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Minerals reflect light to a very varying degree. The lustre of a mineral depends partly on its ability to reflect light and partly on the degree of its transparency, its refractive index and the nature of its surface. Lustre is described by terms which compare the mineral’s surface to that of well-known objects. It may thus be metallic, adamantine (i. e. like diamond used for diamond rings), vitreous, waxy, pearly, porcellanous, resinous or silky, to mention just a few common examples. Minerals with metallic lustre are usually opaque and often have a mirror-like surface. The degree of intensity of the lustre is thus also recorded, and we talk of a mineral being shining, glistening, glimmering or dull. The degree of lustre of a mineral is greatly reduced if its surface is covered with a thin film or incrustation of another mineral, or if there are signs of weathering. Earthy mineral aggregates always have a dull lustre.

If rocks are pushed into the deeper parts of the earth’s crust, or if they are sufficiently long in contact with molten magma, they may eventually come to be completely or partially melted. In such cases, the minerals with the lowest melting point become liquid first, but eventually the whole rock is converted into magma and the cycle thus begins anew. During the rise of the magma in the crust, various exchange reactions between it and the adjoining wall-rock may take place. For instance, the molten portion of a partially fused rock may be squeezed out and may re-crystallise later somewhere else; ‘rafts’ detached from the walls of the magma chamber may be incorporated in the magma; or the solution remaining after the crystallisation of a magma is almost complete may permeate into the surrounding rock and eventually crystallise to form veins some distance away from its parent mass.

Minerals can thus form in a large variety of environments whose physical conditions, such as pressure and temperature, can vary enormously. They are formed and destroyed in environments ranging from magma chambers deep in the crust to the ocean floor or river-bed on its surface. The environments in which the perfect mineral clusters of fine unweathered crystals are formed are, of course, much more rare. This is perfect place for formation of diamond engagement rings.

In only a few cases is there sufficient space to permit the undisturbed growth of large crystals, as the simultaneous formation of many crystals in a fluid uses up the material so quickly that perfect crystals do not have time to form.

It is comparatively rare to find a single mineral forming large geologically separate masses; but though a given rock type or mineral vein usually contains several different minerals, these are always more or less the same throughout the world. Only the relative proportions of the different minerals vary in accordance with the physical conditions that prevailed during their formation. Minerals which were formed together at the same time are called paragenetic or associated minerals. Some mineral associations may occupy many square miles of the earth’s surface.

Granite, the most common deep-seated igneous rock, which is composed of orthoclase, plagioclase, quartz and biotite, forms large parts of the earth’s surface. Other mineral associations, again, are found in only a few isolated localities, as, for instance, emerald associated with mica-schist. Finally, some rock formations result from conditions which are, as far as we can tell, virtually unique, and thus produce extremely rare mineral associations. Certain mineral types are present in many mineral assemblages—quartz, calcite, and pyrite, for instance. Thus minerals like pyrite, which are equally at home in igneous, sedimentary and meta-morphic rocks, do not signify any particular mode of formation. Other minerals, again, can only be formed under a very restricted range of temperature and pressure, or require the presence of particular minerals in the adjoining rock or the crystallising solution. The presence of such minerals is thus confined to very specific areas and mineral associations. But in areas where ideal conditions prevailed, minerals which are usually very rare may be found in quite large quantities. The mineral assemblages which are formed under the same physical conditions are usually composed of a certain definite set of elements, and may also contain distinctive trace elements. The associated minerals in any given locality are thus never brought together just by chance alone, and the number of possible mineral associations is limited.

Minerals from which metals or other useful elements are extracted are termed ores, and those small portions in the earth’s crust in which ores or other useful minerals are concentrated are called ore deposits. In these areas the processes of mineral formation were particularly intense or took place over an extended period of time.

Water and wind can also cause the weathered rock to be conveyed from one place to another. Rivers and streams carry away material both in solution and as gravel, sand, silt or mud, and the disintegration of the particles continues in the rivers and lakes and on the sea-shore. Wind can carry the smaller and lighter products of weathering; and the glaciers of the high mountains and the ice-sheets of the Arctic and Antarctic can carry enormous masses of rock debris. The material which has been weathered by rain, wind and frost, or transported by river, sea, wind or ice, is finally deposited in sea, river-bed, sand dune or glacier moraine. If not disturbed further, it will here become gradually harder and more compact, and will eventually form a new rock.

Material carried away in solution may be deposited in warm seas and inland lakes where the evaporation of water leads to a concentration of the solution. It is also used by many organisms for building their shells and skeletons, which, in the case of coral reefs, may reach vast dimensions. The accumulations which are thus formed by deposition in the sea, in rivers, in lakes or in deserts are termed sediments, and when compacted, sedimentary rocks. After their deposition, the newly-formed sediments are gradually cemented and hardened, and during this process new minerals are formed and existing minerals may be re-crystallised. Like all rocks on the earth’s surface, the newly formed sedimentary rocks may again be exposed to the agents of erosion, and the cycle begins again. If igneous and sedimentary rocks are involved in the processes of mountain building, or have their temperature and pressure raised by ascending masses of magma, the minerals of which they are made may be completely altered. Rocks formed in this way are termed metamorphic.

