Mineralogy. Elemental composition and formulas of minerals.

For finding-out of an elemental composition of a mineral yield its chemical analysis. As a result of recalculation of data of the analysis spot the chemical formula of a mineral. Formulas can be empirical, showing only an elemental composition, and structural, giving representation about a spatial arrangement of atoms in a mineral and their communication among themselves.

For some minerals structural formulas are not erected yet. But thanks to X-ray research techniques in many cases it was possible to spot mutual relations of atoms in crystalline structures of minerals. With a structure of substance and its properties the crystal chemistry is engaged in these questions of communication of a chemistry.

In minerals it is important to reveal cations and the anionic complexes characterising types of crystalline structures. At a writing of formulas of minerals anionic complexes separate from cations square brackets, for example, siderite Fe [CO3].

It is necessary to mean, that empirical formulas of minerals do not map features of their interior structure and in mineralogy they are exchanged now by structural formulas. So, the empirical formula of a mineral of muscovite H2KAl3Si3O12, and structural KAl2 [AlSi3O10] (OH, F) 2. Last shows, that in muscovite structure there is the difficult anionic complex and that water in a muscovite is not in view H2O, and in the form of hydroxyl (OH) - and this hydroxyl can be in turn replaced F-.

In mineralogy quite often distinguish anhydrous and water minerals (sulphates, phosphates, carbonates, etc.). Those minerals which have in the composition electricallies neutral molecules of water concern the water. Water as a part of minerals can be related and free. Relate, or crystallizational, water enters into a crystalline lattice of minerals, taking in it certain places. Some carbonates and sulphates, for example gypsum can be examples. The free water does not participate in a structure of a crystalline lattice of minerals, its quantity can be various in dependence, for example, from temperature. Examples of the free water is water of zeolites. And, of course, all hygroscopic water retained in microscopic fractures of minerals and soils by forces of the superficial tension, also is free and leaves at heating to 110°С.

Hydroxyl containing minerals in strict sense cannot be termed by the water. Between electricallies the basic odds exists a neutral molecule of water H2O and subzeroly charged ion of hydroxyl (HO)-. Hydroxyl (HO) - can replace in minerals such ions as Cl - and Fe - it is strongly retained in crystalline lattices, these properties of a molecule of water do not possess.

Morphology of minerals and units. Twinned joints of crystals.

For some minerals formation not only single crystals, but also their twinned joints - doubles is characteristic. These are field spars, rutile, cassiterite, aragonite, cinnabar and many other things minerals.

In the presents, not casual, joints, individuals grow together on identical flat nets of their spatial lattices. Geometrical individuals in the double it is possible to combine mentally with each other either reflexion in a plane of symmetry or rotational displacement round axis L2. Doubles can consist of a pair of crystals (simple) or from nonsinglely iterated individuals. Prominent feature of a facet of doubles are reentrant angles between facets; on the single ideally developed crystals of such corners does not happen.

It is necessary to distinguish doubles of accretion and doubles of germination. In the first individuals are differentiated on a plane, they as though adjoin with each other. In second crystals as though acquire each other or through get one into another, adjoining on the difficult sinuous (stage) surface.

Doubles are formed for various reasons. In a solution when crystals are in a germinal state and under the influence of those or other forces are developed rather each other. At transition of one polymorphic updating in another. At mechanical actions on growing crystals.

False crystals - pseudomorphosises. Pseudomorphosis is a crystal or grain of the mineral replaced without change of its shape by other mineral or an intermixture of minerals, from here and the name false (pseudo) (morph) shape. At these formations the smallest parts of a surface of initiating crystals and grains are maintained often even.

On pseudomorphosises it is possible to judge a formation mineral as it is visible simultaneously both an initial mineral (grain) and a transformation end-product chemical responses. Pyrite crystals in the superficial requirements are replaced with limonite - the dense brown powdery mass, an intermixture of various hydroxides Fe3 +. Crystals of potash feldspar are replaced with the powdery unit of a kaoline. One more method of formation pseudomorphosis - polymorphic transmutations of substances at temperature and pressure change, they are termed paramorphs.

