Fehrite

Fehrite is the only magnesium copper sulfate known so far. It is isostructural with ktenasite and gobelinite, with magnesium instead of zinc and cobalt. It is found as radiating aggregates of elongated, tabular, ribbon-like crystals with a maximum length of 200 μm. In addition to the elements of the formula, its composition includes significant proportions of Zn and traces of Mn.^[2]

Fehrite is a secondary mineral produced by the altering of sulfides. It seems to be a very rare mineral, and so far, it has only been found in two localities: At the type locality, the Casualidad mine, in Pechina (Almeria), Spain, it was seen as an alteration product of tetrahedrite.  It is associated with kapellasite, connellite and serpierite. Its appearance is very similar to serpierite, in which the presence of slight greenish tones in the fehrite can be differentiated. In contrast, the colour of serpierite is sky blue, without green tones.^[3] It is also found in the Les Ferreres mine, Rocabruna, Camprodón (Gerona), Spain.^[4]