Mineral Formulae
Note: know the formulas for those listed in your lab, marked $. If you cannot memorize a
long formula, try to characterize it. Examples marked with an asterisk, *.
Secondary sedimentary occurrences are not mentioned for the silicates.
NESOSILICATES (alternate name Orthosilicates)
YSiO4
$ Olivines: simple solid solution Y2SiO4 or (Mg,Fe)2SiO4
Forsterite Mg2SiO4
Fayalite Fe2SiO4
OCC: Partial melting of mantle => Peridotites, Basaltic melt, Ol first to xtalz,
Basalts:1. MOR, Rift decompression melting of mantle; 2. Subduction Zone ~ 150 km, dewatering of
subducted Ocean Lithosphere; 3. Partial melting of deep mantle at mantle – core boundary => Basaltic
Plume
USE: 90% of Forsterite is used as a slag conditioner, 10% as a refractory.
Common Appearance: Tiny olive grains, weathers rusty.
$ Garnets:
Y3Al2Si3O12
OCC: Metamorphism of Mudstones
Uses: abrasives
Common Appearance: rhombic dodecahedra or cubic, H ~7+ , no consistent cleavages.
$ Zircon ZrSiO4
OCC: Common as tiny xtals in Granite, large in Alkaline Pegmatites
USE: ceramics; metamict: radiometric dating; source U, Th
Common Appearance: Tetragonal H 7.5, rarely metamict (black from radiation)
Aluminum Silicate Polymorphs
$ Sillimanite Al2SiO5
$ Kyanite Al2SiO5
$ Andalusite Al2SiO5
Yellow-brown needles
Blue or white long Blades
Dark, often cross in xsec
OCC: Metamorphism of Mudstones
Uses P,T index minerals, refractory
Common app: Given above
$ Staurolite (Fe,Mg)2Al9Si4O22(OH)2
*An Fe,Mg hydroxylated Aluminosilicate
OCC: Metamorphism of Mudstones
Uses P,T index minerals, refractory
Common App: brown Laths in side view, elongate diamond-shape or el. hexagon xsec
$ Topaz Al2SiO4(OH)2
* a hydroxylated Aluminosilicate
OCC: Late fractionation of magma (Granite & Rhyolites, Pegmatites)
Uses refractory see: www.dpi.nsw.gov.au/__data/assets/pdf_file/0019/.../Topaz.pdf
Common App: H 8, Prismatic crystals with faces striated parallel to long dimension, wht-yellow
SOROSILICATES contain Si2O7
$ Epidote Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH)
* a Ca Fe+3 Al Sorosilicate
OCC: Retrograde Metamorphism and hydrothermal alteration of common minerals
(feldspars, micas, pyroxenes, amphiboles, garnets).
Uses Index mineral for low pressure (= shallow depth)
Common App: Usually Dark Green, with striated long Prismatic xtals, weathers to
granular powder with distinctive light yellow-green color
$ Hemimorphite Zn4(Si2O7)(OH)2.H2O
* a Zinc Sorosilicate
OCC: Oxidation of the upper parts of Sphalerite ZnS bearing ore bodies, in the gossan
of weathered Zinc Sulfide deposits.
Uses Mined for Zinc
Common App: crystals are terminated by dissimilar faces.
CYCLOSILICATES 3,4 or 6-member rings. Ours all have 6-Silica rings
$ Tourmaline (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4 * a hydroxylated Boron
Cyclosilicate
OCC: in granite and granite pegmatites and in metamorphic rocks such as schist and
marble.
Use: Refractory; Boron is used to dope quartz to make a semiconductor chip in computers
Common Appearance: Elongated crystals, trigonal to hexagonal xsec, usually black but
sometimes colored, even multi-colored.
CHAIN SILICATES (INOSILICATES)
Pyroxenes (single chains) SiO3, the tetrahedra alternate direction in pairs.
Two prominent cleavages, meeting at angles near 90 degrees.
Cpx
$ Diopside CaMgSi2O6
OCC: In peridotite xenoliths from the Earth's mantle; contact metamorphism of
Dolomites
Use: Ceramics, Geothermometer
Common Appearance: Usually medium green, two prominent cleavages, meeting at
angles near 90 degrees, H 5-6
$ Augite (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6. * a solid solution of Diopside and Hedenbergite,
can also have Na and Al substituting
OCC: in mafic = basaltic rx, e.g. Basalt and Gabbro, most commonly at MORs and Rift
Valleys.
Use: Geothermometer, and major component of Basaltic Rx used as Trap rock.
