) expressedby closely spaced fractures that cause thesample to split along thin plates. If thesample exhibits a parallel arrangement ofplaty minerals in thin layers and has asilky or metallic reflection, the sample hasschistosity and is called a schist. If thesample exhibits alternating streaks orbands of light and dark minerals of differ-ing composition, then the sample hasgneissic layering and is called a gneiss.

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Very Fine-Grained. If the sample appearspitted or spongy, it is called frothy. The pitsare called vesicles and are the result of hotgases escaping from magma at the top of alava pool. If the sample is light enough tofloat on water and is light-colored, it is calledpumice. If the frothy rock sample is dark-colored and appears cindery, it is calledscoria.

If the sample has the appearance of brokenglass, it is called either obsidian or quartz.Obsidian is a dark-colored natural volcanicglass that cooled too fast for any crystals todevelop. Quartz is not a glass but is identifiedon the flow chart as light-colored and glassy.Quartz is a mineral and not one of the ag-gregates classifed for use in militaryconstruction. It is often found in its crystal-line form with six-sided crystals. It iscommon as veins in both igneous andmetamorphic rock bodies.

A hardness test is conducted to determinewhether a stony sample is hard or soft. If thesample can be scratched with a knife or nail,then it is said to be soft. If it cannot bescratched, then it is said to be hard.

Fine-grained rocks maybe glassy, frothy, orstony. The term “stony” is used to differen-tiate them from “glassy” and “frothy” rocks.

If the sample is soft, a chemical determina-tion must be made using dilute HC1, HC1tests determine the presence or absence ofcalcite (calcium carbonate) which compriseslimestone/dolomite and marble. Very fine-grained rocks that are soft and stony inappearance and have an acid reaction arecomposed of calcium carbonate. If the sampleis dull and massive, it is called a limestone.Dolomites are also dull and massive and arenot separated from limestones on this chart,but they do not readily react to acid. Theirsurface must be powdered first by scratchingthe sample with a nail. The dolomite powderthen readily reacts to the acid. If the sampleexhibits a sugary appearance, then it is themetamorphic rock, marble. The sugary ap-pearance is due to the partial melting orfusing of calcite crystals by heat and pressure.

It is similar to the appearance of the sugarcoating on a breakfast cereal. These rocks aresusceptible to chem ical attack by acidic solu-tions that form when carbon dioxide (CO2) isabsorbed in groundwater. This produces amild carbonic acid that dissolves the calcite inthe rock. This is the process that producescaves and sinkholes. If there is not an acidreaction (no effervescence) and the samplehas a platy structure, it is a shale. A shalemay be any color. It is normally dull andseparates into soft, thin plates. When freshlybroken, it may have a musty odor similar toclay. Shales are derived from lithification ofclay particles and fine muds. Shale is asedimentary rock and should not be confusedwith its metamorphic equivalent, slate.Shales often occur interbedded within layersof limestone and dolomite. Because they areoften found with carbonate rocks, they mayexhibit an acid reaction due to contaminationby calcium carbonate.

If the sample has no acid reaction and has arelatively low density, with small pieces ofglass in its fine-grained matrix, the rock iscalled a tuff. Tuff is a volcanic sedimentaryrock comprised mostly of ash particles thathave been solidified. By the flow chart, it ischaracterized as soft; however, it may be hardif it has been welded by hot gases during theeruption. If it is hard, tuff may exhibit physi-cal properties that make it suitable for someconstruct ion applications.

Fine-grained, stony samples that are hardmay be waxy, dull, or sandy. Waxy samplesresemble candle wax and have a conchoidalfracture and sharp edges. These rocks arecalled chert. They often weather into soft,white materials. Dull samples are eitherlight- or dark-colored igneous extrusive rocks.If the sample is light-colored, it is a felsite.Felsites are normally massive but may ap-pear banded or layered. Unlike gneiss, thelayers are not made up of alternating lightand dark minerals. Felsites may also havecavities filled with other lighter-coloredminerals. They represent a variety of lavarocks that are high in silica. Because of theirgreat variety, they may be hard to identify


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properly. The dark counterpart to the felsitesis basalt. Basalt is normally black and verytough. It is also a lava rock and often exhibitscolumnar jointing. It often occurs as dikes orsills in other rock bodies.

Hard, sandy, very fine-grained samplescomposed mostly of quartz may be eithersandstone or quartzite. If the sample feelsgritty like sandpaper, it is called sandstone.Sandstones are sedimentary rocks composedof sand grains that have been cementedtogether. If the sample appears sugary anddoes not feel like sandpaper, the rock is calledquartzite. Quartzite is the metamorphicequivalent of sandstone. Like marble, it has asugary appearance, which is due to the par-tial melting and fusing of the crystal grains.

Coarse-Grained. Coarse-grained rocksrefer to those that have either crystals or ce-mented particles that are large enough to bereadily seen with the unaided eye. Samplesmay be hard or soft. Hard samples may ap-pear sandy, mixed, or fragmental. A sandysample would be a sandstone or a quartzite.Because coarse sands grade into fine sands,there are coarse sandstones and finesandstones. The sugary-appearing meta-morphic version of a coarse sandstone iscalled a quartzite.

