Publication Date:
2018
Description:
〈div data-abstract-type="normal"〉〈p〉Beusite-(Ca), ideally Ca〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190128104130349-0707:S0026461X18001202:S0026461X18001202_inline2.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉(PO〈span〉4〈/span〉)〈span〉2〈/span〉, is a new graftonite-group mineral from the Yellowknife pegmatite field, Northwest Territories, Canada. It occurs in a beryl–columbite–phosphate rare-element pegmatite where it is commonly intergrown with triphylite–lithiophilite or sarcopside, and may form by exsolution from a high-temperature (Li,Ca)-rich graftonite-like parent phase. It occurs as pale-brown lamellae 0.1–1.5 mm wide in triphylite, and is pale brown with a vitreous lustre and a very pale-brown streak. It is brittle, has a Mohs hardness of 5, and the calculated density is 3.610 g/cm〈span〉3〈/span〉. Beusite-(Ca) is colourless in plane-polarized light, and is biaxial (+) with α = 1.685(2), β = 1.688(2), γ = 1.700(5), and the optic axial angle is 46.0(5)°. It is non-pleochroic with X || 〈span〉b〈/span〉; Y ˄ 〈span〉a〈/span〉 = 40.3° in β obtuse; Z ˄ 〈span〉a〈/span〉 = 49.7° in β acute. Beusite-(Ca) is monoclinic, has space group 〈span〉P〈/span〉2〈span〉1〈/span〉〈span〉/c〈/span〉, 〈span〉a〈/span〉 = 8.799(2), 〈span〉b〈/span〉 = 11.724(2), 〈span〉c〈/span〉 = 6.170(1) Å, β = 99.23(3)°, 〈span〉V〈/span〉 = 628.3(1) Å〈span〉3〈/span〉 and 〈span〉Z〈/span〉 = 4. Chemical analysis by electron microprobe gave P〈span〉2〈/span〉O〈span〉5〈/span〉 41.63, FeO 19.43, MnO 23.63, CaO 15.45, sum 100.14 wt.%. The empirical formula was normalized on the basis of 8 anions pfu: (Ca〈span〉0.94〈/span〉Fe〈span〉0.92〈/span〉Mn〈span〉1.13〈/span〉)〈span〉Σ2.99〈/span〉(PO〈span〉4〈/span〉)〈span〉2.00〈/span〉. The crystal structure was refined to an 〈span〉R〈/span〉〈span〉1〈/span〉 index of 1.55%. Beusite-(Ca) is a member of the graftonite group with Ca completely ordered at the [8]-coordinated 〈span〉M〈/span〉(1) site.〈/p〉〈/div〉
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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