THAUMASITE [CA3SI(OH)6SO4CO3.12H2O] IN A HYDROTHERMALLY-ALTERED CALCSILICATE SKARN FROM NORTH-CENTRAL CHIHUAHUA, MEXICO

The host rocks for the thaumasite originated from blocks of Cretaceous limestone and Oligocene felsic ashflow tuffs. The blocks were incorporated into a shallow glomerocrystic andesite intrusion that had invaded a major eruptive vent zone for a widespread glomerocrystic, low-silica, ashflow rhyolite. Contact metamorphism and metasomatism at high temperatures was followed by prolonged water-rock reactions that continued to low (ambient) temperatures. During the waning stages of the hydrothermal event, calcite was widely precipitated, and decomposition, hydration, and/or dissolution of high temperature calcium-rich silicates, such as wollastonite, gehlenite, and cuspidine, provided dissolved calcium and silica to the low temperature hydrous fluids.

The thaumasite-bearing rock consists of dark, small-pebble size, CaFeTi garnets embedded in a fine to very fined grained white matrix. Weakly indurated portions of the matrix are monomineralic thaumasite; strongly indurated portions contain thaumasite and calcite. The thaumasite occurs as very fine grained, prismatic crystals. They are uniaxial negative (length-fast with parallel extinction), have relatively low refractive indices for a silicate mineral (1.470-1.504), and bright 3rd order birefringent colors. The x-ray diffraction pattern confirms thaumasite and the microprobe analysis is devoid of Al, showing that the mineral is not an ettringite-thaumasite solid solution.

Laboratory syntheses, spurred by the detrimental effects of thaumasite crystallization in concretes, have in common slow reaction times, low temperatures (5° to 50° C), and an aqueous medium supplied with dissolved silica and calcium, carbonate, and sulfate ions. Gypsum and calcite may crystallize during the syntheses.

The thaumasite formed at low temperatures some time following the Oligocene hydrothermal event. Possible sulfate sources include: 1) oxidation of very sparse, widely dispersed pyrite during weathering; 2) gypsum originally present in the limestone protolith, and 3) gypsum precipitated due to SO2 degassed from the andesite intrusion.