The near surface crust of the Earth is mostly composed of Carbonates and related nitrates and borates. This Carbonate group of minerals is considered as structurally related and chemically related group. This Carbonate mineral class has its basic anionic unit that is negatively charged consisting a triangle wherein the center portion is resided by either a carbon, boron, or nitrogen atom. Then an oxygen atom can be found at every corner of the triangle. The trigonal symmetry of all the members of this mineral class can be explained by the threefold symmetry of the triangle. The mineral will certainly have a trigonal symmetry as long as the triangles of the anionic group fall in plane parallel to the plane of the triangle and to all other bonds in the structure, and when viewed perpendicular to this plane, are multiples of three and are usually found evenly separated from each other.
It may seem complicated but it is in fact the simplest condition of the carbonate minerals. The highest symmetry is often expressed by simplicity. A more complex carbonate is less symmetrical than a simple carbonate. The simplest carbonate in fact has the highest symmetry of this class, bar 3 2/m.
Although the properties of the minerals that belong in this mineral class are usually variable, it can be generalized more so than any other classes. A typical carbonate mineral is usually found transparent in appearance. It is usually lightly colored with marvelous appearance when viewed closely under petrographic polarizing microscope. It usually leaves a white streak when rubbed on a white porcelain streak plate. The specific gravity measure of the mineral is usually found average to above average. It is typically soft when evaluated using the Mohs scale method hardness test. When viewed meticulously under petrographic polarizing microscope, it is commonly found having good to perfect cleavage. Carbonate minerals are also found soluble to at least some degree in acidic solutions. Carbonate minerals tend to originate in sedimentary and oxidizing environments with the exception of course of carbonatite igneous intrusions. The common chemistry shared by the group leads to these most common characteristics of Carbonate minerals. And the members of this mineral class that diverge from the norm do so due to the effect of metal cations like iron, lead, manganese, and copper.
Carbonates mineral class is an important group of minerals that can be found in a wide variety of environments. The members of this class consists carbonate groups that consists a single carbon atom in the center of the three oxygens arranged in a triangle. There are three important groups classified under carbonates mineral class. These are the Calcite Group, Dolomite Group, and Aragonite Group.
The structure of the members of the Calcite group is analogous to the structure of halite. The unit cell can be found flattened to form rhombohedral symmetry by just shortening the cube along the diagonal through the center, which may then become the c crystallographic axis and the optic axis. The resulting structure usually consists alternating layers of cations and carbonate groups parallel to (0001). The structure of the minerals that belong in this group can accommodate cations up to about 1 anionic radius. This is because the cations are in sixfold coordination with oxygen in the carbonate groups. The larger cations require the orthorhombic structure of the aragonite group.
The Dolomite Group on the other hand has the same structure as calcite. It is also rhombohedral in form. The only difference is that it must accommodate two distinctly different sized cations.
The Aragonite Group is orthorhombic in form. It contains cations that are larger than about 1 ionic radius. The arrangement of the cations is in layers in what amounts to an open version of hexagonal close packing. It is usually found with layers of carbonate groups in between, so that the cations coordinate with nine oxygens. Because the corners of the triangular carbonate groups do not all point in the same way, but the crystals are often found possessing pseudo-hexagonal shapes, the symmetry is reduced to orthorhombic system.
As a whole, Borate minerals are considered more complex in their structures than the typical carbonates. Borates could be also considered their own class for that reason.
