A systematic study of the minerals can never be complete without reference to their physical properties, since these are the characteristic and sometimes, specific features of the different minerals.
In the study of minerals, their colours are always carefully noted although the colour is seldom a distinguishing character of any mineral, The colour of a mineral is produced due either to transmission of the mineral or upon its surface, as the case may be. The true colour of a mineral, however, is vitiated to a great extent due to the presence of minute quantities of impurities of different types. Thus the mineral quartz should, in the pure state, be absolutely colourless, but it occurs commonly in shades of pink, yellow or even grey due, perhaps, to the presence of variable amounts of impurities of different types. Moreover, the colour of a large lump of a mineral may be deep while, in smaller fragments, the same specimen may exhibit a lighter shade or may even be colourless. It is apparent, therefore, that the colour of a mineral is, by no means, dependable or distinguishing character.
The streak of a mineral is defined as the colour of its own powder and is determined ordinarily by producing a mark, with the mineral, upon the surface of an unglazed, white porcelain plate, as is done with a piece of chalk upon the blackboard. The mineral, thus, forms a fine powder of its own and the colour of the same can be examined carefully and conveniently against the white background of the porcelain plate. If a mineral, on the other hand, is harder than porcelain, it would not produce a mark upon the plate. On the contrary, the plate itself would wear out, producing its own white powder and thus offering a false impression about the streak of the mineral. The colour and streak of a mineral may be similar in some cases and radically different in some other cases. The mineral haematite is commonly steel grey in colour, but possesses a cherry red streak which is very characteristic for the mineral. Similarly, the mineral chalcopyrite is golden yellow in colour but exhibits a shining greenish black streak. The mineral corundum, being harder than porcelain, cannot produce a streak in the usual way. The streak of a mineral is often a very distinctive feature and forms a very useful criterion in the identification of minerals.
In defining the hardness of a mineral, the popular concept associated with the term has to be done away with. Thus, to a mineralogist, a piece of glass is harder than a piece of iron, since the former can easily scratch the latter. Defined precisely, the hardness of a mineral is the property by virtue of which it offers resistance to any force which might tend to cause abrasion on its surface. Diamond is the hardest of all minerals since this cap scratch them all while no mineral is capable of scratching on the surface of diamond. Talc is softest since all minerals can Scratch upon its surface. With this concept in mind, a suitable scale of hardness, of the minerals was proposed by Mohs as follows ;
1. Talc 6. Orthoclase
2. Gypsum 7. Quartz
3. Calcite 8. Topaz
4. Fluorite 9. Corundum
5. Apatite 10. Diamond
In the above scale, the minerals have been arranged from 14 to 10 in order of increasing hardness.
In the Laboratory, the hardness of any mineral is determined by comparing the same with that of the members of the scale mentioned above. This property is very often found to be a distinctive characteristic and is commonly taken recourse to, in mineral identification.
The specific gravity of a mineral is the ratio of the weight of the mineral to that of an equal volume of water and can be determined conveniently by any standard method. If the weight of the mineral in air be given by W, and its weight in water by W1 then W1—W2, is the weight of an equal volume of water and the specific gravity of the mineral is given by the expression :
Specific gravity = W1/W1 - W2
In determining the specific gravity of minerals, recourse may commonly be taken to Chemical Balances, Walker’s Steelyard Balance, Jolly’s Spring Balance or to Specific Gravity Bottles. The specific gravity of a mineral forms an important and distinguishing character only when the mineral is either too heavy or too light In case of minerals of moderate specific gravity, therefore, this character is not of much importance. Moreover, this property of minerals is variable, within wide limits, depending upon the nature and amount of the impurities present in them.
Cleavage is property of a mineral by virtue of which it develops a tendency to break down, along a particular direction, offering plane and smooth surfaces. The resulting cracks, which are formed due to this tendency, are known as cleavage craks. A mineral may or may not possess any cleavage and, if present, the same may occur in one or more directions. Only crystalline minerals are capable of possessing cleavage and amorphous substances are necessarily devoid of this characteristic. In crystalline minerals, however, there exists no rule governing the presence or absence or the nature ef the cleavage. Thus, the mineral quartz has no cleavage, while galena, calcite etc., have well-developed cleavage. The phenomenon of development of cleavage in crystalline minerals is ascribed to their particular internal atomic structures. Within the atomic structure of any mineral if the bonds, along a particular direction, happen to be weaker than those in the other directions, a suitable blow in the appropriate direction would, naturally, tend to break down the mineral more easily, offering smooth surfaces at right angles to the direction of weak bonds in the structure (Figure). This mineral would, thus, develop a cleavage under such conditions. If, on the other hand, the bonds are of appreciable and equal strength along all.
It is apparent, therefore, that a mineral possessing cleavage is definitely crystalline but all crystalline minerals do not necessarily have cleavage and further, the absence of cleavage is by no means suggestive of want of crystallinity in a mineral.
