CARBONATES CLASS:
The carbonates and related nitrates and borates are common constituents of the earth's near-surface crust.
This is a structurally-related as well as chemically-related group. The basic anionic (negatively charged) unit of this class
consists of a triangle where at the center resides either a carbon, nitrogen or boron atom. At every corner of the triangle
sits an oxygen atom. The threefold symmetry of the triangle explains the trigonal symmetry that many members of this class possess.
As long as the triangles of the anionic group fall in a plane parallel with the plane of the triangle and all other bonds in the
structure, when viewed perpendicular to this plane, are multiples of three, and are evenly separated from each other, the mineral
will have a trigonal symmetry.
As complicated as this seems it is in fact the simplest condition of the carbonates. Simplicity often expresses
the highest symmetry.
As a sphere is more symmetrical than a football; a simple carbonate is more symmetrical than a more complex carbonate and in fact has
the highest symmetry of this
class, bar 3 2/m.
Although somewhat varied, this class' properties can be generalized more so than the other classes. Typical carbonates are transparent,
lightly colored with a white streak, average to above average in density, soft with good to perfect cleavage, soluble to at least some
degree in acidic solutions, and tend to originate in sedimentary and oxidizing environments with the exception of carbonatite igneous
intrusions. Most of these common characteristics are due to the common chemistry the group shares and members that diverge from the
norm do so because of the effects of metal cations such as lead, copper,
manganese and iron.
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