Minerals generally occur in aggregates of more or less imperfectly developed crystals. ‘Structure’ is the usual term used to denote the state of aggregation or shape of the minerals, which may .be of the following type:
(i) Crystalline:
When the minerals are in the form of imperfectly developed crystals.
(ji) Amorphous or massive:
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When the mineral does not possess crystalline form.
(iii) Earthy:
It is a uniform aggregate of exceedingly minute particles like those of china-clay.
(iv) Columnar:
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An aggregate of more of less parallel imperfect prismatic crystals, e.g., amphibole.
(v) Bladed:
An aggregate of flattened imperfect prismatic crystals, e.g., kyanite.
(vi) Fibrous:
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An aggregate of fibres, which may or may not be separable, e.g., asbestos.
(vii) Foliated:
An aggregate of thin separable sheets; also known as miraculous, e g., micas.
(viii) Lamellar:
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Mineral made up of separable plates or leaves which may be curved or straight, e.g., gypsum.
(ix) Tabular:
When the mineral shows broad flat surfaces as feldspar.
(x) Granular or sacchroidal:
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An aggregate of crystalline particles of about the same size, e.g., chromite.
(xi) Oolitic:
An aggregate of small spheres (fish scales).
(xii) Pisolitic:
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An aggregate of large spheres (shot like).
(xiii) Botryoidal:
An aggregate like bunch of grapes.
(xiv) Acicular:
An aggregate of needle-like crystals.
(xv) Reniform:
Kidney-shaped aggregate.
(xvi) Mammillary:
It consists of larger and mutually interfering prominences, e.g., Malachite.
(xvii) Stalactitic:
Cylindrical or conical form of minerals generally due to deposition by dripping water.
2. Colour:
It depends upon the absorption of some and reflection of others of the coloured rays which constitute white light. Some minerals show distinctive colours as follows.
(a) White:
Calcite, Barite, Magnesite, Aragonite, Opal, Talc, Chalk etc.
(b) Blue:
Azurite, Sodalite, Covellite, Lazulite, Lazurite, Apatite etc.
(c) Green:
Fluorite, Beryl, Malachite, Microcline, Olivine, Epidote, Chlorite, Serpentine etc.
(d) Yellow:
Sulphur, Marcasite, Chalcopyrite, Orpiment, Citrine (Quartz), Siderite etc.
(e) Red:
Realgar, Jasper, Orthoclase, Pyrope, Zircon, Cinnabar etc.
(f) Lead gray:
Galena, Graphite, Molybdenite etc.
(g) Steel gray:
Hematite;
(h) Brass yellow:
Pyrite.
(i) Colourless:
Halite, Quartz, Calcite, Zeolite etc.
Variation in colour may be due to
(i) Surface alteration, (ii) difference in composition, (iii) presence of impurities and, (iv) inclusion of foreign matter.
3. Streak:
It is the colour of the powder of a mineral in small amount and sometimes it is quite different from the colour of the mineral in mass. For example,
(a) siderite shows the streak colour as ‘white’,
(b) hematite shows the streak colour as ‘cherry red’,
(c) chalcopyrite shows black streak colour.
4. Lusture:
It is the appearance of the surface of a mineral in reflected light. Lusture also depends upon absorption and reflection of light. Lusture of minerals differ both in intensity and kind depending upon the amount and manner of reflection respectively. Lusture may, be of the following types.
(a) Metallic lusture:
Gold, silver, copper, galena, graphite, molybdenite etc.
(b) Non-metallic lusture:
(i) Vitreous lusture:
It is the lusture of a broken glass, e g., Quartz.
(ii) Greasy lusture:
It is lusture of an oily glass, e.g., Nepheline.
(iii) Resinous lusture:
The lusture of resin as in sphalerite.
(iv) Admantine lusture:
It is the lusture of a diamond.
(v) Silky lusture:
It is shown by minerals possessing fibro structure like asbestos, fibrous gypsum, fibrous calcite etc.
(vi) Pearly lusture:
It is the lusture of a pearl, as in talc, opal, ‘gypsum, kyanite etc.
(vii) Earthy lusture:
It is a dull lusture as of kaoline, chalk.
When the degree of lusture is more, the surface shines like a mirror and it is known as splendent or brilliant lusture.
5. Diaphaneity:
It is the degree of transparency of a miner and is of the following types:
(i) Transparent:
When almost all light falling on a miner is transmitted through it.
(ii) Semi-trans parent:
When the objects are seen through, but the outlines are not clear.
(iii) Translucent:
When light is transmitted through a mineral but the objects are not seen through it.
(iv) Sub-translucent:
When merely the edges are translucent.
(v) Opaque:
When no light is transmitted through a mineral, it is said to be opaque.
