Basics of Mineralogy

Mineralogy is the scientific study of minerals, which are naturally occurring inorganic substances with a specific chemical composition and a characteristic crystalline structure. 

Here are some basics of mineralogy:

1. Definition of Minerals: Minerals are solid, naturally occurring substances that have a specific chemical composition, a defined atomic structure, and are formed through geological processes.

2. Chemical Composition: Minerals are composed of elements in specific proportions. Each mineral has a unique chemical formula that identifies the types and quantities of elements it contains.

3. Crystalline Structure: Minerals exhibit a regular and repeating atomic arrangement known as a crystalline structure. This structure is a result of the way atoms or ions are arranged in a three-dimensional pattern.

4. Physical Properties: Minerals have various physical properties that can be used for identification. These include hardness, cleavage (the way a mineral breaks along planes), luster (the way it reflects light), color, and streak (the color of the powdered mineral).

5. Formation: Minerals form through geological processes such as cooling and solidification of molten rock (igneous processes), precipitation from solution (sedimentary processes), and alteration of pre-existing minerals (metamorphic processes).

6. Classification: Minerals are classified into groups based on their chemical composition and crystal structure. 

7. Occurrence: Minerals are found in a variety of geological environments, including igneous rocks, sedimentary rocks, and metamorphic rocks. Some minerals are also found in hydrothermal veins, in soils, and as part of biological structures.

8. Use and Importance: Minerals have numerous practical applications in various industries. For example, quartz is used in the production of glass, feldspar in ceramics, and metals like iron and copper are extracted from mineral ores for manufacturing purposes.

9. Mineral Identification: Geologists use various tools and techniques to identify minerals in the field and laboratory. These include visual observation, hardness tests, specific gravity measurements, and advanced analytical methods like X-ray diffraction.

10. Mineralogical Research: Ongoing research in mineralogy contributes to our understanding of Earth's processes, history, and the formation of minerals. Mineralogists play a crucial role in studying the composition and behavior of minerals to gain insights into geological and environmental processes.

The 6 Crystal Systems:

All have 3 axes, except for 4 axes in Hexagonal system

Isometric  all axes equal length, all angles 90o

Hexagonal -- 3 of 4 axes equal length, three angles 90o, three , 120o

Tetragonal -- two axes equal length, all angles 90o not common in rock forming minerals)

Orthorhombic -- all axes unequal length, all angles 90o

Monoclinic -- all axes unequal length, only two angles are 90o

Triclinic -- all axes unequal length, no angles 90o

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 

Luster

Transparency or Opacity

Crystal System

Crystal Habit

Twinning of crystals

Cleavage

Fracture

Hardness

Specific Gravity or density

Streak

Moh’s Scale of Hardness

1. Talc

2. Gypsum

3. Calcite

4. Fluorite

5. Apatite

6. Orthoclase

7. Quartz

8. Topaz

9. Corundum (ruby)

10. Diamond

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

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