Beusite-(Ca), ideally CaMn22+(PO4)2, a new graftonite-group mineral from the Yellowknife pegmatite field, Northwest Territories, Canada: Description and crystal structure. (29th May 2018)

• Record Type: Journal Article
• Authors: Hawthorne, Frank C.; Wise, Michael A.; Černý, Petr; Abdu, Yassir A.; Ball, Neil A.; Pieczka, Adam; Włodek, Adam
• Is Part Of: Mineralogical magazine. Volume 82:Number 6(2018)
• Page Start: 1323
• Page End: 1332
• Publication Date: 2018-05-29
• Subjects: beusite-(Ca), -- new mineral, -- chemical analysis, -- crystal structure, -- Raman spectrum, -- Yellowknife pegmatite field, -- Northwest Territories, -- Canada; Mineralogy -- Periodicals; Mineralogy -- Great Britain -- Periodicals; 549.05
• DOI: 10.1180/mgm.2018.120
• Languages: English
• ISSNs: 0026-461X

Abstract

ABSTRACT: Beusite-(Ca), ideally Ca ${\rm Mn}_{\rm 2}^{2 +} $ (PO4 )2, 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 3 . 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 || b ; Y ˄ a = 40.3° in β obtuse; Z ˄ a = 49.7° in β acute. Beusite-(Ca) is monoclinic, has space group P 21 /c, a = 8.799(2), b = 11.724(2), c = 6.170(1) Å, β = 99.23(3)°, V = 628.3(1) Å 3 and Z = 4. Chemical analysis by electron microprobe gave P2 O5 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: (Ca0.94 Fe0.92 Mn1.13 )Σ2.99 (PO4 )2.00 . The crystal structure was refined to an R 1 index of 1.55%. Beusite-(Ca) is a member of the graftonite group with Ca completely ordered at the [8]-coordinated M (1) site.