Micas are minerals that are abundant in nature and make up approximately 3.8% of the Earth’s crust. The most abundant variety at the Penouta Mine is white mica, more specifically muscovite.
Muscovite is found in the mineralized granite, in the greisen and the metamorophic host rock. In the metamorphic layer in a gneis called Ollo de Sapo, there is also biotite mica. Both muscovite and biotite can appear in the Penouta Mine, even though there is significantly higher proportion of muscovite.
Various types of muscovite have been found in the albitic granite at Penouta using an electron microprobe, which give the granite different tonality:
The petrographic study conducted on granite samples reveals that muscovite crystallized late in the sequence. This falls in line with the antagonistic character of the white mica's distribution compared to potassium feldspar. This peculiar feature could indicate that the white mica was formed by an alteration of the potassium feldspar.