With Allochromasie (from Greek allos : different, different, and chroma : color ), the foreign color of a substance called. An allochromatic substance is not or only slightly colored, but is colored by the contamination with a strongly coloring substance. The line color of most allochromatic substances is white , unlike idiochromatic substances such as malachite or copper sulfate , which are inherently colored.
These coloring impurities are mostly chromophores , which contain elements of the transition metals . Vanadium , chromium , manganese , iron , cobalt , nickel or copper are often found . Depending on the oxidation level and complex formation , these elements can show very different colors. An example is corundum ( Al 2 O 3 ): As a pure substance, corundum is colorless. If it is contaminated with iron, it becomes a blue sapphire , if it is chromium it becomes a red ruby . Further examples are quartz and feldspar , which show very different colors depending on the contamination.
Allochromatic and idiochromatic substances are colored by the same chromophores. The difference is that the coloring element occurs only in traces in the former, so that they are not mentioned in a chemical formula , but so often in the latter that they have to be included in the formula. One example is trivalent chromium (Cr 3+ ), which replaces aluminum cations (Al 3+ ) in the corundum to a small extent, making it a red ruby. As pure chromium (III) oxide (Cr 2 O 3 ), it is used as the green pigment, chromium oxide green .
A third group is formed by pseudochromatic substances, which owe their color to optical effects, in particular to interference in thin layers . Examples are the shimmering ammolite or the temper colors of the bornite .