Nontronite, the iron-rich end member of Smectite Group clays, has previously been reported as a hydrothermal alteration product, as a constituent of soil clays formed by weathering of a variety of rock types, and as an alteration product of volcanic rocks. This paper describes the chemical, optical, and X-ray properties of a new occurrence of the mineral where its origin is apparently related to direct crystallization from high temperature waters. The mineral is further unique in that it contains only a very small percentage of alumina and thereby closely approaches the previously unreported wholly iron end-member.

Significant differences were found in the optical properties of the Venezuelan nontronite when compared with those of an earlier study, especially with regard to the variation in refractive index with Fe2O8 content. It now appears that this variation has been over-simplified and may not be as orderly as was previously believed. The various lines of evidence that argue against an origin by either hydrothermal alteration or chemical weathering are discussed, as well as those supporting an origin by direct crystallization from high temperature waters of probable magmatic or metamorphic origin.

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