Major Rock Groups
Minerals are distinguished by their physical and chemical properties. The same properties
are responsible for the many of the mechanical characteristics of rocks. Most common
minerals can be recognized from one or two characteristics.
- Colour The colour of minerals is rarely diagnostic when used alone but
some minerals (native sulphur - YELLOW, amethyst quartz - PURPLE, hematite - RED, azurite - BLUE GREEN) are very distinctively coloured.
- Lustre Lustre is the way in which light is reflected from mineral surfaces and is more frequently diagnostic than colour. Metallic lustre is often found in sulphide minerals, non-metallic lustres include glassy, dull and earthy.
- Transparency The ability to see through a mineral is a measure of transparency. Reflecting surfaces are called translucent.
- Crystal System Crystal symmetry is a very important diagnostic aid. Minerals fall into one of seven crystal classes. For more details click here.
- Crystal Habits This is the name given to the form or shape of crystals. Cubic, dodecahdral, octahedral, rhombohdral, prismatic, columnar, pinacoidal and pyramidal are a few of the many forms that crystal can display.
- Cleavage This refers to the characteristic manner in which minerals split along planes determined by their crystal structure. Mica has a perfect basal cleavage in one direction and splits into thin sheets. Feldspars commonly show two strong cleavages. Cubic minerals such as halite often display three mutually orthogonal cleavage directions.
- Fracture Fracture is mineral breakage which is unrelated to crystal structure. Quartz has no cleavage but can often show conchoidal fracture patterns.
- Hardness Mineral hardness is measured on a non-linear relative scale called Mohs Scale of Hardness
- Specific Gravity S.G. is an easily measured physical property that can be readily estimated. In general, sulphides and oxides have much higher specific gravities than silicates. Click here for additional information.
- Streak Streak is the name given to the colour of a mineral when powdered by abraision against a stronger material (usually a ceramic or porcellain plate). Streak is a much more useful diagnostic characteristic than colour.
- Other Characteristics: Some minerals have special characteristics that are diagnostic for only a few species. Refractive index (calcite), effervescence with dilute acids (calcite), fluorescence (fluorite), phosphorescence, piezoelectricity (quartz), resistivity (halite), taste (halite) and magnetic properties (magnetite) are all used to identify particular minerals.
- Best Field Indicators To successfully identify minerals it is necessary only to learn the two or three physical properties that are particularly diagnostic.
Major Rock Groups
(c) Department of Civil and Geological Engineering, University of Saskatchewan,
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