Contribution to the mineralogy of Norway. No. 12. Hisingerite in "Dark" Larvikite.
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Magnesium- and calcium-bearing hisingerite, typically with extremely fine magnetite inclusions, occurs spatially associated with pyroxene (and very possibly as an alteration of olivine) in "dark" larvikite. It has a specific gravity of ca. 1.80, a waxy to subvitreous luster, is black to dark brown in reflected light and golden- to wine-brown by transmitted light, and has a greenish brown to olive green streak. Optical studies indicate the presence of intimately intermixed amorphous and crystalline phases. The amorphous phase has an index of refraction that ranges from 1.480 to 1.486. The crystalline phase has a beta index that ranges from 1.470 to 1.478, is biaxial negative, ha:s a 2V of 60° to 70°, and has oblique extinction. X-ray diffraction patterns consist of three broad reflections with d values of their central portions of ca. 4.44, 2.575, and 1.535. Differential thermal analyses show the following reactions: large endothermic with peak at ca. 125° C., medium-sized exothermic with peak at ca. 450° C., and small exothermic at ca. 970° C. Chemical analyses, all of which have necessarily been made on contaminated material, are consistent with previously published analyses.
This study has pointed up the fact that relationships among materials called hisingerite, chloropal, nontronite, natural iron oxide gel., etc. need much further investigation and clarification. It is suggested that for a start the name hisingerite could be restricted to either the amorphous or the crystalline phase with the listed characteristics.