Abstract

The crystal structures of five monoclinic alkali amphiboles from Kajlidongri, Madhya Pradesh, India, have been refined to R indices of <2% using ~1100 observed (5σ) reflections measured with Mo X-radiation; the crystals used in the collection of the X-ray data were subsequently analyzed by electron microprobe techniques. Both site-scattenng refinement and an ion microprobe analysis show major Li in these crystals (up to ~1.3 wt% Li2O), which fall in the series ferri-eckermannite-leakeite [NaNa2(Mg4Fe3+)Si8O22OH2-NaNa2(Mg2Fe23+Li)Si8O22(OH)2]. HRTEM examination showed a well-behaved amphibole structure, relatively free of defects. These amphiboles contain significant Mn (up to 0.5 apfu), and all Fe and Mn are in the trivalent state. These amphiboles are indicative of strongly oxidizing conditions. In agreement with this, one crystal shows an oxy-substitution that is characterized by the occurrence of Ti4+ at the M1 site. Li is completely ordered at the M3 site, and the compensating trivalent cations are ordered primarily at M2. This means that [6]Li-bearing alkali amphiboles can easily be identified in the absence of AA or ion microprobe analysis by the anomalously low X-ray scattering at the M3 site. Significant C-group Li has only been found in alkali amphiboles, where it seems to be locally associated with Na at the A site and Si at all neighboring T sites. Local charge-balance considerations suggest that [6]Li does not occur in sodic-calcic and calcic amphiboles, in agreement with the coexistence of holmquisite and hornblende or pargasite.

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