Minerals are the “stuff” of which all rocks are made. Whether Igneous, Metamorphic, or Sedimentary, all rocks are composed of assorted sizes and unique combinations of minerals.
The unique recipe of minerals in rocks tells geologists the story of the origin, history, and environments of rocks and the changes to which rocks have been subjected over time. Geologists are scientists who have learned to read and share the stories that rocks tell. These changes in rocks and minerals enable the Geologists to reconstruct and study the past environments of the Earth.
What is a Mineral?
To meet the definition of "mineral" used by most geologists, a substance must meet five requirements:
- Naturally occurring
- Inorganic
- Solid
- Definite chemical composition
- Ordered internal structure
Groups of Minerals
Minerals are grouped by the elements they are made of
1. Silicates
- The most abundant group of minerals.
- Characteristics: Contain oxygen & silica.
- Making up more than 90% of the earth.
- Example: Feldspar, Quartz, Mica etc
Mica
Quartz
2. Non-Silicates:
Make up only 5% of the Earth’s crust
Include some of the most important metallic minerals .
Example: iron, copper, gold, silver, diamonds, rubies etc
Copper
Iron
Ruby
Gold
3. Carbonates
Characteristics: Contain Carbon & oxygen and a positive ion, such as calcium .
Example: Calcite (CaCO3)
Calcite Crystal
4. Oxides
Characteristics: Contains Metallic ion and oxygen.
Example: Hematite (Fe2)O3 Magnetite Fe3O4
Hematite Jasper Quartzite
Magnetite
5. Sulfides
Characteristics: Sulfur and a metallic ion.
Example: Galena (PbS), Pyrite (FeS2), Chalcopyrite (CuFeS2), Bornite (Cu5FeS4) etc
Cubic Pyrite Crystals
Chalcopyrite
Bornite or Peacock ore
6. Native Elements
Characteristics: Single elements
Example: Gold (Au), Diamond (C), Silver (Ag).
Diamonds crystals
Gold Pieces
How do minerals form?
- Cooling of magma (hot, liquid rock and minerals inside the earth (from the mantle).
- Fast Cooling = No Crystals (mineraloids)
- Medium Cooling = small crystals
- Slow Cooling = large crystals
2. Elements dissolved in liquids (usually water).
Physical Properties of Minerals Used for Identification (I.D.)
Color: The most obvious property of a mineral, its color, is unfortunately also the least diagnostic. Color comes from the chemical composition of a mineral. Easy to identify, but not always reliable, one mineral can take on several different colors if there are impurities in the chemical composition, such as quartz, which can be clear, smoky, pink, purple, or yellow.
Streak: Streak is the Color of a mineral’s powder produced when it is dragged across an un-weathered surface/ Porcelain plate. Much more reliable than mineral color, because each mineral has a diagnostic streak color, no matter the color of the mineral itself.
Streak of Hematite and Magnetite
Hardness: A mineral’s resistance to being scratched. In 1812 a scale of mineral hardness was devised by the German mineralogist Frederich Mohs (1773-1839), called Mohs Hardness Scale. The scale ran from 1-10. The hardest mineral known, diamond, was assigned the number 10 while the softest mineral known, Talc was assigned the number 1.
How does the hardness scale work? The Mohs Hardness Scale ranks the order of hardness of minerals and some common objects. For example, your fingernail can scratch the minerals talc and gypsum, with a hardness of 2 or lower. A copper penny can scratch calcite, gypsum, and talc.
Mohs Hardness Scale
Cleavage: The ability of a mineral to break along definite weak planes. Not all minerals have cleavage. Some minerals break or fracture in irregular ways.
Fracture: Breaks irregularly, jagged edges
Fracture Modes
Magnetism
- Magnetism is the ability of some minerals to behave like magnets. Magnetism is strongly related to iron content.
- Assess magnetism by holding a magnet up to a sample and feeling if there is a “pull” when you move the mineral farther away.
Luster : Luster refers to How light shines off a mineral. Mineral luster is classified generally as metallic, non-metallic, glassy, shiny, greasy, waxy, dull, earthy .
Metallic Vs Non Metallic Minerals
Metallic Minerals:
- Metallic minerals contain metal elements in their chemical formula.
- Metallic mineral are those minerals which can be melted to obtain new products.
- These are generally associated with igneous rocks.
- They are usually hard and have shines or luster of their own.
- When hit, they do not get broken.
- Metallic minerals are good conductors of heat and electricity.
- Metallic minerals are less abundance compare to non-metallic minerals.
- Metallic minerals are malleable and ductile in nature.
- Examples of metallic minerals are chalcopyrite (CuFeS2), Gold, Hematite (Fe2O3), Molybdenite (MoS2), Native copper (Cu), Pyrite (FeS2), and Sphalerite (Zn, FeS) etc.
Non Metallic Minerals
- Non-Metallic Minerals do not contain metal elements in their inorganic chemical formula
- Non-metallic minerals are those which do not yield new products on melting.
- These are generally associated with sedimentary rocks.
- They are not so hard and have no shine or luster of their own.
- Non-metallic minerals not malleable and ductile in nature.
- When hit, they may got broken into pieces.
- Non-metallic are more abundance compare to metallic minerals .
- Non-metallic minerals are good insulators of heat and electricity and poor conductors of heat and electricity.
- Non-metallic minerals are generally associated with sedimentary rocks such as coal, salt, clay, marble, limestone, magnesite, dolomite, phosphorite, talc, quartz, mica, clay, silica sand, gemstones, decorative and dimension stones, construction materials, kaolin, brine, calcite, lignite, limonite, mica, potash, rock phosphate, radioactive minerals, soapstone, sulphur, rock salt, vermiculite and sulphur.
Stay tuned - Next Monday we are digging deeper!
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About the Author
This article is the second in the "Mineral Monday Series". It is written by Exploration Geologist & Geophysicist, Azeem Khel. By profession, Azeem researches and facilitates the processes of mineral extraction. Azeem is a member of Destination Gold's Facebook community, and has agreed to share his knowledge, work and, experience in the "Mineral Mondays Series".