Minerals I Elements of the Earth’s crust Properties of minerals Crystal form; Cleavage/Fracture; Color, streak, luster; Hardness, tenacity; Specific gravity; Acid test, magnetism, etc.

Why Study Minerals?

  Building blocks of Earth materials

  Economically important

  Essential to understanding rock formation

  Minerals are cool

naturally occurring - materials synthesized in laboratory do not count, must be formed by natural processes in wild

inorganic - not formed by organic chemistry (e.g., sugars, etc.)

homogeneous solid - single substance in solid phase which cannot be physically separated into simpler compounds

definite chemical composition - composition can be represented as chemical formula (e.g., NaCl), although variation in exact composition is possible (e.g., (Mg, Fe)2SiO4 )

ordered atomic arrangement – repeating structure at the atomic level, which expresses as symmetry in large specimens, i.e., crystalline structure.

http://www.uoregon.edu/~jrice/geol311

Mineral - A naturally occurring, inorganic, homogeneous solid with a definite chemical composition and an ordered atomic arrangement.

Mineral Formation Minerals found at the Earth’s surface formed from natural

processes both within the Earth and at the Earth’s surface. Environments vary greatly, and include:

  High temperature and pressure typical of the upper mantle.

  Evaporation basins

  Ocean bottoms

  Volcanic calderas and lava flows

  A host of stressful environments in mountain building zones

And thousands of others!

Mineral Properties

  Crystal Form   Cleavage/Fracture   Color   Streak Color   Luster

  Hardness   Tenacity   Specific Gravity   Acid Test   Magnetism

Most useful properties for mineral identification

Mineral Properties

 Crystal Form •  Cleavage/Fracture •  Color •  Streak Color •  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Crystal symmetry is controlled by the atomic structure of mineral

4-fold axes (90o)

90o

Symmetry

4-fold symmetry (90o)

http://www.a-m.de/englisch/lexikon/mineral/halogenide/halit-bild1.htm

3-fold symmetry (120o)

3-fold axes (120o) Symmetry

http://home.planet.nl/~bartdw/sphalerite.htm

Crystal Form There are only 6 symmetry classes; each mineral belongs to only ONE

symmetry class, and thus all crystals exhibit that symmetry only!

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isometric tetragonal orthorhombic

hexagonal monoclinic triclinic

Crystal Form Caveat One: Crowding of other crystals can mask symmetry

Caveat Two: Crystals are often too small to see

Symmetry can always be seen using techniques like X-ray diffraction

Mineral Properties

•  Crystal Form  Cleavage/Fracture

•  Color •  Streak Color •  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Cleavage - tendency to split along planes of weakness corresponding to weaker chemical links in internal structure of crystal

A single crystal can display several cleavage directions.

The cleavage directions are not always at right angles to each other. They also are not necessarily parallel to the crystal faces of the mineral.

1 direction 2 directions

Cleavage

Can be parallel to crystal faces, but is often at an angle

Parallel Cleavage

Fig. 02.19a

One Direction of Cleavage

One Direction of Cleavage

Fig. 02.21

Two Directions of Cleavage

Three Directions of Cleavage

Fig. 02.22 http://www.geology.sdsu.edu/visualgeology

http://www.greatsouth.net/http://www.xpsdata.com/

Fluorite Crystals

Four Directions of Cleavage

Mineral Properties

•  Crystal Form  Cleavage/Fracture

•  Color •  Streak Color •  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Fracture - any breakage that is not cleavage

conchoidal fracture

Characteristic of quartz, chert and flint

Fig. 02.23

conchoidal fracture

Mineral Properties

•  Crystal Form •  Cleavage/Fracture  Color

•  Streak Color •  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Characteristic for some minerals, useless for most

Spinel MgAl2O4

Hardness = 8

One mineral can come in a rainbow of colors

Quartz SiO2

Hardness = 7

Rose Quartz

Smokey Quartz

Quartz Citrine

Amethyst Amethyst

Carnelian

Onyx Jasper

Alexandrite

sunlight

artificial light

One mineral can appear to be different colors under different lighting

BeAl2O4 Hardness = 8.5

Chrysoberyl

Mineral Properties

•  Crystal Form •  Cleavage/Fracture •  Color  Streak Color

•  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Streak Color - Color of powdered mineral, which may or may not be the color of the crystal

hematite streak color

Mineral Properties

•  Crystal Form •  Cleavage/Fracture •  Color •  Streak Color  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Luster - quality of light reflected from mineral surface

Luster - quality of light reflected from mineral surface

metallic pyrite

gold

silver

Luster quality of light reflected from mineral surface

resinous pearly

vitreous (glassy) non-metallic

Mineral Properties

•  Crystal Form •  Cleavage/Fracture •  Color •  Streak Color •  Luster

 Hardness •  Tenacity •  Specific Gravity •  Acid Test •  Magnetism

Hardness - resistance of mineral to scratching or abrasion

Mohs Hardness Scale Austrian mineralogist F. Mohs, 1824

Based on relative hardness of 10 common minerals hard

soft

10. Diamond C 9. Corundum Al2O3 8. Topaz Al2SiO4(F,OH)2 7. Quartz SiO2 6. Orthoclase KAlSi3O8 5. Apatite Ca5(PO4)3(F,Cl,OH) 4. Fluorite CaF2 3. Calcite CaCO3 2. Gypsum CaSO4 1. Talc Mg3Si4O10(OH)2

Higher numbered minerals scratch lower numbered minerals

fingernail copper penny

glass plate

Mineral Properties

•  Crystal Form •  Cleavage/Fracture •  Color •  Streak Color •  Luster

•  Hardness  Tenacity

•  Specific Gravity •  Acid Test •  Magnetism

Tenacity - resistance of the mineral to breaking

•  Crystal Form •  Cleavage/Fracture •  Color •  Streak •  Luster

•  Hardness •  Tenacity  Specific Gravity

•  Acid Test •  Magnetism

Specific Gravity - density of the material (g/cm3)

Mineral Properties

Specific Gravity

Related to both the atomic weight of the contained elements, and how tightly the atoms are packed

Orthorhombic Carbonates All minerals have same structure, only cation differs Mineral Formula Cation g/cm3

Aragonite CaCO3 20 2.95

Strontianite SrCO3 38 3.76

Witherite BaCO3 56 4.29

Cerrusite PbCO3 82 6.55

Specific Gravity Related to both the atomic weight of the contained

elements, and how tightly the atoms are packed

Both are minerals composed of the element carbon Ggraphite = 2.23 (loose) Gdiamond = 3.51(tight)

http://geology.uvm.edu/vtlandforms/gl_graph.htm

Graphite Diamond

Mineral Properties

•  Crystal Form •  Cleavage/Fracture •  Color •  Streak •  Luster

•  Hardness •  Tenacity •  Specific Gravity  Acid Test

•  Magnetism

Reaction to dilute acid; especially useful in identifying carbonate minerals

Minerals like calcite react with weak acid, dissolving the mineral and producing lots of bubbles (effervescence)

Mineral Properties

•  Crystal Form •  Cleavage/Fracture •  Color •  Streak •  Luster

•  Hardness •  Tenacity •  Specific Gravity •  Acid Test  Magnetism

Some minerals strongly attract magnets. Characteristic of magnetite and some other minerals