Mineral#Colour and streak
With Allochromasie (from the Greek allos: other, different, and chroma: color ) refers to the foreign coloration of a substance. A allochromatischer substance is not or only slightly colored, but colored by the contamination with a highly colored substance. The stroke color of most allochromatischen substances is white, unlike the idiochromatischen substances such as malachite or copper sulfate, which are colored by itself.
These coloring impurities are usually chromophores containing elements of the transition metals. Often you can find vanadium, chromium, manganese, iron, cobalt, nickel or copper. Depending on the oxidation state and complexation of these elements can have very different colors. An example is aluminum oxide ( Al2O3): As a pure substance corundum is colorless. He is the blue sapphire, ruby red by chrome on by contamination with iron. Other examples are quartz and feldspar, depending on how dirty the show very different colors.
Allochromatische and idiochromatische substances are colored by the same chromophores. The difference is that the coloring element present in the former only in trace amounts, so that they are not mentioned in a chemical formula in the latter but so often that they must be listed in the formula. An example is trivalent chromium ( Cr3 ), which is a small portion of aluminum cations replaced ( Al3 ) in the corundum and so makes him a red ruby. As a pure chromium (III ) oxide ( Cr2O3 ), it is used as a green pigment, chromium oxide.
A third group is formed by the pseudo- chromatic substances which owe their color to optical effects, in particular the interference in thin layers. Examples are the iridescent ammolite or the tarnishing of Bornits.