Geotechnical Engineering A

2. Minerals(Reference to notes: www.geology.csupomona.edu

Textbook: A Geology for Engineers by Blyth & Freitas)

Contents1. Minerals2. Mineralogy

2.1 Chemical Mineralogy 2.3 Crystallography Mineralogy2.2 Physical Mineralogy

2.2.1 Colour2.2.2 Streak2.2.3 Lustre2.2.4 Hardness2.2.5 Cleavage2.2.6 Fracture2.2.7 Tenacity2.2.8 Specific Gravity2.2.9 Crystal Shapes – Forms

2.4 Other Properties

3. Rock Forming Minerals 4. Identification of Unknown Minerals

1. Minerals • A mineral is a naturally occurring, inorganic (non-living)

substance. • Each mineral has a particular chemical composition which can

be expressed as a chemical formula. • Although most minerals are chemical compounds, a small

number (e.g. sulfur, copper and gold) are elements. • However, the majority of minerals are compounds comprising

two or more elements, for example NaCI which comprises sodium and chlorine, forming sodium chloride or Halite - Salt

• Minerals are usually of uniform composition and minerals form the building blocks which make up rocks.

• Within the rock mass, however, the proportion of minerals will vary from one sample to another.

• Some rocks comprise mainly one mineral type but the majority are a mixture of a number of different minerals.

• Mineralogy is that branch of geology which is concerned with the study of minerals.

2. Mineralogy • Mineralogy is the identification of minerals and the study of

their properties, origin, and classification.

Approximately 3,000 mineral exist in nature.

The most important properties for identifying minerals and distinguishing them from one another are:

1. Chemical mineralogy

2. Crystallography

3. Physical mineralogy

2.3 Crystallography• When minerals form, they can grow without interference from neighboring

grains.– They commonly develop into regular geometric shapes, called crystals

• The crystalline form of individual crystal which make up minerals is related to the atomic structure of the crystals.

• A very simple example of how the atomic structure influences the crystal shape is Sodium chloride NaCl (Salt)

This model show the atomic structure of salt - the balls represent the positions of the atoms within salt crystal

Note how the structural model closely resembles a real salt crystal – both look like cubes due to the packing of the atoms

• If mineral crystals grow freely form a liquid state, they assume their own characteristic shape, with the angles between adjacent crystal faces constant for similar crystals of a particular mineral.

• The faces are referenced to crystallographic axes

• Crystallographic axes intersect at a common origin and they form the framework on which the faces of the crystal are erected.

• Crystal systems are:

2.2 Physical Mineralogy• The physical properties of minerals are used to identify and

characterise them. • Most of the physical properties can be recognised at sight or

determined by simple tests. – Colour – Streak– Lustre– Hardness– Cleavage– Fracture– Tenacity– Specific Gravity– Crystal Shape - Forms

Not all of these properties are normally required to identify any one mineral; 2 or 3 of them taken together

may be sufficient

• Many other minerals show a variety of colours depending on the extent to which the base colour has been affected by trace substances.

• A typical example of this is fluorite which can be brown, purple, pink, green or blue when it contains impurities.

2.2.1 Colour• Some minerals have a distinctive colour which

clearly aids identification. – For example, sulphur is bright yellow.

• Minerals of one colour are said to be idiochromatic whereas minerals of variable colour are allochromatic.

• Streak is usually produced by rubbing the mineral on a piece of unglazed porcelain or on a streak plate.

2.2.2 Streak

• Streak is useful in clearly identifying some minerals but is not a useful diagnostic property of the many silicate minerals which are often too hard to produce streak. Black – Graphite

Black – Pryite

Black – Magnetite

Black – Chalcopyrite

Grey – Galena

Limonite – Yellow-brown

Hematite – Red-brown

• A mineral in a powdered condition has a much more reliable colour than its colour in the mass.

