This is part of the materials used in the Solid Earth class.
Minerals are the basic building blocks of Earth materials and are defined as naturally occurring, inorganic solids with definite chemical compositions and regular, internal crystalline structures. Minerals are important constituents of rocks, and recognizing distinctive minerals will greatly improve your ability to recognize rocks (coming up in the next 2 weeks). Approximately 4500 different minerals have been observed in nature, but there are relatively few common rock forming minerals and important ore minerals.
Helpful notes on mineral identifications
The following notes are gathered during my mineral class in 2019 Fall and the preparation for the mineral identification lab.
What is a mineral?
A mineral is a naturally occurring solid with a highly ordered atomic arrangement and a definite (but not fixed) chemical composition. It is usually formed by inorganic processes. Rocks are aggregates of minerals (or non-minerals). The minerals present in a rock are dependent on the intensive properties of the system:
- Temperature
- Pressure
- Chemical Composition And also kinetic factors if the rock is not in chemical equilibrium. Mineral properties reflect crystal chemistry and structure
Properties for Identifying Minerals in Hand-Sample
Light
A. Color
Color must be used carefully because differences in grain size, minute amounts of impurities and other variations may cause changes in a mineral’s normal color. Many minerals are the same color and a single mineral can be many colors.
- Color produced by absorption of light
- Idiochromatic – azurite
- Allochromatic – quartz
- Pseudochromatic – labradorite, bornite
- Color produced by production of light
- Fluorescence – fluorite
- Phosphorescence – calcite (rarely)
** One problem with using mineral color to identify a mineral is that it is often highly variable for a given mineral. Impurities, crystallinity, and lots of factors may have major effects. The color of a finely powdered mineral, known as its streak, however, does not vary much.
B. Luster
Luster is the way a mineral reflects light. There are two main classifications of luster, metallic and nonmetallic.
- Metallic – copper A luster is metallic only if it resembles a metal (such as iron, bronze, lead, silver or gold) so much that it might be confused with it. Metallic minerals are opaque – that is, no light penetrates the mineral.
- Non-metallic
These minerals are those which do not look like metal. Their luster is further described by the following terms:
- Adamantine - zircon
- Vitreous - quartz
- Sub-vitreous – forsterite
- Resinous – garnet
- Pearly – muscovite
- Silky – asbestos
- Greasy – opal
- Waxy – chalcedony
- Dull/ Earthy – kaolinite
Shape
C. Crystal Systems
D. Crystal Habit
If you can see many individual grains or a mass that show no recognizable arrangement, identify it as one of the following aggregate specimen structures:
- Granular – made of grains large enough to recognize some of their properties.
- Massive - applies to a mass of tiny little grains (crystals), so small that the individual properties of the grains cannot recognized.
- Microcrystalline - crystals too small to be seen with a hand lens. Can often be recognized by conchoidal fracture and waxy luster.
- Fibrous (or acicular) - means long thin crystals which may be parallel, radiating, or crisscrossing.
- Elongated - crystals which are longer in one direction than the others and larger than fibrous. If shaped like the blade of a sword, they are called bladed.
- Tabular, platy or in laminae - applies to crystals that grow originally in thin flat shapes, usually intergrown in different positions. If they are all lying the same way and have been squeezed together like a pile of wet leaves, they are called foliated.
Mass
E. Density
Specific Gravity (SG) is the ratio of the density (mass of a unit volume) of a substance to the density of water.
- Light (SG 1-2) – sulfur, graphite
- Medium (SG 2-3) – quartz, gypsum
- Medium Heavy (SG 3-4) – fluorite, beryl
- Heavy (SG 4-5) – corundum, most metal oxides
- Extreme (SG > 10) – gold, platinum
Mechanical Cohesion
F. Tenacity, Structural varieties
- Flexible - can be bent, but will not return to original shape when released.
- Elastic - will return to original shape after bent and released.
- Sectile - can be cut with a knife into thin sheets.
- Malleable - can be hammered into shapes other than that of the original.
- Brittle - easily broken or shattered.
Examples:
- Brittle – quartz
- Malleable – silver
- Ductile – gold
- Flexible but inelastic – copper
- Flexible but elastic – crysotile
G. Hardness
Hardness is how resistant a substance is to scratching. If one mineral can scratch another, the one that does the scratching is harder. Two minerals of equal hardness will either both scratch each other or both fail to scratch each other. The following minerals form a standard scale of relative hardness for minerals called the Moh’s Scale: Moh’s Scale:
- 1 – talc
- 2 – gypsum
- 3 – calcite
- 4 – fluorite
- 5 – apatite
- 6 – orthoclase
- 7 – quartz
- 8 – topaz
- 9 – corundum
- 10 – diamond
Compared to the minerals above, fingernails have a hardness of 2.5, a penny is 3-4, knife blades are about 5.5, and streak plates are about 7.
H. Cleavage
Cleavage is the tendency of a mineral to break along a specific direction or plane. It is a break between planes of tiny particles in a mineral’s internal structure. Some minerals have no cleavage; some have only one direction of cleavage; others have two or more directions, producing surfaces that are not parallel. Each cleavage direction may produce many parallel surfaces. When there are two or three directions, it is important to note whether they are perpendicular of oblique to each other. Count every cleavage on a single crystal that is not parallel to one already counted.
- Cleavage strength
- Perfect – biotite mica
- Good – fluorite
- Fair - augite
- Poor - apatite
- Cleavage directions
- Basal cleavage – mica
- Prismatic cleavage – amphibole
- Cubic cleavage – halite
- Rhombohedral cleavage – calcite
- Octahedral cleavage – fluorite
- Dodecahedral cleavage – sphalerite
I. Fracture
Any break which is not a cleavage is called a fracture. A fracture is not predetermined by the arrangement of atoms in the mineral and is usually irregular.
- Conchoidal – quartz
- Fibrous – asbestos
- Splintery – actinolite
- Hackly - copper
- Granular
- Even
- Uneven – anhydrite
Miscellaneous Properties
- A. Transmission of light
- Transparent - can see distinctly through the mineral.
- Translucent - can see light but not objects through the mineral.
- Opaque – no light passes through the mineral.
- B. Taste – halite is salty for example
- C. Smell – sulphur smells like rotten eggs for example
- D. Feel - mineral is gritty, soapy, etc. when rubbed with your finger.
- E. Magnetism - Magnetic minerals are attracted to a magnet.
- F. Twinning - appears as one crystal growing through the other (or as in the case of plagioclase appears as lines on the surface perpendicular to the polysynthetic twinning).