1973 Volume 4 Issue 4 Pages 175-191
A clayey material consisting predominantly of the trioctahedral 2: 1 phyllosilicate which is similar to talc rather than stevensite, with a basal spacing of 9.7 A, was found in hydrothermal-alteration products of wollastonite-rich skarn at the Akatani copper-iron mine, Niigata Prefecture, Japan, by one of the present authors (N. I.). The material occurs as a small pocket in the wollastonite-rich skarn, and is pseudomorphous after wollastonite. Field relations indicated that the small pocket is located in an intermediate subzone between inner talc and outer stevensite zones.
Chemical analysis of the clay fraction less than 2μ leads to the following structural formula for the mineral in question per unit-cell-layer on the basis of O20 (OH) 4 anion unit;
(Mg5.81 Mn2+0.40 Fe3+0.06)(Si7.73 Al0.15) O20 (OH) 4 (Na0.02 K0.02)-EnH2O.
X-ray diffraction, thermal and infrared absorption analyses revealed that the mineral in question is a variety less hydrated than stevensite, and may be named as “hydrated talc”.
Based upon the field relations, information on the formation temperatures of minerals from the mine obtained by means of fluid-inclusion thermometry, and the synthetic data for the system MgO-SiO2-H2O, the mineral in question was identified as an alteration product of the preexisting wollastonite at temperatures of 250°-300° by the action of magnesium-bearing hot water, which journeyed through a channel-way immediately before ore-formation and simultaneously dolomitized limestone wallrock.