Hard, coarse-grained rocks that are com-prised of mixed interlocking crystals have asalt-and-pepper appearance due to their lightand dark minerals. If the rock is predomi-nantly light in color, it is a granite. If it ispredominantly dark in color, it is called agabbro-diorite. Both of these rocks are ig-neous intrusive rocks that cooled slowly,allowing the growth of large interlockingcrystals. If the sample appears to be made ofround, cemented rock fragments similar tothe appearance of concrete, it is called a con-glomerate. If the rock fragments are angularinstead of round, the rock is called a breccia.

If a coarse-grained sample is soft, it mayeither be fragmental or have an acid reaction.If it appears fragmental and has a low densityand small pieces of glass, it is a tuff. Tuff has

already been described as a very fine-g-rained,stony, soft rock material. This entry on thechart is for the coarse-grained version of thesame material. The coarse-grained rocksamples that are soft and have an acid reac-tion are coarse-grained limestones andmarbles, as already described.

The system of identification of rock typesused by military engineers serves to identifymost common rock types. This method re-quires the user to approach rock identificationin a mnsistent and systematic manner; other-wise, important rock characteristics may gounnoticed. Many important rock featuresmay not appear in small hand specimens. Toenable better identification and evaluation,personnel who are in charge of geologic ex-ploration should maintain careful notes onsuch rock features as bedding, foliation,gradational changes in composition orproperties, and on the associations of rocks inthe field. During preliminary reconnaissancework, geologic maps or map substitutesshould be used to make preliminary engineer-ing estimates based on the typical rockproperties.

Engineering PropertiesThe following engineering properties and

tests are provided to help make engineeringjudgments concerning the use of rockmaterials as construction aggregate:

Toughness.Hardness.Durability.Crushed shape.Chemical stability.Surface character.Density.

Preliminary engineering estimates can bemade based on the typical rock properties as-certained from these tests. If a rock samplecannot be identified using the flow chart, adecision can be made as to its suitability foruse as a construction material by thesetests.

Toughness. This is mechanical strength, orresistance to crushing or breaking. In the


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field, this property may be estimated by at-tempting to break the rock with a hammer orby measuring its resistance to penetration byimpact drills.

Hardness. This is resistance to scratchingor abrasion. In the field, this may be es-timated by attempting to scratch the rockwith a steel knife blade. Soft materials arereadily scratched with a knife, while hardmaterials are difficult or impossible to scratch(see Table 1-6).

Durability. This is resistance to slaking ordisintegration due to alternating cycles ofwetting and drying or freezing and thawing.Generally, this may be estimated in the fieldby observing the effects of weathering onnatural exposures of the rock.

Crushed Shape. Rocks that break into ir-regular, bulky fragments provide the bestaggregates for construction because the par-ticles compact well, interlock to resistdisplacement and distribute loads, and are ofnearly equal strength in all directions. Rocksthat break into elongated pieces or thin slabs,sheets, or flakes are weak in their thin dimen-sions and do not compact, interlock, ordistribute loads as effectively (see Figure1-13).

Chemical Stability. This is resistance toreaction with alkali materials in portland ce-ments. Several rock types contain impureforms of silica that react with alkalies in ce-ment to form a gel. The gel absorbs water and

expands to crack or disintegrate the hardenedconcrete. This potential alkali-aggregatereaction may be estimated in the field only byidentifying the rock and comparing it toknown reactive types or by investigatingstructures in which the aggregate has pre-viously been used.

Surface Character. This refers to the bond-ing characteristics of the broken rock surface.Excessively smooth, slick, nonabsorbent ag-gregate surfaces bond poorly with cementingmaterials and shift readily under loads. Ex-cessively rough, jagged, or absorbent surfacesare likewise undisturbed because they resistcompaction or placement and require exces-sive amounts of cementing material.

Density. This is weight per unit volume. Inthe field, this may be estimated by “hefting” arock sample (see Table 1-7). Density reflectson excavation and hauling costs and may in-fluence the selection of rocks for specialrequirements (such as riprap, jetty stone, orlightweight aggregate). Among rocks of thesame type, density is often a good indicator ofthe toughness and durability to be expected,Table 1-8, page 1-24, lists the general ratingsof rock properties for each of the 18 typicalmilitary construction aggregates. These


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ratings serve only as a guide; each individualrock body must be sampled and evaluatedseparately. Table 1-9, page 1-24, providesgeneral guidance to determine the suitabilityof an unidentified rock sample for generalmilitary construction missions based on theevaluation of its physical properties.

Table 1-10, page 1-25, provides a rating forselected geologic materials concerning their

suitability as an aggregate for concrete or as-phalt or for their use as a base coursematerial. Rock materials that typically ex-hibit chemical instability in concrete aremarked with an asterisk. These materialsmay cause a concrete-alkali reaction due totheir high silica content. In general, felsites,chert, and obsidian should not be used as con-crete aggregate. The end result of thesereactions is the weakening, and in extremecases the failure, of the concrete design. Ap-parently, silica is drawn out of the aggregateto make a gel that creates weaknesses withinthe mix. The gel may expand with tempera-ture changes and prevent the proper bondingof the cement and aggregate.

Rock types with two asterisks may not bondreadily to bituminous materials. They re-quire special antistripping agents to ensurethat they do not “strip,” or separate, from thepavement mix. Stripping severely reducespavement performance.


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chapter 1 rocks and minerals section i. mineralsrocks and minerals 1-2 cleavage cleavage is the...

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