a major industry of the ottosdal area is the pyrophyllite mines located on the farm gestoptefontein. these mines produce a very high quality pyrophyllite, which is a phyllosilicate mineral composed of aluminium silicate hydroxide: alsiooh. the pyrophyllite is a metamorphosed clay. the clay was formed by the alteration of volcanic ash, which apparently accumulated at the bottom of a quiet body of water. it forms a dense, but soft light grey, or darker, fine-grained stone, which splits into well-defined slabs. the strata, in which it occurs, is a 3.0 to 3.1 billion year-old, over 2 kilometer-thick assemblage of arenaceous sediments, conglomerates, grits, basic volcanics, tuffs, coarse pyroclastic rocks and quartzite called the "dominion group".