2.2.3 Lustre• Lustre is the is the way its surface reflects light.

Metallic Lustre - Pyrite

Non-metallic Lustre - Kaolinite

Glassy Lustre - Obsidian

– Non-metallic is further described as:• Vitreous – glassy – as in Obsidian

• Adamantine - Sparkly such as diamonds

• Resinous - Lustre of resin such as an opal• Pearly - As a pearl, talc being an example• Silky - Used to describe a fibrous structure• Greasy - Lustre produced by a mineral

with many tiny irregularities on the surface• Earthy - No lustre, also called dull

• It is subdivided into:– Metallic – as in Pyrite

– Sub-metallic as shown by semi-opaque oxides such as haematite

– Non-metallic as shown by transparent minerals – as in Kaolinite

Ref: www.geology.csupomona.edu

2.2.4 Hardness• This is a measure of resistance to abrasion or scratching.

• It is measured relative to a reference set often minerals which is known as Mohs’ Scale of Hardness:

MINERAL HARDNESS COMMON TESTS

Talc 1

Gypsum 2

Calcite 3 Scratched by copper coin

Fluorite 4

Apatite 5

Feldspar 6

Quartz 7

Topaz 8

Corundum 9

Diamond 10 Scratches all common materials

Scratched by fingernail

Steel Nail

Scratches a knife balde or window glass

• Hardness is tested by scratching the minerals of Mohs’ Scale with the specimen.

• Hence, a mineral which scratches 4 Fluorite but not 5 Apatite, has a hardness H = 4-5.

• This is a simple useful test and an example of its use is in distinguishing between Calcite (H=3) and Quartz (H=7).

2.2.5 Cleavage • Many minerals will break easily in certain directions.

• This feature is known as cleavage and it is dependant upon the atomic structure of the mineral.

• Cleavage can occur in one or more directions.

• The split will usually leave a smooth, plane surface called the cleavage plane.

• Mineral cleavage is described as perfect, good, distinct or imperfect.

Weak layers of bonds between atoms = cleavage planes

NaCl (salt)

2.2.6 Fracture• Fracture refers to the irregular, broken

surface of a mineral.

Cleavage – weak bonds

Fracture – strong bonds

• Types of fracture are:

Conchoidal - Breaks with a convex or concave surface

Even - Nearly flat

Uneven - rough

Rough – Small, shape, irregularities on the surface.

2.2.7 Tenacity

• This describes the response of a mineral to hitting, cutting or bending.

• Terms used to describe tenacity are:– Malleable – e.g. gold. copper.

– Brittle - Fracture when hit with a hammer

– Sectile - Can he cut with a knife

– Flexible and elastic - Mica bends and returns to a flat tabular state when left

– Flexible and inelastic - Gypsum flakes are flexible but do not regain their original shape

2.2.8 Specific Gravity

• Specific gravity is the ratio of the mass of a body of the solid to that of an equal volume of water.

• Most minerals and rocks have a specific gravity of between 2 and 5– but the Specific Gravity can range from about one to more than 20.

• Specific Gravity can be quite useful as some minerals feel unusually light or heavy.

halite 2.16 muscovite 2.8-3.0 rutile 4.2glauconite 2.3 apatite 3.2 zircon 4.7gypsum 2.32 hornblende 3.2 haematite 4.72feldspar 2.56-2.7 tourmaline 3.0-3.2 ilmenatite 4.8clays 2.5-2.8 sphene 3.5 pyrite 5.01quartz 2.65 topaz 3.6 monazite 5.2calcite 2.71 kyanite 3.6 magnetite 5.2dolomite 2.85 stauralite 3.7 cassiterite 6.9chlorite 2.6-3.3 garnet 3.7-4.3

Specific gravity of common minerals

Dendritic - Moss or tree like forms

Botryoidal - Spheroidal aggregations (resembling a bunch of grapes)

Reniform - Kidney shaped

Tabular – Based flat surfaces

Concretionary or nodular - Detached masses of spherical, ellipsoidal or irregular shape

Acicular - Fine needle like crystals, also known as filoform.

• Terms used to describe clusters of minerals is ‘Form’:

• Typical commonly occurring crystal shapes are:Drusv - closely packed small crystals growing into a cavity

Radiated - Needle like crystals radiating from a central point

Fibrous - Thread like strandsGranular -

In coarse or fine grains Reticulated - A

mesh of crossed crystals

2.2.9 Crystal Shapes - Forms

2.4 Other Properties (CAUTION)• Magnetism – a few minerals are

attracted to a magnet or are themselves capable of acting as magnets (e.g. magnetite)

• Feel – e.g. talc and graphite feel greasy or slippery when you rub your fingers over them – the pressure of your finger is sufficient break bonds and to slide planes of atoms past each other.