For example, alpha quartz paramorphs on beta quartz (t transmutations 575 C at 10*5 Pases, or 100 Kpa).

Process of pseudomorphic displacement of minerals can occur as a chain of consecutive chemical responses. Limonite (an intermixture of hydroxides Fe3 +) on calcite (a calcium carbonate): the intermediate stage was, probably, displacement of calcite by siderite or dolomite (ferriferous carbonates) on "chain" CaCO3-> FeCO3-> hydroxides Fe.

There are also hollow pseudomorphosises - impressions in mined rock of crystals of the dissolved minerals which place remains unoccupied.

Some units of crystals. Granular units - continuous masses of any way accrete grains of one or several minerals. Each grain - the neofacot crystal which has grown in constrained requirements.

Druses (brush) - groups of crystals, accrued perpendicularly or nearly so it is perpendicular to a surface of fractures, walls of a lode or a vacuity in rock. The major phenomenon - geometrical takeoff. The single differently oriented crystals at first accrue, expanding they adjoin with each other, rest each other, to itself prevent to grow. Prolong to grow only those crystals which vector of growth is oriented towards the free space, i.e. on a normal line to a fracture surface. Secretions are formed, when any vacuity in rock is filled with mineral substance. Often at centre of secretions druses settle down. Most often than secretion of chalcedony with quartz druses inside, dated for amygdales in basalt.

Concretions - spherical, sometimes the flattened, incorrectly approximated units of a radially-radiant structure. In their centre quite often there is a grain which served as a seeding agent at concretion growth. More often they are formed in porous sedimentary soils - sand and clays. (Concretions of calcite, pyrite and phosphorites). The sizes from millimetre to tens centimetres.

Parallelly-shestovatye and fibrous units are usually formed in fractures. These are veins of silky gypsum, serpentine-asbestos, sparry calcite. In one cases these units crystallise in open fractures: at first on walls by a principle of geometrical takeoff druses accrue; expanding towards each other they are closed and form parallelly-shestovatye or fibrous units. In others - such units are shaped in gradually slightly opened fractures when velocity of opening less or is equal to growth rate of individuals. At first the fracture is filled with the granular unit of a mineral in the form of a continuous thin vein. Then, in process of grain opening, resting each other, can grow only after moved apart walls of fracture. They are gradually extended normally to walls, shaping the parallelly-sparry or fibrous unit. In other cases (parallelly-shestovatye (more often fibrous) units are formed at their growth from hair fractures in both legs, growth goes on by a principle of formation of units of the first and second sort.

Oolites are formed when the mineral crystallises from a solution on any kernel, as though covering it cockleshell, leaning against each other. They have concentricallies - shelly a structure obliged to rhythmical gang a mineral of formation. Most often oolites are shaped in hot radiants, in benthonic lake and sea slimes.

They are characteristic for some versions of bauxites, manganous and iron ores. The sizes of oolites - from millimetres to several centimetres.

Spherolites and reniform units are termed so on the morphology. Spherolites very often have is almost ideal-spherical shape and the size from shares to 1-2 sm and more. They as blobs accrue on other minerals and and on walls of different cavities in ores and rocks. Spherolites are formed or as effect of the splitted growth of crystals, or in them as in a concretion, there is a kernel-grain (or granular mass) on which the mineral accrues. Owing to geometrical takeoff or the constrained requirements crystals can expand, only raying from spherolite centre.

Reniform units consist of set of adjoining "kidneys", each of which has, like a spherolite, a radially-radiant structure, the truth it not always considerably with open years. Reniform units of goethite HFeO2 X H2O and malachite Cu2 (CO3) (OH) 2 have especially typical structure. Their formation occurred on a rough surface at the expense of batch growth and geometrical takeoff of spherolites; those spherolites which were on protuberances of a substratum remained and expanded only. In some reniform units considerably not only radially-radiant, but also concentricallies-zone a structure, as reflexion of gang of requirements at unit growth. Most often reniform units are formed in various cavities in near-surface bands of fracture and airing of ores and rocks.