Stevensite and Talc—Hydrothermal Alteration Products of Wollastonite
Magnesium-bearing hydrothermal solutions moving along channelways created by faulting in the Willsboro, New York, wollastonite deposit have effected the conversion of wollastonite to layer-lattice silicates. Alteration products sheath faults in wollastonite and may be divided into three gradational but regular zones: (1) least altered (fresh) country rock composed of wollastonite with minor diopsidic clinopyroxene and grossularite-andradito garnet; (2) moderately altered rock rich in montmorillonite; and (3) strongly altered rock composed predominantly of talc.
X-ray diffraction, oscillating-heating X-ray diffraction, electron microscopy, infrared absorption, base exchange, and chemical analyses substantiate identification of the montmorillonitic alteration product, pseudomorphous after wollastonite, as stevensite. Stevensite, a trioctahedral magnesium montmorillonite, has been reported heretofore only in basaltic rocks where it has been formed by the action of magnesium-bearing solutions on pectolite. Pectolite and wollastonite have similar compositions and structures and may be similarly converted to stevensite in a magnesium-rich hydrothermal environment.
Talc, the dominant mineral in the most intensely altered zone bordering the solution channelway, was formed by the continuing action of magnesium-bearing solutions on stevensite. Stevensite has a defect structure, i.e., a deficiency in the total number of ions in octahedral coordination. The addition of magnesium from hydrothermal solutions to the octahedral layer of stevensite converts this mineral to talc, concomitantly freeing interlayer cations and water.
Stevensite is unstable and is converted to talc above a temperature of 275°C at water pressures of 690 bars. It is believed that hydrothermal alteration of wollastonite took place at temperatures near or below 275°C with stevensite forming in the cooler part of the reaction zone, and talc forming from stevensite at slightly higher temperatures nearer the solution channelway.
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