What is Minerals and properties.

Minerals

Minerals are the solid constituents of all rocks, igneous, sedimentary and metamorphic, and occur as crystals. A mineral can be defined as a natural inorganic substance having a particular chemical composition or range of composition, and a regular atomic structure to which its crystalline form is related. Before beginning the study of rocks it is necessary to know something of the chief rock-forming minerals. The average composition of crustal rocks is given in Table given below and has been calculated from many chemical analyses.
       


Table 1 .Average composition of crustal rocks
                                                      %
    SiO2                                               59.26
    AI2O3                                            15.35
    Fe2 O3                                            3.14
    FeO                                                3.74
    MgO                                               3.46
    CaO                                                5.08
    Na2O                                             3.81
    K2O                                                3.12
    H2O                                               1.26
    P2O5                                              0.28
    TiO2                                               0.73
    rest                                                0.77
                         Total:                       100
      
      The last item includes the oxides of other metals; and also gases such as carbon dioxide , sulphur dioxide, chlorine, fluorine, and others; and trace elements, which occur in very small quantities. Table 2 lists eight elements in their order of abundance in crustal rocks. Thus silicon and oxygen together make up nearly 75 per cent of crustal rocks and the other elements over 98 per cent. Since silicon and oxygen preponderate in the rocks, the chief rock forming minerals are silicates. Although over three thousand different minerals are known to the mineralogist, the commonly occurring silicates are relatively few.

Table 2. The most abundant Elements
Oxygen                             (46.60%)
Silicon                               (27.72%)
Aluminum                         (8.13%)
Iron                                   (5.00%)
Calcium                             (3.63%)
Sodium                              (2.83%)
Potassium                         (259%)
Magnesium                       (2.09%)

   
  • Physical characters
Included under this head are properties such as color, lustre, form, hardness, cleavage, fracture, tenacity, and specific gravity. Not all of these properties would necessarily be needed to identify any one mineral; two or three of them taken together may be sufficient, apart from optical properties . Other characters such as fusibility, fluorescence, magnetism, and electrical conductivity are also useful in some cases as means of identification, and will be referred to as they arise in the descriptions of mineral species. In a few instances taste (e.g. rock-salt) and touch (e.g. talc, feels soapy) are useful indicators.
  • Colour
Some minerals have a distinctive colour, for example the green colour of chlorite, but most naturally occurring
minerals contain traces of substances which modify their colour. Thus quartz, which is colourless when pure, may
be white, grey, pink or yellow, when certain chemical impurities or included particles are present. Much more constant is the colour of a mineral in the powdered condition, known as the streak. This may be produced by rubbing the mineral on a piece of unglazed porcelain, called a streak-plate, or other rough surface. Streak is useful, for example, in distinguishing the various oxides of iron; haematite (Fe2O3) gives a red streak, limonite (hydrated Fe2O3) a brown, and magnetite (Fe3O4) a grey streak.
  • Lustre
Lustre is the appearance of a mineral surface in reflected light. It may be described as metallic, as in pyrite or galena; glassy or vitreous, as in quartz; resinous or greasy, as in opal; pearly, as in talc; or silky, as infibrousminerals such as asbestos and satin-spar (fibrous gypsum). Minerals with no lustre are described as dull.
  • Form
Under this heading come a number of terms which are commonly used to describe various shapes assumed by minerals in groups or clusters .
Acicular - in fine needle-like crystals (also described as filiform), e.g. schorl, natrolite.
Botryoidal - consisting of spheroidal aggregations, somewhat resembling a bunch of grapes; e.g. chalcedony. The curved surfaces are boundaries of the ends of many crystal fibres arranged in radiating clusters.

Concretionary or nodular - terms applied to minerals found in detached masses of spherical, ellipsoidal, or irregular shape; e.g. the flint nodules of the chalk.

Dendritic - moss-like or tree-like forms, generally produced by the deposition of a mineral in thin veneers on
joint planes or in crevices; e.g. dendritic deposits of manganese oxide.

Reniform - kidney-shaped, the rounded surfaces of the mineral resembling those of kidneys; e.g. kidney ironore, a variety of haematite.

Tabular - showing broad flat surfaces; e.g. the 6-sided crystals of mica.
Note that the above terms do not apply to rocks.
table 2. illustrates five commonly occurring mineral relationships, as follows.

·        Drusy - closely packed small crystals growing into a cavity, such as a gas bubble preserved in solidified lava.

·        Radiated - needle-like crystals radiating from a centre: the illustration is of a pyrite (FeS) concretion.

·        Fibrous - consisting of fine thread-like strands; e.g. asbestos and the satin-spar variety of gypsum.

·        Granular - in grains, either coarse orfine;the rock marble is an even granular aggregate of calcite crystals.

·        Reticulated - a mesh of crossing crystals.

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