Common Appearance: Usually Dark Green, , two prominent cleavages, meeting at angles
near 90 degrees, H 5-6.5
Pyroxenoids also SiO3, but tetrahedra do not alternate direction in pairs
$ Rhodonite MnSiO3
OCC: Hydrothermal Deposits
Use: Minor ore of Manganese
Common Appearance: Rose-red, oxidizes brown.
Amphiboles (double chains) X2Y5Si8O22 (OH)2. Two cleavages at about 120 and 60 degrees.
$ Hornblende Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 a mixture of Amphiboles with Aluminum
OCC: The Mafic mineral in intermediate magmas, e.g. Andesite and Diorite
Use:
Common Appearance: Dark green, straight stringy appearance in one
direction, two cleavages at about 124 and 56 degrees.
SHEET SILICATES (PHYLLOSILICATES)
Micas
Phyllosilicates with (AlSi3O10)(OH)2
$ Muscovite KAl2(AlSi3O10)(OH)2 Light Brown sheets
$ Biotite
K(Mg,Fe)3(AlSi3O10)(OH)2 Dark Brown to Black Sheets
OCC: In Granites and Metamorphosed mudrocks
Use: Finishing of drywall, insulation, electrical insulator
Common Appearance: see above.
Clays *hydrated aluminosilicates
$Kaolinite Group, polymorphs of Al2Si2O5(OH)4
OCC: Hydrolysis weathering of Feldspars
Use: Porcelain, paper, food, light bulbs. Kaopectate.
Common Appearance: Soft white powdery, swells when wet.
Miscellaneous Phyllosilicates
$ Talc Mg3Si4O10(OH)2
OCC: Metamorphism of ultramafic (mantle) rocks, and in blueschist provinces (metam. wet ophilites
from subduction zones.)
Use: Paper, Paint, Electric Cable.
Common Appearance: White, greasy slippery feel, soft
FRAMEWORK SILICATES (TECTOSILICATES)
A. Feldspars
1. Plagioclase
XAl (Al,Si)3O8
$ Anorthite CaAl2Si2O8
$ Albite
NaAlSi3O8
Note the coupled solid solution, Ca++ and Al+3
or Na+ and Si+4
OCC: Anorthite in Basaltic Rocks, Albite in Granitic Rocks
Use: glassmaking, ceramics
Common Appearance: Hardness ~6, 2 good cleavages at 90o , Anorthite is Gray, Albite white.
Plagioclase is often Striated
2. K-spars
$ Microcline KAlSi3O8
Low T
OCC: Granitic Rocks and Pegmatites
Use: glassmaking, ceramics
Common Appearance: Hardness ~6, 2 good cleavages at 90o , Microcline is pink to tan with light
often with streaks that are exsolved Albite.
B. SiO2 Group
$ Quartz SiO2
OCC: In Granitic Rocks
Use: glassmaking, ceramics, abrasives, semiconductor chips
Common Appearance: Hardness 7, NO CLEAVAGE just Conchoidal fractures, Many colors
Plagioclase is often Striated
C. Feldspathoids form when Silica SiO2 is not abundant.
$Sodalite Na8Al6Si6O24Cl2
OCC: Veins in Nepheline Syenites
Use: Carvings
Common Appearance: Royal Blue color is distinctive
OXIDES
$ Corundum Al2O3
OCC: Metamorphosed mudstones.
Use: abrasive
Common Appearance: Scratches Quartz, Center of crystals appear swollen
$ Hematite Fe2O3
OCC: most is weathered Magnetite
Use: Ore of Iron
Common Appearance: Black to red, RED STREAK
$ Magnetite Fe3O4
OCC: Common Acc Min in Igneous Rx
Use: Ore of Iron
Common Appearance: Black, MAGNETIC i.e. it diverts the needle of a cheap compass.
HYDROXIDES
$ Gibbsite Al(OH)3
OCC: usually as a component of the mixture ore “Bauxite”
an ore of Aluminum, from secondary enrichment of Al from laterites
Use: Aluminum is a light structural metal: aircraft, etc.
Common Appearance: Wavy opaque white, looks like white agate.
SULFIDES
$ Pyrite FeS2
OCC:
Use: Manufacture of Sulfuric Acid
Common Appearance: Shiny gold, NO CLEAVAGES
$ Chalcopyrite CuFeS2
OCC: Sulfide deposits from Hydrothermal cells, e.g. MORs
Use: Ore of Copper
Common Appearance: Like tarnished Pyrite, tarnish is rusty colored.
