In contrast to the modifications , which are counted as separate minerals, the basic lattice structure of the crystal is identical in each of the varieties, but there are differences between coarse crystalline and fine crystalline phases. The chemical composition can also vary slightly.
For example, the chemical composition of the mineral corundum ( Al 2 O 3 ) is colorless in its pure form. A small addition (contamination) with chromium , however, produces the well-known red variety ruby and small amounts of iron , titanium and / or vanadium produce the blue, yellow to orange, green and purple varieties of sapphire . A variety of different colors can also be found in the varieties of the mineral spinel and tourmaline , due to the addition of trace elements during crystal growth. See also the article on spectral colors .
Quartz (crystalline silicon dioxide SiO 2 ) is particularly rich in varieties in terms of shape and color . In its pure form, it is called "rock crystal" and occurs macrocrystalline as yellow to red-brown citrine and purple amethyst . In microcrystalline formation, it occurs in nature with monochrome to bluish-white banding as chalcedony and multi-colored banded as agate . The amorphous silicon dioxide modification of the opal is created by the storage of crystal water and the associated loss of crystallinity .
- Rudolf Graubner: Lexicon of geology, minerals and rocks . Emil Vollmer Verlag, Munich 1980, ISBN 3-87876-327-1 .
- Mineral Atlas: Variety (Wiki)