The fracture of a mineral may be defined as the nature of the surface produced due to fracturing of the mineral mass. If the mineral possesses cleavage in several directions, fracturing would necessarily reduce the lump into smaller fragments produced due to the breaking down of the mass along the cleavage planes. The broken surfaces, thus, would continue to be smooth and the characteristic pattern of fracture of the mineral cannot be exposed. In minerals which have no cleavage, the fracture can be determined by breaking the mineral along any direction. In case of minerals with cleavage, however, the fracture should necessarily be determined by breaking the mineral in any direction other than the cleavage direction. The broken surface of the mineral, thus produced, is often found to offer very characteristic patterns of fracture. Thus, minerals like quartz produce a fractured surface which is made up of a number of curvatures, both concave and convex, like those developed on a piece of broken glass. This type of fracture, which involves the development of more or less smooth curved surfaces, is defined as conchoidal fracture. If the fractured surface is rough and uneven the fracture is more or less smooth and plane, the fracture is even or regular. In case of minerals of remarkably uneven fracture, it may so happen that needle like projections are developed on the broken surface of. the mineral. Such extremely irregular fractures have been named hackly. Many of the common minerals have their own characteristic fracture patterns.
The lustre of a mineral is the property by virtue of which if is capable of reflecting light. Defined precisely, the lustre of a mineral is the nature and amount of shine that the mineral can possibly offer, due to reflection of light upon its surface. The different degrees of lustre are qualified as
(1) splendent
(2) shining
(3) glistening
(4) glimmering and
(5) dull
The above five are arranged in order of decreasing amount of lustre, i.e., splendent forms the highest degree and glimmering, the lowest while the term dull is suggestive of absence of lustre. The different kinds of lustre, again, are studied from the point of view of the nature of the shining surface. Thus a mineral may show metallic lustre if its lustre or nature of the shining surface is similar to that of broken pieces of metals. Within the category of non metallic lustre there are different varieties like:
(a) adamantine
(b) vitreous
(c) resinous
(d) pearly
(e) silky
(f) greasy, etc.
The term adamantine indicates the nature of the shine of diamond while vitreous is the lustre of glassy substances. Resinous, pearly, silky and greasy lustres necessarily simulate, as is apparent from their names, the nature of shine of the respective substances. Many minerals have very characteristic lustres of their own. Thus, the mineral quartz exhibits. typically vitreous lustre, the mineral sphalerite is characterised by its resinous lustre, while minerals like pyrite, chalcopyrite, galena, etc., show metallic lustre.
The form and structure of minerals are also given their due Share in their study. It has eben stated earlier that most of the minerals are crystalline while a few are amorphous as well. A mineral, therefore, should first of all be either crystalline or amorphous. If it be crystalline, it may either occur in the form of well-developed crystals or, alternatively, it may be massive, without the development of recognisable crystal forms. Amorphous substances and some of the cryptocrystalline( These are made up of very crystals, which can be distinguished:only under
) bodies are often characterised by the development of colloform structures, i.e., structures shown commonly by drying clloids. Many crystalline minerals show some amount of flaky, fibrous, platy and similar other forms. Thus, the mineral mica is flaky and asbestos is fibrous while the mineral gypsum is often platy in form. The forms and structures of minerals are often characteristic features and are always studied carefully.
Amongst the minerals, the property of magnetism is ordinarily found to vary between wide limits. Thus, minerals like magnetite are highly magnetic, those like chromite, ilmenite etc., are less conspicuously so, while others like quartz, garnet, opal etc., are altogether non-magnetic. If, however, the strength of the magnetic field by considerable, most of the minerals behave at least as feebly magnetic substances. In the Laboratory, a strong bar or horse-sho, magnet is commonly used and with such a magnetic or nonmagnetic, substances. The property of magnetism is studied with metallic minerals only and more so with ferrous minerals.
Some of the minerals are characterised by the presence of some special properties in them which are their specific features. Thus, for example, the minerals graphite and pyrolusite soil hand and the former can mark on paper as well. Again, the mineral talc has q soapy feel. Though these are designated separately as their special properties, these are nothing more than the mere expression of some of the other physical properties in some readily recognisable form. Thus, the soapy feel of talc is due to its extreme softness while graphite can mark paper due to its softness and the mark becomes vivid due to the dark colour of the mineral.
The physical properties, as described above, are individually as well as collectively of great importance in the study and identification of minerals. Thus, for example, the cherry red streak of haematite, together with its steel grey colour and metallic lustre, is its distinguishing criterion. For the mineral pyrite, its bronze yellow colour, black streak and hardness are distinctive. The mineral quartz is characterised by its hardness, absence of cleavage, conchoidal fracture and vitreous lustre and, in a similar manner, each and every mineral can be distinguished with the help of the characteristic physical properties exhibited by them. It may be stated with confidence, therefore, that a proper study and identification of the minerals can be readily done with the help of an assemblage of their common physical properties.