What is a trace element in mineralogy ?

Trace element : definition

This term designates a chemical element present accidentally, or systematically but in very small quantities in a mineral. This element does not exceed a content of 0.1 to 0.2% (ie 1000 to 2000 ppm).
Because of these low contents, a trace element hardly ever appears in the structural formula of a mineral. The presence or absence of a trace element in a mineral varies greatly according to the age and geological context of the deposits, but also according to the overall geochemistry of the mineralization, mineral associations, etc...
Arsenopyrite can for example incorporate in its crystal lattice a few gold atoms in the form of Au3+ to replace iron atoms : the contents only very rarely exceed 1000 ppm (0.1%).