Click the photo for a larger version. Phengite is a catchall name used mostly in microscopic studies for a mica mineral that departs from the ideal attributes of muscovite (specifically, a high α, β and γ and a low 2V). The formula allows considerable iron substituting for the Mg and Al (that is, both Fe+2 and Fe+3). For the record, Deer Howie and Zussman give the formula as K(Al,Fe3+)Al1–x(Mg,Fe2+)x[Al1–xSi3+xO10](OH)2.
Mariposite is a green chromium-bearing variety of phengite, first described in 1868 from the Mother Lode country of California, where it is associated with gold-bearing quartz veins and serpentinite precursors. It generally is massive in habit, with a waxy luster and no visible crystals. Mariposite-bearing quartz rock is a popular landscaping stone, itself often called mariposite. The name comes from Mariposa County. Supposedly the rock was once a candidate for the California state rock, but serpentinite prevailed.