Optical properties of minerals, involve the various characteristics which can be observed through the penological microscope and they include.
1. In Polarised Light:
(a) Form ->
Anhedral- Zig-zag outline
(b) Colour and pleochroism.
(c) Relief, whether positive or negative through Becke-line method.
(d) Cleavage and fracture.
2. Under Crossed-Nicols:
(ii) Birefringence and interference colour.
(ill) Type of extinction.
(iv) Extinction angle, if any.
(v) Twinning and zoning.
(vi) Alteration, as seen by the microscope,
All these optical properties may be seen from the principles of optics described earlier.
Chemical Properties of Minerals:
Reaction with Acid:
Excepting silicates and ferromagnesian minerals, usually, most of the carbonate-minerals and sulphide- minerals produce effervescence whenever they come in contact with acid.
The following chemical phenomena are usually found with minerals of various categories:
Chemical compounds which have an alogous composition and a closely related crystalline form are said to be isomorphous. The members of an isomorphous series, , a gradation in chemical composition, crystal forms, specific gravity, refractive-index, etc. from one extreme to the other. The plagioclase fieldspars constitute an excellent example of an isomorphous series.
This is the phenomenon in which a substance containing the same chemical composition differs from one another by some physical properties like system of crystallization, hardness, density, etc. It is of the following types:
(i) On the basis of number of polymorphic forms:
When the chemical compound exists in two- distinct forms, e g., calcite and aragonite, Pyrite and Marcasite, etc.
When the chemical compound exists in three distinct polymorphic forms, e.g., Anadalusite, kyanite and sillimanite, Rutile, anatase and brookite; orthoclase, microcline and sanidine, etc.
The polymorphic forms in case of elements are known as allotrope.
(ii) On the basis of reversibility:
When the polymorphic substances are interchangeable, it is called enantiotropy, for example, Diamond=Graphite and Quartz<=>Tridymite.
When the changeability is from one substance to the other but not the reverse, e.g., Marcasite <=> Pyrite.
Besides the above, it is to be noted that the high temperatute polymorphs have higher degree of symmetry than its lower temperature polymorph which has lower degree of symmetry.
A mineral is said to exhibit pseudomorphism when it falsely assumes the outer form of a different mineral crystallizing in another system. It is due to: 1. Incrustation’ 2. Infiltration, 3. Replacement, 4. Alteration, etc.
Some minerals closely resemble others in crystal habits although they are different in chemical composition. If the geometry of the arrangement of the dissimilar ions is the similar appearing crystals may result, e.g., Rutile and Zircon, which are tetragonal but have different chemical composition. Such minerals are called homeomorphs and the phenomenon as homeomorphism.
It is the phenomenon in which a crystal whose internal structure has changed to that of a polymorphous form with0ut any change in the external form. Thus rutile changes to brookite and aragonite changes to calcite.