Minerals
Definition: Mineralogy -the study of minerals
Definition:
Mineral -a naturally-occurring inorganic (non-living) material with a unique
crystal structure.
Definition: Rock -a mixture of minerals
4 Characteristics of a Mineral:
1. must be naturally occurring (not artificially made in laboratories)
2. must be inorganic (non-living)
3. must be solid
4. must possess a definite chemical structure (see notes on Atomic Structure)
Properties of Minerals:
a)
Crystal form -orderly arrangement of atoms
-without space restrictions, crystals will form crystal faces (ie.
Pyrite or quartz)
-with space restrictions, crystals become jammed together and
do not form crystal faces
-most inorganic solids are composed of crystals, not clearly
visible to eye
b)
Lustre -light reflected from the surface of a mineral
-minerals that have the appearance of a metal (though they may
not be metal) have a metallic lustre
c)
Color -unreliable property of identifying a mineral, ie. Quartz may be pink, purple, white or black,
depending on impurities
d)
Streak -color of a mineral in its powdered form
-better indication of true color
-use a streak plate made of porcelain
e)
Hardness -very useful property for identification of minerals
-measure of resistance of a mineral to scratching
-geologists use standard hardness scale  Moh’s Scale (from 1 to 10)
Relative Scale
1
2
3
4
5
6
7
8
9
10
Mineral
Talc (softest)
Gypsum
Calcite
Fluorite
Apatite
Feldspar
Quartz
Topaz
Corundum
Diamond (hardest)
(5.5 glass, pocketknife; 3 copper penny; 2.5 fingernail)
f)
Cleavage -tendency of a mineral to break along planes where atomic bonding is weak
g)
Fracture -when minerals do not cleave  they break!
h)
Specific Gravity -compares the weight of a mineral to weight of an equal volume of water
-example: if a mineral weighs 3 times as much as an equal volume of water, its specific gravity is
3 (specific gravity of water is 1)
Mineral Groups:
-over 4000 minerals known
-40-50 new discovered each year
-only a few dozen abundant & make up Earth’s crust
-these are called rock-forming minerals
-only 8 elements mostly compose these minerals
-represent 98% of continental crust (by weight)
Relative Abundance of Most Common Elements in Earth’s Crust
Element
Oxygen
Silicon
Aluminum
Iron
Calcium
Sodium
Potassium
Magnesium
Others
TOTAL
% by Weight
46.6
27.7
8.1
5.0
3.6
2.8
2.6
2.1
1.5
100
-2 most abundant elements are silicon & oxygen  combine to form silicates
-all silicates have the same fundamental building block  silicon-oxygen tetrahedron
-tetrahedron consists of 4 oxygen atoms surrounding a smaller silicon atom
-iron, magnesium, sodium, aluminium and calcium most often link silicates
Common Silicate Minerals:
-feldspars are most abundant group of silicate minerals  making up 50% of Earth’s crust
-quartz only mineral made completely of silicon & oxygen  2nd most abundant mineral
-most silicate minerals form when molten rock cools  cooling occurs at or near Earth’s surface (low
temperature and pressure) or at great depths (high temperature and pressure)
-temperature, pressure & chemical composition affect type of silicate minerals formed  example:
olivine forms at high temperatures; quartz forms at low temperatures
-some silicate minerals form at Earth’s surface from weathering of older silicate minerals
-other silicate minerals form under extreme pressure through orogenesis (mountain-building)
-by examining minerals within rocks, geologists determine conditions under which rocks were formed
Feldspar
Quartz
Dark Silicate Minerals:
-those silicate minerals that contain iron and/or magnesium
-darker color due to iron
-have greater specific gravity (between 3.2 and 3.6)
-common dark silicate minerals are olivine, pyroxenes, amphiboles, biotite
Olivine
Pyroxene
Amphibole
Light Silicate Minerals:
-those silicate minerals that contain aluminum, potassium, calcium and/or sodium
-lighter color
-have specific gravity about 2.7
Common Light Silicate Minerals:
a) Feldspars
i. Orthoclase feldspars contain potassium and have creamy to pink color
ii. Plagioclase feldspars contain sodium and calcium and have white to grey
color
Orthoclase Feldspar
Plagioclase Feldspar
Definition: Striations
-fine parallel lines found on cleavage faces
-plagioclase feldspars have striations; orthoclase feldspars do not
b) Quartz
-only silicate mineral consisting of silicon & oxygen only
-hard, resistant to weathering, does not have cleavage
-most common varieties are milky (white), smoky (gray), rose
(pink), methyst (purple) & rock crystal (clear)
Amethyst
Rose Quartz
c) Muscovite
-common member of the mica family
-light in color  pearly lustre
-has excellent cleavage in one direction
-can make thin sheets of muscovite  used as
glass in Middle Ages
Important Non-Silicate Minerals:
-carbonate structure contains Carbon & Oxygen (1 carbon bonded with 3
oxygen)
-simpler structure than silicate
-most common minerals are calcite (calcium carbonate) & dolomite
(calcium/magnesium carbonate)
Calcite
Dolomite
-difficult to tell calcite from dolomite
-adding hydrochloric acid to each can be used to tell the difference  acid reacts quickly with calcite &
slowly with dolomite
-calcite & dolomite are primary ingredients in sedimentary rocks limestone (mostly calcite) & dolostone
(mostly dolomite)
-other non-silicate minerals often found in sedimentary rocks are halite (table salt) & gypsum (found in
gyp-rock)
Halite
Gypsum