A mineral is an inorganic natural substance that has a specific chemical makeup. Each mineral is unique.
A common example of a mineral is Calcite. Calcite consists of calcium, carbon and oxygen atoms in respective ratios 1:1:3. The geometry of calcite is Rhombohedral. Another substance, whose structure is identical to calcite but has manganese instead of calcium is Rhodochrosite.
So we can see how different molecular configurations give crystals their differences in texture, colour, and appearance. There is, however, an exception to this rule, when two distinct minerals have the same chemical makeup but their geometry differs due to variations that were present during their formation. When iron and sulphur atoms arrange themselves into cubes they make Pyrite, but when they stretch into shoebox-like shapes they make Marcasite.
Crystals are born when minerals accumulate under high pressure to form flat surfaced shapes arranged in geometric patterns. This occurs in hollows, fissures and caves inside the Earth. Fluid, either water or molten rock flows into the hollow under extreme heat and pressure. In transit the fluid leaves deposits of its mineral content dissolved on the walls of the hollow. ( Water cannot evaporate away under any temperature if it is enclosed in the Earth, it is like a pressure cooker without a safety valve). If the walls do not fill in completely crystals will grow in the spaces. But if the hollow is filled in ordinary rock forms. Ordinary rock retains it’s crystal inner structure at micro levels, but the crystals are crammed and jumbled. It is called Crystaline when the outer structure does not reflect the shape of it’s inner structure, i.e. Granite, a combination of Quartz, Feldspar, and Mica.
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Marcasite photo. from: http://en.wikipedia.org/wiki/Marcasite
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