• Taste – e.g. salt, however some minerals can are poisonous!

• Reaction with Dilute Hydrochloric Acid –– Minerals containing the

carbonate anion (CO3)2-

effervesce (fizz) when a drop of dilute hydrochloric acid is placed on them.

– Carbon dioxide is released from the mineral & bubbles out through the acid – creating a fizz

3. Rock Forming Minerals

• The minerals within a rock sample are described as:

– Essential constituents whose presence is implied by the rock name

– Accessory which are present in smaller quantities than the essential minerals.

– Secondary minerals are those present in the rock which have resulted from the decomposition of previous minerals.

• Minerals in hand specimens of rock are usually observed and identified using a hand lens.

• The characteristics most often used in identifying minerals in rocks are:

– General shape of grains

– Colour / transparency

– Cleavage– Twinning (two crystals that

have grown together)

– Hardness

• Minerals combine with each other to form rocks– e.g. Granite (rock) = feldspar + quartz + mica + amphibole (minerals in varying ratio)

4. Identification of Unknown Mineralswww.geology.csupomona.edu/alert/mineral/id1.htm

Step 1 – Lustre (metallic or non-metallic)

Sheen of metal

Glassy (allow light to pass), dull or waxy

Step 2 – Hardness

Lustre = metallic

Fingernail (hardness = 1-2)

Steel Nail (Hardness = 5)

Knife blade (Hardness = >5)

Lustre = metallic; Hardness >5

Step 3 – Streak test

Black Red-brown

Lustre = metallic; Hardness >5; Streak = Red-brown

Step 3 – Cleavage

= no apparent cleavage

Hematite Fe2O3

Identification of MineralsMinerals

Step 2 – Hardness

Metallic (Step 1 – Lustre )

Fingernail (hardness 1-2)

Steel Nail (hardness 5)

Knife blade (hardness >5)

GRAPHITE

One direction

3 direction 900

GALENA

6 directions

SPHALERITE

Magnetic Non-magnetic

No Cleavage

No Cleavage

No Cleavage

No Cleavage

LIMONITE

CHALCOPYRITEGOLD

HEMATITE

Step 4 – CleavageStep 4 – Cleavage Step 4 –

Cleavage

Step 3 – StreakStep 3 – Streak Step 3 – Streak

No Cleavage

MAGNETITE PYRITE

Black Yellow-Brown Black Yellow Black Red-Brown

3 directions 900

GALENA

Nonmetallic (Step 1 – Lustre )

Dull, Vitreous, Adamantine,Resinous, Pearly, Silky, Greasy & Earthy

Step 2 – Hardness

Fingernail (hardness 1-2)

Steel Nail (hardness 5)

Knife blade (hardness >5)

Step 4 – Cleavage

One direction

TALC

No Cleavage (earthy smell)

Step 4 – Cleavage

Step 3 – StreakStep 3 – Streak Step 3 – Streak

Yellow Green BrownWhite

Small flakes slippery feel

Small green flakes

Larger, silvery or transparent

flakes

Colourless to chalky white, poor cleavage

(blocky)

CHLORITEMOSCOVITE GYPSUM

KAOLINITE

No Cleavage

SULFUR

One direction

CALCITE

White Yellow Brown

3 directions not 900

Fizzes vigorously in hydrochloric acid

Only slight fizzes in hydrochloric acid - fizzes

more when mineral is crushed into a power

BIOTITE

DOLOMITE

3 directions 900

HALITE

4 directions

FLUORITE

No Cleavage

LIMONITE

White colourless

Red BrownPale Green - Colourless

2 directions 900

PLAGIOCLASE FELDPAR

ORTHOCLASE FELDPAR

No Cleavage

QUARTZ GARNET

No Cleavage

HEMATITE

Title: Minerals Rocks & Fossils

Author: Bishop, Woolley & Hamilton

Pub: Philip’s

ISBN: 0-540-07429-2

Price: £9.99

Modules: Geotechnical Engineering A – coursework number 1