Potassium micas: a new treatment of literature data

The present data set includes results of 1460 mica analyses, of which 1157 had passed crystal-chemical screening. Their composition was plotted in a projection pyramid alongside with eight potassium mica end-members. The biotite and zinnwaldite faces of the pyramid, together with the basal projection (from the muscovite vertex), are most informative, not only to illustrate main composition trends, but also to appreciate the distribution of dioctahedral + trioctahedral mica pairs. The two-mica subset (87 pairs, 61 of which passed crystal-chemical screening) might be promising for T-p applications. Physical properties are available for a limited number of micas: refractive indices β, γ (N = 620), mass density (N = 284), and unit-cell parameter b (N = 176). Corresponding regression equations were fitted as functions of orthogonal coordinates in the basal projection and offer estimates of what researchers may expect for a given mica composition.