Rocks and Minerals Quiz: Primary and Secondary Minerals

Mica Group

• Micas are complex potassium aluminosilicates that often contain iron and magnesium.
• Two important types of micas are:
  • Muscovite: a white mica, important source of potassium and silicate clays, with a hardness of 2.0-2.5.
  • Biotite: a black mica, containing iron, magnesium, and potassium, and a source of silicate clays.

Amphibole Group

• Amphiboles are silicates of iron, calcium, and magnesium.
• They have a hardness of 5-6.
• Two important types of amphiboles are:
  • Hornblende: a black-colored amphibole with distinct cleavage.
  • Augite: a dull green pyroxene with distinct cleavage.

Apatite

• Apatite is the most common primary mineral, carrying phosphorus.
• It has a hardness of 5.0.

Carbonate Group

• The carbonate group consists of primary minerals made up of carbonate of calcium and magnesium.
• Examples include:
  • Calcite (CaCO3)
  • Dolomite (CaMg(CO3)2)

Iron Group

• The iron group consists of oxides of iron in several hydration states.
• Examples include:
  • Limonite: with a color varying from brown to black, and a yellow-brown streak.
  • Hematite: also known as "paintrock" or red ochre, and a source of hydroxide of clays, iron, and soil color.

Aluminum Group

• The aluminum group consists of oxides of Al in several states of hydration.
• Examples include:
  • Corundum (Al2O3): with a hardness of 9.
  • Boehmite (AlOOH)
  • Gibbsite (Al(OH)3): largely found in highly weathered, old, and infertile acidic soils.

Gypsum

• Gypsum is a secondary mineral with the formula CaSO4.2H2O.
• It has a hardness of 2.0.
• It is a source of calcium and sulfur.
• It can be used as a soil ameliorant and a retarder in cement.

Clay Group

• The clay group consists of hydrated aluminosilicates.
• They are colloidal in nature.
• Examples include:
  • Kaolin group: Al4Si4O10(OH)8 (e.g., kaolinite, dickite, nacrite, anauxite).
  • Montmorillonite group: Al4Si8O20(OH)4 (e.g., montmorillonite, beidellite, vermiculite, nontronite).
  • Hydrous mica group: e.g., illite.

Minerals

• A mineral is a naturally occurring inorganic substance with a fairly definite chemical composition and specific physical properties.
• Minerals can be classified into:
  • Primary minerals: originating from the original rocks and appearing prominently in the soil.
  • Secondary minerals: originating from the chemical weathering of less resistant primary minerals.

Quartz

• Quartz is the most abundant primary mineral, making up about 13% of the earth's crust and 30-40% of the average soil.
• It has a hardness of 7.0.
• It is colorless in its pure state, insoluble in water, and slightly soluble in alkaline solutions.
• It contributes no plant nutrients to the soil and is very resistant to chemical weathering.

Feldspars

• Feldspars are aluminosilicates containing potassium, sodium, and/or calcium.
• They form clay minerals upon weathering.
• Examples include:
  • Orthoclase feldspar (KAlSi3O8): an important source of potassium in soils, with a hardness of 6.0.
  • Plagioclase feldspar: Albite (NaAlSi3O8) and Anorthite (CaAlSi2O8).