X-ray examination of geocronite from several localities shows that it is monoclinic, probable space group C2h2P21/m, with cell dimensions varying slightly near a0 9.0, b0 31.9, c0 8.5, β 118°. Geocronite is isostructural with jordanite and presumably constitutes the intermediate member of a series with varying proportions of antimony and arsenic, and possibly other compositional variation. Jordanite is the arsenic end-member of this series, the antimony end-member not being certainly known. As with jordanite there is uncertainty as to the cell content of geocronite and no simple formula can be assigned with assurance. Polished-surface observations indicate that geocronite, jordanite, and boulangerite are not distinguishable from one another with certainty by these methods alone and suggest that in a number of respects previous descriptions may be unreliable.

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