Some optical, chemical and x-ray studies have been made on several chromium micas (fuchsite and mariposite) of the type associated with many pre-Cambrian gold deposits.
which closely approaches the accepted muscovite formula, K4Al8(Al,Si3)40OH8, representing the ideal unit cell content. In fuchsite Cr replaces Al up to 4.81% Cr2O3, corresponding closely to 1 atom of chromium per unit cell. Optical properties of fuchsite show the following variation: α=1.559 to 1.569, β = 1.593 to 1.604, γ =1.597 to 1.611; 2V 30°-46°, indices increasing with increase in chromium. X-ray studies confirm the structural identity of fuchsite with muscovite. The varietal name chromian muscovite is preferred to the name fuchsite.
Available analyses of mariposite show a decidedly lower content of Cr2O3 and Al2O3 and higher SiO2. The highest observed chromium content is 0.78% Cr2O3. The aluminum-silicon ratio in tetrahedral groups is between 2:14 and 1:15, compared with a ratio of 4:12 in fuchsite and muscovite. Published optical data show mariposite with higher indices than fuchsite and 2V 0°-14°. X-ray studies have not yet established the structural type of mica to which it belongs, but it is tentatively classed as chromian phengite.
Consideration is given to the association of these micas with hydrothermal carbonate zones and quartz-sulfide gold deposits.