6. Phenomena depending on light:
Some minerals exhibit colours which are not in the minerals themselves but are produced by the effects of certain structures present in the minerals on white light. These are as
(i) Play of colour:
A series, of colours are seen at various angles like a rainbow.
(ii) Change of colour:
Succession of colours produced when the mineral is turned about.
(iii) Opalescence:
It is a pearly reflection from the interior of a mineral.
(iv) Iridescence:
It is the brilliant colour shown by copper and pyrite.
(v) Schillerization:
It is a kind of metallic colour shown by non-metallic hypersthene.
(vi) Fluorescence:
Those minerals which after being exposed to ultra-violet light, emit light are known as fluorescent.
(vii) Phosphorescence:
It is the property of continued emission f light after a substance has been subjected to rubbing, heating or electric radiation etc.
7. Cleavage:
This is the property that some minerals exhibit f breaking along definite smooth planes. The presence of these planes is a simple indication of the difference in strength of bonds between atoms in the crystal: thus the property of cleavage is intimately connected with the atomic structure of minerals.
In number, there may be one or as many as six directions of cleavage. The atomic structures of minerals do not permit cleavage in five directions or in more than six directions.
Type’s of cleavages:
(i) Pinacoidal:
Parallel to the pinacoidal faces. It is unidirectional:
(а) ‘a’ pinacoidal. Kyanite.
(b) ‘b’-pinacoidal. Gypsum.
(c) ‘c’-pinacoidal. Also known as basal, cleavage, e.g., micas graphite, talc, etc.
In feldspar, there are ‘b-pinacoidal’ and ‘c-pinacoidal’ cleavages nearly at right angles.
(ii) Prismatic:
Parallel to prism faces. It is bi-directional, which may or may not be at right-angles. Pyroxene and amphiboles show this type of cleavage.
(iii) Cubic:
It is tri-directional at right angles as in galena, halite etc.
(iv) Rhombohedral:
It is tri-directional, parallel to the faces of the rhombohedra as in the calcite group of minerals.
(v) Octohedral:
Parallel to the faces of an octahedron. It is 4-direction as in case of diamond and fluoiite.
(vi) Pyramidal:
It is 4-direction, parallel to the pyramidal faces, e g., scheelite.
(vii) Dodecahedral:
Parallel to the faces of the dodecahedron, it is in 6-direction, as in sphalerite.
8. Fracture:
(i) Conchoidal:
When a mineral breaks with curved concavities, more or less deep, as in a broken glass, e.g., Quartz.
(ii) Even:
When the fracture surface approximates to a plane.
(iii) Uneven:
When the fracture surface is rough.
(iv) Hackly:
When the surface is studded with jagged elevations and depression.
(v) Splintery:
When the mineral separates out in fibres as in asbestos.
Fracture is, thus, the character of the surface obtained when a mineral is broken in a direction other than that of the cleavage.
9. Tenacity:
It is the behaviour of a mineral under stress and it may be
(i) Brittle:
When parts of mineral separates in powder, e.g., calcite.
(ii) Sectile:
When a mineral can be cut with a knife, but the slices yield under pressure, e.g., Graphite, Gypsum etc.
(iii) Malleable:
When the mineral can be cut with knife which flattens out under a hammer, e.g., gold.
(iv) Flexible:
When a mineral bends without breaking even when the force is removed, e.g., micas, chlorite etc.
(v) Elastic:
When the mineral attains its previous position after the withdrawal of the force, e.g., mica.
10. Hardness:
It is the resistance of a mineral offers to abrasion ‘or scratching.
‘Sclenometer’ is an instrument used for determining hardness.
11. Specific gravity:
It is the ratio of the weight for the mineral to the weight of an equal volume of water. It is determined through:
(a) Walker’s steel yard balance. For large specimen.
(b) Specific gravity bottle (Pycnometer). For small mineral grains.
(c) Chemical balance method. For small fragments of minerals.
12. Magnetic property:
A mineral capable of being attracted by a strong magnet is called magnetic, e.g., magnetite and pyrrohite.
13. Electricity:
(a) Pyroelectricity:
The development of positive and negative charges of electricity on different parts of the same crystal when its temperature is suitably altered is called pyroelectricity, e.g., Quartz.
(b) Piezo-electricity:
The property of development of electric- charges on crystallized mineral by pressure or by tension is called piezo-electricity, e.g., Tourmaline, Quartz.
(c) Photo-electricity:
When some minerals are exposed to radiation they produce electricity, e.g., fluorite,
14. Radioactivity:
Minerals containing elements of high atomic weights are called radioactive, because of their emissions. ‘Greiger Counter’ is an instrument used for the detection of radioactivity.