Argentine is a siliceous variety of calcite, or carbonate of lime containing a little silica, having a silvery-white, pearlylustre, and a waving or curved lamellar structure. It is found in primitive rocks and frequently in metallic veins. Research Argentine
Calcite is a common mineral composing such rocks as chalk and marble. It has the formulae CaCO3 and a relative hardness of 3. It effervesces vigorously with HCl. Clear specimens exhibit double refraction. Occurs as widespread sedimentary rock masses such as limestone. Crystalline metamorphosed limestones are called marbles. Argentine is a pearlylamellar variety; aphrite is foliated or chalklike; dogtooth spar, a form in acute rhombohedral or scalenohedral crystals; calc- sinter and calc-tufa are lose or porous varieties formed in caverns or wet grounds from calcareous deposits; agaric mineral is a soft, white friable variety of similar origin; stalactite and stalagmite are varieties formed from the drillings in caverns. Iceland spar is a transparent variety, exhibiting the strong double refraction of the species, and hence is called doubly refracting spar. Research Calcite
The majority of minerals can, under certain conditions, occur in regularly shaped crystals. Crystals vary greatly in shape and size. Some are minute, visible only through a microscope; others may measure several metres. The shape of a crystal can be as thin as a needle (acicular), or columnar, tabular, fibrous or lamellar. A closer look at crystallized minerals shows that their crystals are structurally nearly always the same. It is then possible to conclude that a mineral has a definite crystal structure, which is not accidental but is determined by certain laws. The science which pursues the study of these natural laws and explains and describes crystal structures of different minerals is called crystallography. Natural scientists have studied crystal structure since ancient times, but crystallography only gained recognition as a true science in the 17th and 18th centuries. Crystallography, as we know it today, is based upon the findings of many other scientific fields, especially mineralogy, chemistry,
physics and mathematics. The basic difference between crystalline and non- crystalline matter does not lie only in the regularity of arrangement of the external faces. After all, crystalline grains in rock minerals often have irregular shapes and yet they are crystals. The basic difference is in their internal structure, the arrangement of the molecules, atoms and ions. These tiny particles are chaotically arranged in gases, liquids and non- crystalline solids. In crystals, however, they have a regular, repeating pattern. Some minerals crystallize in a definite characteristic structure. But the structure of many crystallized minerals varies. Calcite crystals, for example, occur in various forms: high or low rhombohedra, columnar, acicular or tabular forms. These shapes share one characteristic, however; they are symmetrical, and their symmetry is in line with the group symmetry of the smallest particles of the substance. According to this symmetry, crystals are divided into seven major groups, called crystal systems: 1) triclinic, 2) monoclinic, 3) orthorhombic, 4) tetragonal, 5) trigonal, 6) hexagonal, 7) cubic. Crystals are classified as belonging to a particular system according to their axes of symmetry which are, basically, imaginary lines passing through the centre of a crystal, and also according to their number and kind. All crystals belonging to an individual system must have a certain characteristic form, which corresponds to the relative symmetry. This means, in fact, that each crystal system has its own individual crystal form. The crystal structure of every mineral is determined mainly by its chemical composition and by physical conditions during its development, especially temperature and pressure. Crystals with completely even faces do not often occur, for the various external conditions make such perfect growth of faces impossible. The size and the development of the individual crystal faces are not so vital for classification as the angle between faces, which is the same for every crystal of the same system. Crystallography is
largely based on the study of these interfacial angles. Instruments used for measuring the angles are called goniometers. Distortions of crystal faces, which develop either during the growth of a crystal, or through the effects of weathering, can be an aid in identifying a mineral (the faces of pyrite crystals are, for instance, often striated). The conditions of environment affect the speed of growth of crystals, and also their physical properties. When a cluster of crystals grows in a confined space, they crowd each other and impede each other's development. If a growing crystal becomes entangled with a neighbouring crystal, it cannot continue to grow in the original direction, but can expand in other directions, This is why the grains of rock have an irregular shape. On the other hand, crystals which originate in a soft, yielding environment, such as volcanic tuff, or in certain sediments, are often able to develop perfect faces. Crystals also form in rock crevices, where they are deposited either by solutions or gases. Here the conditions
for development are very different and much less favourable. The growing crystal cannot develop faces upon the rock surface to which they adhere. They can continue in their growth only towards the centre of the cavity. Clusters of crystals which grew from a common base in fairly parallel lines are called druses. Such parallel growth can occur only if the wall of the crevice is fairly even. If the rock crevice is round, the adhering crystals protrude with their free ends towards its centre, and are called geodes. It is common for minerals to occur grouped in druses and geodes. They are frequently found in ore-veins and in cracks and crevices of the most varied types of rocks. Quartz, calcite, fluorite, barites and similar minerals are often grouped in druses. In some large cavities and crevices truly magnificent crystals can develop, as can be seen, for instance, in the ' crystal cellars' in the Alps. The assemblage of crystals in druses and geodes is usually in a fairly regular pattern. But if the crystals grow in
an interlocking, irregular clump, it is called an aggregate. These aggregates are often made up of multitudes of tiny crystals, more often than not with faces imperfectly bounded. Apart from aggregates there are also mineral clusters which are basically made of parts of individual crystals not visibly bounded externally. The minerals which usually appear in this form are those which commonly develop large crystals, such as quartz. For some irregular or round crystal formations in rocks the term nodule is used in mineralogy and petrography. Usually of quartzitic composition, nodules are often found in limestone beds. Chalcedonic and opal varieties also develop nodules, which originate through deposition of marine plankton. The laws governing the growth of crystals can often be applied to whole groups of crystals. Sometimes two or more crystals of the same chemical composition grow together with definite rules, governing their relative positions. Depending on the number of the twinned crystals, we talk of twins, triplets,
etc. The intergrowth of two different minerals, though perhaps of the same chemical composition but with a different internal structure is not often found in nature (pyrite and marcasite, or even some minerals with a completely different chemical composition, such as haematite and rutile, are some examples). Another mineral form is a so-called pseudomorph or mineral mimic which is frequently found. For instance, limonite, which has no crystal form of its own, is sometimes found in the shape of a perfectly bounded pyritecrystal. This unusual happening occurs because the original crystal of pyrite has become decomposed through the effects of water and has been replaced by limonite. In other cases one mineral changes into another mineral of identical chemical composition. For instance aragonite (CaCO,) has an orthorhombiccrystal form, and it changes slowly and gradually into rhombohedral calcite, but the original crystal form remains unchanged. Sometimes a pseudomorph is the result of incrustation which is produced by a
powder like coating of one mineral being deposited on the crystals of another. It is rarer to find in nature minerals which have no traces of crystalline structure and which are said to be amorphous (opal and amber are examples). Research Mineral Structure
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Abbreviations
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