Basics of Mineralogy

Geology 200

Geology for Environmental Scientists

Terms to Know:

• Atom
• Molecule
• Proton
• Neutron
• Electron
• Isotope
• Ion
• Bonding
• ionic
• covalent
• metallic

The 6 Crystal Systems

• All have 3 axes, except for 4 axes in Hexagonal system
• Isometric -- all axes equal length, all angles 90^o
• Hexagonal -- 3 of 4 axes equal length, three angles @ 90^o, three @ 120^o
• Tetragonal -- two axes equal length, all angles 90^o not common in rock forming minerals)
• Orthorhombic -- all axes unequal length, all angles 90^o
• Monoclinic -- all axes unequal length, only two angles are 90^o
• Triclinic -- all axes unequal length, no angles @ 90^o

Crystal systems and example minerals:

Isometric - diamond, garnet, halite, pyrite

Hexagonal - quartz, calcite, dolomite

Tetragonal - not common in rock forming minerals

Orthorhombic - anhydrite, olivine, staurolite

Monoclinic - orthoclase, biotite, muscovite, amphibole, pyroxene, gypsum, kaolinite

Triclinic - plagioclase (Na-Ca-feldspar), microcline, kyanite 

Chemical Classification of minerals and some examples:

Native elements - gold, silver, copper

Sulfides - pyrite (FeS₂), galena (PbS)

Oxides - hematite, limonite (iron oxides)

Halides - rock salt or halite, fluorite

Carbonates - calcite, dolomite

Sulfates - gypsum, anhydrite

Silicates - quartz, biotite, K-feldspar, plagioclase, pyroxene, amphibole

Chemical Composition of Minerals

Many minerals have a precise chemical formula. Examples include:

quartz, SiO₂

calcite, CaCO₃

Other minerals have a variable formula because of ionic substitution, which does not change crystal structure. Examples include:

olivine (Mg, Fe)₂SiO₄

pyroxene (Mg, Fe)SiO₃

plagioclase NaAlSi₃O₈ or CaAl₂Si₂O₈

Major Silicate Mineral Groups based on tetrahedral configurations:

Isolated: olivine, garnet, kyanite

Double: uncommon in rocks

Rings: uncommon in rocks

Single chains: pyroxenes

Double chains: amphiboles

Sheets: micas, chlorite, clay minerals

Framework: feldspars and quartz

These are the physical properties most useful for mineral identification:

Color (be careful, not always diagnostic)

Luster

Transparency or Opacity

Crystal System

Crystal Habit

Twinning of crystals

Cleavage

Fracture

Hardness: Table 3.1 in Textbook

Specific Gravity or density

Streak

Moh’s Scale of Hardness

Density -- some examples

water 1 g/cm₃

quartz 2.65 g/ cm₃

olivine 3.37-4.40 g/ cm₃

lead 11 g/ cm₃

gold 20 g/ cm₃

Rock Forming Minerals

• Sialic Minerals
• Mafic Minerals
• Clay Minerals
• Non-silicate Minerals

Sialic Minerals - rich in Si and Al

• Feldspars
• K-feldspar: KAlSi₃O₈
• plagioclase: NaAlSi₃O₈ - CaAl₂Si₂O₈
• Quartz: SiO₂
• Muscovite: KAl₃Si₃O₁₀(OH)₂

Mafic Minerals - rich in Mg & Fe

• Olivine: (Mg,Fe)₂SiO₄
• Pyroxene: (Mg,Fe)SiO₃
• Amphibole: Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
• Biotite: K(Mg,Fe)₃AlSi₃O₁₀(OH)₂

Clay Minerals

• Form from weathering of silicate minerals; common in shale, mudstone, and soil.
• Kaolinite: Al₄Si₄O₁₀(OH)₈
• Montmorillonite or bentonite: (Al,Mg)₈(Si₄O₁₀)₃(OH)₁₀^.12H20
• Illite: more complex than montmorillonite

Non-Silicate, Sedimentary Minerals

• Calcite: CaCO₃
• Dolomite: CaMg(CO₃)₂
• Halite: NaCl
• Gypsum: CaSO₄^.2H₂O