Most of the sparkling gemstones and colourful clusters of minerals which adorn our collections were formed many millions of years ago and seem to us to symbolise the everlasting and indestructible. Compared with the span of human life they are indeed everlasting, but looked at from a geologist’s point of view they form but part of the geological cycle of continuous formation and destruction. New minerals are constantly being formed in countless areas within and on the earth, and at the same time others are being changed or completely destroyed.

Minerals can be formed in a number of ways: they may crystallise from molten magma, volcanic gases or aqueous solutions, or they may be re-crystallised from solid material. The rocks formed by crystallisation from magma within the crust, or from the lava which has been brought to the surface through volcanic necks or fissures, are termed igneous or eruptive rocks. On the earth’s surface these rocks are attacked by the agents of weathering—wind, frost, water and ice. These are responsible for their chemical solution or mechanical disintegration.

Certain minerals can readily be told apart just by weighing them in the hand. The natural minerals with the highest specific gravity are native gold and platinum, which are nearly twenty times as dense as water. The specific gravity of most ’stony’ minerals, which include all the silicates, ranges from 2-5 to 3-5. The metallic ores are somewhat heavier, and many sulphides and oxides of the heavy metals have a specific gravity ranging from 4 to 8. The Veinstones’ or gangue minerals which form a large part of most ore veins, such as calcite, ankerite and dolomite, have a density around 3, and the similar but distinctly heavier mineral barytes has a density of 4-5.

In amorphous substances, gases, and liquids, as well as in minerals crystallising in the cubic system, hght entering from the outside is refracted equally in all directions. In all other minerals the refracted light is broken into two rays vibrating at right angles and travelling at different velocities. These minerals are said to be doubly refracting or birefringent. Double refraction can be particularly well demonstrated with a rhombohedral cleavage-fragment of the clear variety of calcite called Iceland spar. In the case of most minerals, however, the property of double refraction can only be observed with optical instruments. A mineral can be identified by the extent of its double refraction, the direction of the two refracted rays within the crystal, as well as by its other optical properties. It is not surprising, therefore, that the examination of thin sections of transparent minerals and polished sections of opaque ore minerals with the petrographic microscope is one of the most important aspects of the science of mineralogy.

Double refraction

The hardness of minerals has long been used as a means of identification. The scale of hardness in general use is known as Mohs’s Scale, which has ten grades. The minerals used as standards for this scale are as follows: i, talc; 2, gypsum; 3, calcite; 4, fluorspar; 5, apatite; 6, feldspar; 7, quartz; 8, topaz; 9, corundum; and 10, diamond. Any one mineral in this scale will scratch all the minerals which precede it, and will itself be scratched by all the succeeding minerals. Minerals of hardness 1 can be scratched with the fingernail; they usually also have a soapy feel like talc or graphite. Minerals of up to hardness 5 can be scratched with the blade of a pocket-knife, and those of hardness 7 and over can scratch window glass. It should be noted that the difference in hardness between successive grades is by no means uniform, and that that between the last two grades, corundum and diamond, is particularly great.

When minerals are cut and polished into gems or ornaments, only the harder ones resist abrasion in everyday use and escape the small scratches from minute quartz particles in the atmosphere. For this reason it has been customary to class minerals with a hardness above 7 (i.e. those which cannot be scratched by quartz) as precious stones. In certain minerals the hardness of the crystal varies according to the direction in which its face is scratched. Kyanite, for example, whose hardness ranges from 4- 5 to 7, can be scratched by a penknife in only one direction. Aggregates made up of more than one crystal usually appear to have a lower degree of hardness than a single crystal of the same mineral. It is, for instance, easy to scratch most sandstones with a knife, in spite of the fact that they consist essentially of quartz grains whose hardness is 7.

The streak of a mineral is the colour of its powder as seen on a white surface. It is often quite different from that of a larger piece of the mineral, and can be a diagnostic feature used in mineral identification. Pyrite crystals, for instance, are yellow, but the powder of pyrite is greenish-black. The streak of the black minerals, blende, limonite and haematite, is respectively leather-brown, yellowish-brown and blood-red. Many mineral identification tables are based on the colour of streak, as this provides a simple means of distinguishing between many superficially similar minerals. Streak is readily observed by rubbing the mineral on a tablet of unglazed porcelain.

Streak of a mineral

Minerals which have evenly spaced inclusions of other minerals, or minute cracks, or are intricately twinned, can reflect and bend light in such a way as to produce a peculiar, but for some minerals characteristic, play of colours. The variety of plagioclase called labradorite, for example, often shows a rich play of colours in which blue and green predominate. The colours of labradorite change when the crystal is viewed from different angles, the variation in colour being caused by the interference of light reflected from minute, repeatedly twinned plates of the mineral which are intricately intergrown, as well as from regularly-spaced lamellar inclusions of haematite, magnetite or ilmenite.

Opalescence is the term applied to the play of colours characteristic of opal, and can be attributed to the peculiar internal structure of the mineral through which the light is reflected. When opal is turned around or viewed from different directions, it produces a play of moving rainbow colours on a milky white background.
Iridescent colouration is seen on crystals whose surface is tarnished by exposure to the air. The tarnish is usually due to oxidation or chemical action by other gases, such as sulphur dioxide, in the atmosphere. Tarnish can appear on a large number of minerals and may sometimes lead to some confusion in their identification. Minerals which tarnish very readily are covelline and erubescite. The latter is often called peacock ore.