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Chapter 2

Earth Sciences 1022A/B Chapter Notes - Chapter 2: Conchoidal Fracture, Birefringence, Mohs Scale Of Mineral Hardness


Department
Earth Sciences
Course Code
EARTHSCI 1022A/B
Professor
Stephen R Hicock
Chapter
2

Page:
of 4
Minerals 9/13/2012 9:16:00 AM
Definition of Minerals
Variation of Minerals
Cleavage
Mineralogy
study of minerals
Rock
- any solid mass of mineral that occurs naturally as part of our planet
- most are aggregate of various minerals
Minerals
solid chemical compound that is found in nature
chemical compounds that combine to form rocks
every mineral is a molecule
hardness, cleavage varies depending on element composition
4000 minerals, 20 common
some have same chemical composition but different physical characteristics
polymorphs eg. Graphite and Diamonds
Criteria
1. occur naturally
2. solid at surface temp and pressure
3. posses orderly internal structure, atoms arranged in pattern
4. definite chemical composition
5. usually inorganic
Cleavage
how minerals break along the weakest plains
Crystal Habit (shape)
• external expression of a mineral
Lustre
• quality of light reflected from the surface
Streak
• colour of mineral in powdered form
Hardness **
• mineral’s resistance to abrasion, measured on Mohs scale
Fracture
• minerals that have similar strength in all directions and lack ability to cleavage
can fracture in distinctive manner. Eg. conchoidal fracture (curved like glass)
Specific Gravity
• number representing ratio of weight of mineral to weight of an equal
Double refraction
- when placed over letter, reflects twice
Common Minerals
Most abundant
mineral
Silicon-oxygen
tetrahedron.
Ferromagnesian
kinds.
Olivine
Pyroxene
Amphibole
Biotite
Nonferromagnesian
Muscovite
Common Minerals
Silicates
most abundant;
made of the silicon-oxygen tetrahedron
(silica, (SiO4)-4) bonded to various metallic
cations
tetra = 4, hedron = pyramid
Common Silicate Minerals
Ferromagnesian (dark) silicates:
contain Fe+2 and Mg+2 cations
olivine
- single silica tetrahedral bonded to Fe+2, Mg +2 ions
- grass-green in colour
- found deep in crust
pyroxene
- single chains of tetrahedral
- 90 degree cleavage
- dark looking mineral
amphibole
- double chains of tvetrahedra
- 120° cleavage
- darker looking
biotite (black mica)
- tetrahedral sheets
- perfect (basal) cleavage
- weak bonding between the dbl sheets
- peels easily because of weak bonding, very thin
Nonferromagnesian (light) silicates:
muscovite (white mica)
- perfect (basal) cleavage (weak bond where K is)
Feldspar
Quartz
Carbonate types
Calcite
Dolomite
Minerals that are
formed by
evaporation?
Halite
Gypsum
Ores of Metals
Hematite
Sphalerite
Galena
Gold and Silver
- lighter colour
feldspar network silicate, 2 types
orthoclase (pink K feldspar)
- right angle cleavage
plagioclase (Ca-Na feldspar)
- light to dark variation in colour
quartz
pure all tetrahedral (SiO2)
- no cleavage, will not break in plains
- hardness of 7 (max is 10 diamonds)
- broken beer bottle, like glass, sweeping ridged shiny fractured pattern (conchoidal
fracture)
Important Non silicate Minerals
Carbonate: metals bonded to (CO3)-2 group form, rhombusshaped
Calcite CaCO3; used for lime, cement
Dolomite (CaMg(CO3)2)
Minerals formed by evaporation
halite (NaCl; table salt) cubes shaped, chlorine, baking soda
gypsum (CaSO4· 2H2O; used to make plaster and drywall) are formed by
evaporation of ancient shallow seas and salt ponds
- soft mineral
- basal cleavage
Ores of metals
hematite (Fe2O3; iron ore) hema latin: blood, when scratched, blood like streak
appears
sphalerite (ZnS, zinc ore) heavy
galena (PbS; lead ore) bright shiny saucepan like
- cubic cleavage
native gold, silver (Au, Ag)
Summary:
- many minerals tend to occur together because they were formed under similar
pressure and temperature conditions, but with different combinations of elements
- minerals can be transformed into new minerals by being transported to another part
of the rock cycle - the new minerals form from previous ones as pressure and
temperature conditions change in the new environment