• Color of Minerals
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Idiochromatic color
– color caused by elements in chemical formula--Cu in malachite and azurite and Mn in rhodochrosite

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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.
– 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
MnO
2
• red color in many minerals is caused by hematite inclusions

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•
Play of Colors (Iridescence)
• defracted or reflected light off features on the mineral
– labradorite and opal

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• silky or wavey appearance across the surface of the mineral as it is rotated--

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• a star like figure on surface of mineral caused by reflected or scattered light off included minerals aligned parallel to crystallographic axes

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Luminescence
• caused by impurities (activators) in a mineral--some specimens of fluorite, scheelite, and calcite
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Fluorescence
• emission of wavelengths of light caused by electron excitation in metals during bombardment by ultraviolet or X-rays
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Phosphorescence
• continuous emission of wavelengths after excitation source is removed
• Thermoluminescence
• emissions of wavelengths when subjected to heat at less than “red hot” temperatures

•
• 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
•
• the same as above brought about by stimulation of mineral with heat--quartz and tourmaline

•
• 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
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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

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• 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

•
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

•
• 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

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

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

• Other types pisolitic 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 goethitelimonite

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