Physical Properties of Minerals

Physical Properties• Color of Minerals

• Idiochromatic color– color caused by elements in chemical formula--Cu

in malachite and azurite and Mn in rhodochrosite

• Chromaphore color– color caused by concentrations of

elements not part of chemical formula--Cu and V in beryl (emerald), Fe in amethyst and V in zoisite (tanzanite)

• Electron and Molecular Transitions• Colors caused by Idiochromatic and

chromophore elements can be generated by• 1. Crystal field transitions

– wavelengths of light are absorbed by “d” electrons in transition elements causing emissions of wavelengths of light quanta resulting in color of mineral---Cr+3 in emerald and ruby, Mn+2 in morganite and Fe+2 in peridot

• 2. Molecular Orbital Transitions– ions undergo charge transitions or

electron hopping--Fe+2 to Fe+3 in aquamarine or Fe+2 to Ti+4 in sapphire

• Inclusions of Other Minerals• minerals dispersed or included in minerals can

influence color• chlorite dispersed in quartz (aventurine) can cause

a green color• black calcite can be caused by inclusions of MnO2

• red color in many minerals is caused by hematite inclusions

• Mineral Properties Related to Light, Heat, and Electricity• Play of Colors (Iridescence)

• defracted or reflected light off features on the mineral

– labradorite and opal

• Chatoyancy• silky or wavey appearance across the surface

of the mineral as it is rotated--

• Asterism• a star like figure on surface of mineral

caused by reflected or scattered light off included minerals aligned parallel to crystallographic axes

• Luminescence• caused by impurities (activators) in a

mineral--some specimens of fluorite, scheelite, and calcite

• Fluorescence• emission of wavelengths of light caused by

electron excitation in metals during bombardment by ultraviolet or X-rays

• Phosphorescence• continuous emission of wavelengths after

excitation source is removed• Thermoluminescence

• emissions of wavelengths when subjected to heat at less than “red hot” temperatures

• Piezoelectricity• a flow of electrons in minerals with exertion of

a compression force between the “c” crystallographic axis of a mineral which has no center of symmetry---quartz and tourmaline

• Pyroelectricity• the same as above brought about by

stimulation of mineral with heat--quartz and tourmaline

tourmaline

• Hardness• Moh’s hardness scale is a special list of minerals

with increasing relative hardness• 1. talc 6. feldspar• 2. gypsum 7. quartz• 3. calcite 8. topaz• 4. fluorite 9. corundum• 5. apatite 10. diamond

– glass and knife = 5.5 - 6– steel file = 6 -7– fingernail = 2.5– penny = 3– some minerals have more than 1 hardness

depending on the direction of scratching--kyanite= 5-7 and calcite between 2 and 3

• Tenacity• cohesiveness of mineral or resistance to

breaking, crushing, bending, etc.• 1. brittle--if a mineral powders easily--

quartz• 2. malleable--if mineral can be

hammered into sheets--native Cu, Au

• sectile--if mineral can be cut into thin shavings--talc

• ductile--if mineral can be drawn into wire--Cu, Au

• flexable--if a mineral is bent and does not assume its’ original shape

• elastic--if a mineral is bent and resumes its’ original shape--mica

talc

• Streak and Luster• streak is the color of the powder of the mineral on a

porcelain plate• luster is metallic (dark or black prominent streak--

dense and opaque to light) or nonmetallic ( translucent or transparent with a colorless or white streak) or somewhat inbetween called submetallic• some specific nonmetallic lusters are:

– 1.vitreous--resembling glass--quartz crystals– 2. resinous--resin like--sulfur and sphalerite– 3. pearly--mother of pearl like--talc

talc--pearly

• greasy--like grease or oil--massive quartz• silky--like silk--satin spar gyspum• adamantine--brillant with a high index of

refraction--diamond or clear quartz crystals

• Cleavage• ability of mineral to come apart in a

consistent way• breakage is along atomic planes--consistent

with crystal symmetry--- there can be from one to multidirectional cleavage from mineral to mineral

• Parting• can resemble cleavage• breakage of minerals along planes of weakness such

as twinning planes--minerals which grow around each other, each one forming at a different time

• caused by minerals being subjected to special pressures during formation

• Fracture• inability of a mineral to break in a consistent way• do not break along cleavage planes

• Kinds of fracture are:• concoidal--smooth, curved breakage--quartz• fibrous or splintery• hackly--jagged with sharp edges• irregular--rough surfaces

• Specific Gravity• a number expressing a ratio between a mineral and

the weight of an equal volume of water• same number as density without units• S.G. depends on:

• kinds of atoms (atomic weight) comprising mineral• packing of atoms(close or loosely packed)

• S.G. can be determined with a Jolly Balance • Crystal Habits and Aggregates

• appearance of a single crystal or aggregate of a crystals of a mineral• isolated individual crystals

– bladed--elongated flattened crystal like a knife blade

– acicular--thin needlelike crystal– capillary--hair like or thinner

acicular-- stibnite

• groups of distinct crystals– dendritic--resembling a branching tree

or veinlets on a tree leaf– radiated--crystal appearing in a radial

pattern– drusy--a surface containing very small

crystals

radiated--wavellite

• groups of distinct crystals in parallel or spherical form

– columnar--column like crystals– bladed--many flat knife like crystals– fibrous--parallel fibers– colloform--botryoidal (bunch of grapes,

reniform (kidney shaped), mammillary

fibrous--crysotile (serpentine)

colloform--hematite

• aggregate of scales or lamellae– foliated--easily splits into thin sheets or

leaves– plumose--scaly-feather like

• granular aggregate– equant crystal grains

foliated (micaceous)- micaequant granular mass- pyrite

• Other typespisolitic or oolitic--rounded masses of pea sized grains (pisolitic) or very small grains( oolitic)---this picture is bauxite (pisolitic) and some samples of hematite occur as oolitic

stalactitic--resembling stalactites---this example is goethite-limonite

massive--massive with no form or distinguishing features

geode--rock cavity filling with mineral crystals

banded--mineral showing narrow bands of different colors or textures