$ Sphalerite ZnS
OCC: Sulfide deposits from Hydrothermal cells, e.g. MORs
Use: Ore of Zinc
Common Appearance: Dark Brown YELLOW STREAK
$ Galena PbS
OCC: Sulfide deposits from Hydrothermal cells, e.g. MORs
Use: Ore of Lead, sometimes Silver and Gold.
Common Appearance: 3 good cleavages at 90o, metallic luster.
$ Cinnabar HgS
OCC: a vein-filling mineral associated with recent volcanic activity and alkaline hot
springs.
Use: Ore of Mercury. Mercury is used to make, for example, modern light bulbs
Common Appearance: Cinnabar is usually a bluish pink
$ Bornite Cu5FeS4
OCC: Sulfide deposits from Hydrothermal cells, e.g. MORs
Use: Minor ore of copper
Common Appearance: Dark, with a iridescent purple tarnish
$ Pyrrhotite Fe(1-x)S
OCC: a common trace constituent of mafic igneous rocks, and as sulfide “intrusions” with
pentlandite, chalcopyrite and other sulfides
Use: Pyrrhotite does not have specific applications. It is mined primarily because it is associated
with pentlandite, a sulfide mineral that can contain significant amounts of nickel and cobalt
Common Appearance: Looks like Chalcopyrite, but crystals are pseudo-hexagonal.
$ Realgar As4S4 or AsS
OCC: Hot Springs
Use: Ore of Arsenic; arsenic is used to dope quartz to make a semiconductor chip in computers
Common Appearance: amorphous red, with yellow Orpiment
$ Orpiment As2S3
OCC: Hot Springs above magma.
Use: Ore of Arsenic
Common Appearance: amorphous yellow, with red Realgar
SULFATES
$ Gypsum CaSO4
OCC: Evaporites
Use: Drywall = sheet rock; plaster of Paris
Common Appearance: two good cleavages at 90o , soft, hardness 2 so scratched by fingernail.
Sometimes makes a “desert rose”
$ Anhydrite CaSO4
$ Barite BaSO4
OCC: Hot Springs for example in our own Rift Valley.
Use: Ore of Barium, used as drilling mud.
Common Appearance: Heavy, usually white. Sometimes makes a “desert rose”
CARBONATES
$ Calcite Low Pressure CaCO3
OCC: Limestone from fossil shells, also hot springs: seawater saturated in Ca++ and CO2
Use: Concrete, optics.
Common Appearance:
$ Aragonite High Pressure CaCO3
OCC: Limestone from Seashells, caves.
Use: Pure source of CaCO3 for laboratory use, Concrete
Common Appearance: Cyclic Twins
$ Azurite Cu3(CO3)2(OH)2
OCC: From weathered Copper sulfides, due oxidation of Copper ions in the zone of aeration
Use: Pigment
Common Appearance: Dark Blue. Weathers to green Malachite, with which it is usually associated.
2 Cu3(CO3)2(OH)2 + H2O → 3 Cu2(CO3)(OH)2 + CO2
$ Malachite Cu2CO3(OH)2
OCC: From weathered Copper sulfides, due oxidation of Copper ions in the zone of aeration
Use: Pigment
Common Appearance: Green. Often from weathered Azurite.
$ Dolomite CaMg(CO3)2
OCC: Flooding of limestone islands, seawater infiltrates Calcite, some Ca++ replaced by Mg++
Use: concrete; source of magnesium ( the Pidgeon process).
Common Appearance: Crystal masses look like pink rice or like maggots
PHOSPHATES
$ Apatite Ca5(PO4)3(F,Cl,OH)
OCC: Metamorphosed limestones (i.e. Marble)
Use: Fertilizer, source of Phosphorous; Ore of Rare Earth Minerals; Phosphorus is used to dope
quartz to make semiconductor chips.
Common Appearance: Hardness 5,~ hex xsec, colors vary
HALIDES
$ Fluorite CaF2
OCC: In gangue of weathered sulfides.
Use: Made into Cryolite, a flux for Aluminum smelting. Also made into HF hydrofluoric Acid.
Common Appearance: 4 cleavages, hardness 4 so scratches copper in penny, many colors
$ Halite NaCl
OCC: Evaporites
Use: Food, preservative
Common Appearance: Clear to white, 3 good cleavages at 90o
$ Sylvite KCl
OCC: Evaporites
Use: Potassium Fertilizer
Common Appearance: Colorless to White to Red, Cubic, 3 good cleavages at 90o