1. What Are Minerals?

Minerals
The halides manifest relatively low hardness and moderate-to-high melting points.
In the solid state they are poor thermal and electric conductors, but when molten
they conduct electricity well.
Halogen ions may also combine with smaller, more strongly polarizing cations than
the alkali metal ions. Lower symmetry and a higher degree of covalent bonding
prevail in these structures. Water and hydroxyl ions may enter the structure.
The halides consist of about 80 chemically related minerals with diverse structures
and widely varied origins. The most common are halite (NaCl) and fluorite (CaF2).
5.6. Class 6: Carbonates
The carbonate minerals contain the anionic complex (CO3) 2- , which is triangular in
its coordination, with a carbon atom at the centre and an oxygen atom at each of the
corners of an equilateral triangle. These anionic groups are strongly bonded,
individual units and do not share oxygen atoms with one another. The triangular
carbonate groups are the basic building units of all carbonate minerals and are
largely responsible for the properties particular to the class.
Carbonates are frequently identified using the effervescence test with acid. The
reaction that results in the characteristic fizz, 2H + + CO 2− /3 → H2O + CO2, makes
use of the fact that the carbon-oxygen bonds of the CO3 groups are not quite as
strong as the corresponding carbon-oxygen bonds in carbon dioxide.
The common anhydrous carbonates are divided into three groups that differ in
structure type: calcite, aragonite, and dolomite. The copper carbonates azurite and
malachite are the only notable hydrous varieties.
Members of the calcite group exhibit perfect rhombohedra cleavage. The
composition CaCO3 most commonly occurs in two different polymorphs:
rhombohedra calcite with calcium surrounded by six closest oxygen atoms and
orthorhombic aragonite with calcium surrounded by nine closest oxygen atoms.
The aragonite group displays more limited solid solution than the calcite group.
The type of cation present in aragonite minerals is largely responsible for the
differences in physical properties among the members of the group. Specific
gravity, for example, is roughly proportional to the atomic weight of the metal ions.
The dolomite structure can be considered as a calcite-type structure in which
magnesium and calcium cations occupy the metal sites in alternate layers. The
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