Detrital chrome spinels in terrigenous rocks of the Alkesvozh suite interact with the rock matrix during metamorphism under greenschist facies conditions. This results in the replacement of chrome spinels by Cr-bearing muscovite (fuchsite) or magnetite. The relict portion of the chrome spinels, as a result of component exchange with the environment, is replaced by secondary chrome spinels, which lack Mg but contain significant Zn contents. The composition of all chrome spinel grains is secondary, but the process of change caused by metamorphism of sedimentary rocks is clearly visible only in some of them. The ZnO content of detrital chrome spinels varies both from grain to grain and within a single grain. The minimum ZnO content recorded by us in chrome spinels is 1.3 wt. %, and the maximum is 17.5 wt. %. The final result of the transformation of chrome spinels is the conservation of their relics in magnetite crystals, or their complete disappearance. The former presence of detrital chrome spinelides in the rock is evidenced only by the presence of Cr in metamorphic minerals (mica, chlorite, and hematite), as well as the synthesis of idiomorphic crystals of Zn-containing (14.4—18.1 wt. % ZnO) chromium spinelide in a fuchsite matrix.
Onishchenko et al. (Wed,) studied this question.