Micheelsenite, (Ca, Y)_3Al (PO_3OH, CO_3) (CO_3) (OH)_6 centre dot 12H_2O, a new mineral from Mont Saint-Hilaire, Quebec, Canada and the Nanna pegmatite, Narsaarsuup Qaava, South Greenland

摘要

Micheelsenite, a new member of the ettringite group, is found in pegmatites, so-called silicate cavities, seams in hornfels and marble xenoliths at the Poudrette quarry, Mont Saint-Hilaire, Quebec, Canada and at the Nanna pegmatite, Narsaarsuup Qaava, South Greenland. It occurs as acicular to fibrous crystals in loosely packed radiating groups and as matted fibres. At Mont Saint-Hilaire, the mineral occurs in association with aegirine, albite, ancylite-(Ce), catapleiite, fluorite, microcline, monteregianite-(Y), natrolite, nenadkevichite, rhodochrosite and serandite (pegmatites) or natrolite, titanite, calcite and pyrite (seams in hornfels). At the Nanna pegmatite the mineral occurs with aegirine, astrophyllite, analcime, calcio-ancylite-(Ce), catapleiite, fluorite, galena, gibbsite, leucophanite, microcline, natrolite, nafertisite, orthoclase, polylithionite, sodalite var. hackmanite and todorokite. The mineral is colorless to white, transparent to opaque with a vitreous luster. It has a hardness of 3(1/2)-4, is brittle with a splintery fracture and has good {101-bar0) and {0001} cleavages. It is uniaxial negative with #omega# = 1.532(1) and #epsilon# = 1.503(1) (Mont Saint-Hilaire), #omega# = 1.5265(5) and #epsilon# = 1.5020(5) (Nanna pegmatite) and is nonpleochroic. D_(meas). 2.15(1) and D_(calc). 2.17(1) g/cm~3. The mineral is hexagonal, P6_3 with a 10.828(3), c 10.516(4) A, V = 1067.8(5)A~3 (Mont Saint-Hilaire), a 10.864(4), c 10.539(4) A, V = 1077.2(5) A~3 (Nanna pegmatite) with Z = 2. The strongest lines on the X-ray powder diffraction pattern (d in A (I)(hkl)) are: 9.38(100)(100), 4.59(70)(102), 3.77(50)(112), 3.36(55)(211), 2.491(80)(213), 2.143(65)(223,402). (Mont Saint-Hilaire). Electron microprobe analysis (average of three) for micheelsenite from Mont Saint-Hilaire gave: CaO 16.90, Al_2O_3 6.70, Y_2O_3 18.07, Gd_2O_3 0.84, Dy_2O_3 2.65, Ho_2O_3 0.51, Er_2O_3 1.88, SiO_2 0.07, P_2O_5 7.80, SO_3 0.53, CO_2 (8.38), H_2O (43.01), total 107.34 wt. percent. The ideal formula is (Ca,Y)_3Al(PO_3OH,CO_3)(CO_3)(OH)_6 centre dot 12H_2O based on results from the crystal structure analysis and its relationship with ettringite. The IR spectrum includes the following bands (cm~(-1)): 3240, 1666, 1402, 1030, 924, 579 and 452 cm~(-1) It is a late-stage, hydrothermal mineral, likely developing from highly fractionated Y- and HREE-enriched fluids. It is the newest member of the ettringite group and the first of this group that does not contain essential SO_4. The mineral is named in honor of Dr. H. I. MICHEELSEN (1931-), discoverer of the Nanna pegmatite, for his contributions to the mineralogy of alkaline rocks, specifically those from South Greenland.