Identification of Minerals
INTRODUCTION
Mineral is a naturally occurring inorganic substance having definite chemical composition and with definite internal structure. The branch of geology which deals with the various aspects of minerals such as their individual properties, their mode of formation and mode of occurrence is called as mineralogy. The most common minerals are quartz, feldspar etc. The important index properties of minerals are,
- Habit/Form/Structure
- Colour
- Streak
- Luster
- Hardness
- Cleavage
- Fracture
- Diaphaneity
- Specific Gravity
- Special properties like magnetism
1. Habit/form/structure: It is the internal atomic arrangement of the mineral which is manifested outwardly by development of geometrical shapes or crystals characters. Few structural forms observed in minerals are
| Form | Description | Example |
|---|---|---|
| Lamellar form | Mineral appears as thin separable layers | Different varieties of Mica |
| Tabular form | Mineral appears as slabs of uniform thickness | Feldspars, Gypsum |
| Fibrous form | Mineral appears to be made up of fine threads | Asbestos |
| Pisolitic form | Mineral appears to be made up of small spherical grains | Bauxite |
| Oolitic form | Similar to Pisolitic form but grains are of still smaller size | Limestone |
| Rhombic form | Rhombic shape | Calcite |
| Bladed form | Mineral appears as cluster or as independent rectangular grains | Kyanite |
| Granular form | Mineral appears to be made up of innumerable equidimensional grains of coarse or medium or fine size | Chromite, Magnetite |
| Columnar form | Mineral appears as long slender prism | Topaz |
| Prismatic form | As elongated | Apatite, quartz |
| Spongy form | Porous | Pyrolusite |
| Crystal form | Polyhedral, Geometrical shapes | Garnets, Galena |
| Massive form | No definite shape for mineral | Jasper, Graphite |
| Concretionary Form | Porous and appears due to accretion of small irregularly shaped masses | Laterite |
| Nodular form | Irregularly shaped compact bodies with curved surfaces | Flint |
2. Colour: It is a light dependent property. Colour is not considered as a diagnostic property of mineral. The colour of the mineral may also change due to the exposure to atmosphere.
3. Streak: It is the colour of mineral in the powdered form. It is one of the important diagnostic property of mineral especially those with a metallic lustre. It is found by rubbing the specimen on a piece of unglazed tile, or streak plate.
4. Lustre: The lustre of a mineral is the way its surface shines when held up to the light. Lustre is a property distinct from colour.
| Lustre | Description | Example |
|---|---|---|
| Metallic Lustre | It is the type of shining that appears on the surface of a metal | Galena, Gold, Pyrite |
| Sub metallic Lustre | lf the amount of shining is less when compared to metallic luster | Hematite, Chromite, Magnetite |
| Vitreous Lustre | Shining like a glass sheet | Quartz, Feldspar |
| Sub Vitreous Lustre | Less shining when compared to vitreous lustre | Pyroxenes |
| Pearly Lustre | Shining like a pearl | Talc, Muscovite mica |
| Silky Lustre | Shining like silk | Asbestos |
| Resinous Lustre | Shining like a resin | Opal, Agate |
| Greasy Lustre | Shining like grease | Graphite |
| Adamantine Lustre | Shining like a diamond | Garnet, Diamond |
| Earthy or Dull Lustre | No Shining | Bauxite, Magnesite |
5. Cleavage: The definite direction or plane along which a mineral tends to break easily is called cleavage of that mineral. It occurs as innumerable parallel planes along which the mineral is equally weak. Such parallel planes of weakness are referred to as a set.
| Cleavage | Example |
|---|---|
| One set of cleavage | Mica, Chlorite, Talc |
| Two sets of cleavages | Feldspars, Pyroxenes, Amphiboles |
| Three sets of cleavages | Calcite, Dolomite, Galena |
| Four sets of cleavages | Fluorite |
| Six sets of cleavages | Sphalerite |
| No cleavage | Quartz, Olivine, Garnet |
6. Fracture: The appearance of the broken surface of a mineral in a direction other than that of cleavage. Common types of fracture are
| Fracture | Description | Example |
|---|---|---|
| Even | When the broken surface is smooth and flat | Chert |
| Uneven | When the mineral breaks with an irregular surface | Fluorite |
| Conchoidal | The broken surface of the mineral shows concavities | Quartz |
| Splintery | When the mineral breaks with the rough woody fracture | Kayanite |
| Hackly | The broken surface is highly irregular with numerous sharp, fine projections | Copper |
| Earthy | The surface is smooth, soft and porous | Chalk |
7. Hardness: Hardness is the resistance offered by the mineral to abrasion or scratching. It is determined with the help pH Moh's scale of hardness which consists of ten reference minerals arranged in increasing order of hardness and numbered accordingly.
| Hardness | Example | Hardness | Example |
|---|---|---|---|
| Talc | 1 | Feldspar | 6 |
| Gypsum | 2 | Quartz | 7 |
| Calcite | 3 | Topaz | 8 |
| Fluorite | 4 | Corundum | 9 |
| Apatite | 5 | Diamond | 10 |
8. Specific Gravity/ Density: Specific gravity or Density of minerals depends on their chemical composition and atomic structure.
| Density | Specific Gravity | Example |
|---|---|---|
| Low density | Specific gravity less than 2.5 | Gypsum (2.3), Graphite (2-2.3) |
| Medium density | Specific gravity between 2.5 and 3.5 | Quartz (2.7), Feldspar(2.5) |
| High density | Specific gravity greater than 3.5 | Chromite (4.5- 4.8) |
9. Diaphaneity: The property possessed by some minerals to transmit light is known as Transparency/ Diaphaneity.
| Diaphaneity | Description | Example |
|---|---|---|
| Transparent | When the outlines of an object seen clearly though the mineral | Rock crystal |
| Translucent | A mineral which can transmit light only at edges | Calcite, Fluorite, Quartz, |
| Opaque | When no light is transmitted through the mineral | Hematite, Magnetite, etc |