Minerals and Rocks

Minerals
A mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes
Physical properties of minerals commonly used are:

  • Crystal structure and habit
  • Hardness: the physical hardness of a mineral is usually measured according to the Mohs scale.

Mohs hardness scale
1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond

  • Luster indicates the way a mineral’s surface interacts with light and can range from dull to glassy (vitreous).

o Metallic -high reflectivity like metal: galena and pyrite
o Sub-metallic -slightly less than metallic reflectivity: magnetite
o Non-metallic lusters:
-Adamantine – brilliant, the luster of diamond also cerussite and anglesite
-Vitreous -the luster of a broken glass: quartz
-Pearly – iridescent and pearl-like: talc and apophyllite
-Resinous – the luster of resin: sphalerite and sulfur
-Silky – a soft light shown by fibrous materials: gypsum and chrysotile
-Dull/earthy – shown by finely crystallized minerals: hematite

  • Color indicates the appearance of the mineral in reflected light or transmitted light for translucent minerals.
  • Streak refers to the color of the powder a mineral leaves after rubbing it on an unglazed porcelain streak plate.
  • Cleavage describes the way a mineral may split apart along various planes. In thin sections, cleavage is visible as thin parallel lines across a mineral.
  • Fracture describes how a mineral breaks when broken contrary to its natural cleavage planes.

o Chonchoidal fracture is a smooth curved fracture with concentric ridges of the type shown by glass.
o Hackley is jagged fracture with sharp edges.
o Fibrous
o Irregular

  • Specific gravity relates the mineral mass to the mass of an equal volume of water, namely the density of the material.

Minerals may be classified according to chemical composition. Here they are:
-Silicate class
-Carbonate class
-Sulfate class
-Halide class
-Oxide class
-Sulfide class
-Phosphate class
-Element class
-Organic class

Rocks
According to the processes of forming, there are 3 group of rocks:
Igneous rocks are formed when molten magma cools and are divided into two main categories: plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earth’s crust, while volcanic or extrusive rocks result from magma reaching the surface.

magma

Sedimentary rocks are formed by deposition of either clastic sediments, and non clastic sediments, followed by compaction of the particulate matter and cementation during diagenesis. Sedimentary rocks form at or near the Earth’s surface

kinds of sedimentary rocksurface.

Metamorphic rocks are formed by subjecting any rock type (including previously-formed metamorphic rock) to different temperature and pressure conditions than those in which the original rock was formed. These temperatures and pressures are always higher than those at the Earth’s surface and must be sufficiently high so as to change the original minerals into other mineral types or else into other forms of the same minerals.

metamorphic rocks

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