Abstract

Norsethite, BaMg(CO3)2, was found in dolomitic black oil shale below the main trona bed in the Westvaco trona mine in Wyoming, associated with shortite, labuntsovite, sear-lesite, loughlinite, pyrite, and quartz. It also occurs there in gray shale with abundant shortite and northupite, some searlesite and loughlinite, in a fine-grained matrix consisting essentially of quartz and pyrite.

Norsethite occurs as clear to milky-white circular plates or flattened rhombohedral crystals, 0.2-2.0 mm. across. The mineral is insoluble in water but is readily decomposed by cold dilute hydrochloric acid. Norsethite has hardness about 3.5; density 3.837 ± 0.005 (meas.), 3.840 (calc.); luster vitreous to pearly; fracture hackly; good rhombohedral cleavage. It is infusible before the blowpipe. Norsethite is uniaxial negative, and the indices of refraction are ω = 1.694 and ε = 1.519. X-ray crystallographic studies show that the crystals are rhombohedral and have the following characteristics: possible space groups, R3m, R3m, or R32, the most probable being R32 (D37), a subgroup of R3c, the space group of calcite; hexagonal a = 5.020 ± 0.005 Å, c = 16.75 ± 0.02; rhombohedral arh = 6.29 ± 0.01 Å, α = 47°02’ ± 05’; volume 365.6 Å3 (hex.); cell contents: 3[BaMg(CO3)2], in the hexagonal unit. Crystal forms observed are c {10001}, a {1120 ], m {1010}, and r {1011}. The strongest x-ray lines are: 3.015 Å (100), 3.860 (35), 2.656 (35), 2.512 (35), 2.104 (35), 1.931 (35), 1.864 (35).

Norsethite has a structure similar to that of calcite. The structural relations of norsethite, dolomite, and calcite are discussed.

Chemical analysis of a 0.1 gm. sample gave: BaO 52.9, CaO 0.5, MgO 13.9, FeO 0.4, MnO 0.1, CO2 31.2, Na2O 0.2, SiO2 0.3, insoluble 0.4, total 99.9 per cent.

Norsethite is named in honor of Mr. Keith Norseth, engineering geologist of the trona mine at Westvaco, Sweetwater County, Wyoming.

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