The Nature of Garnierites—I Structures, Chemical Compositions and Color Characteristics

Abstract

X-ray diffraction patterns of garnierites indicate that most samples resemble serpentine- group minerals or a talc-like mineral, or a mixture of these forms, and give respectively 7 A and l0 A basal reflections. From a survey of some 40 garnierites, four of predominantly serpentine type and seven of predominantly talc-like type were selected for detailed study. The talc-like garnierites exhibit little variation of the l 0 A basal spacing with low-temperature heating or with immersion in liquids, though some may contain a small proportion of expandable layers. Chemical analyses show consider- able deviations of octahedral/tetrahedral cation ratios from the values 3/2 and 3/4 for normal serpen- tine and talc minerals, and may be interpreted in terms of mixed 1 : I and 2 : 1 layer types, either as separate phases and/or as interstratifications, or as defect structures of various kinds. The H20+ contents of the talc-like forms of garnierite are considerably greater than that of normal talc and point to a mineral of composition 3(Mg, Ni)O'4SiO2.2H20 or ((Mg, Ni)3Si4010(OH)2)'H20-a talc mono- hydrate formula. The green color of garnierites is related to the NiO weight per cent and a color index is derived based on the Munsell color charts.