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Investigations of what was previously called “metamorphic vermiculite” from the Upper Precambrian metagreywackes surrounding the massive sulfide deposit of Rouez, France, have led to the characterization of this material as an interstratified chlorite/smectite (C/S). X-ray powder diffraction data obtained from oriented mounts of small quantities of material (< 0.001 mg) drilled from thin sections confirmed the presence of smectite layers (< 20%) randomly interstratified with chlorite. This interstratified C/S has a composition that differs from chlorite in that it contains more titanium and alkali elements; however, the absolute amounts of these elements vary from grain to grain. Mössbauer spectrometry indicated a low Fe3+ content for this material.

In thin section, the C/S occurs as disseminated, brown flakes, optically similar to biotite or stilpno-melane. The C/S is apparently authigenic, and has been partly replaced by the assemblage chlorite + phengite during low-grade Hercynian regional metamorphism.

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