ABSTRACT

     Micheelsenite, ideally (Ca,Y)₃Al(PO₃OH,CO₃)(CO₃)(OH)₆ · 12H₂O, originally designated as UK32 has been approved as a new mineral by the Commission on New Minerals and Mineral Names, IMA. The type localities are the Nanna pegmatite, Narsaarsuup Qaava, South Greenland and the Poudrette quarry, Mont Saint-Hilaire, Quebec.  It was named in honour of Dr. H.I. Micheelsen (1931- ), discoverer of the Nanna pegmatite, for his contributions to the mineralogy of alkaline rock, specifically those from South Greenland.  Cotype material is preserved in the Canadian Museum of Nature (Ottawa, Canada) CMNMC 83049 and CMNMC 83050, and the Geological Museum of the University of Copenhagen (Copenhagen, Denmark) G.M.2000.250.

     Micheelsenite was first encountered at MSH in April 1971 by Mssrs. David Richerson, Jacques Bradley, and Col. Quintin Wight.  Mr Richerson had demonstrated that the species was potentially new to science and it was tentatively referred to as UK32 and WMC-1 (Worchester Mineral Club).  It was subsequently found at the Narssârssuk pegmatite in 1988 and referred to as an unidentified mineral.  Better study material was subsequently found at the Nanna pegmatite, Narsaarsuup Qaava, South Greenland.

     At MSH it is found in pegmatites (the largest concentration), so-called silicate cavities, seams in hornfels, and marble xenoliths.  Associated minerals include aegirine, albite, ancylite-(Ce), catapleiite, fluorite, microcline, monteregianite-(Y), (sometimes referred to as "rhodesite"), natrolite, nenadkevichite, rhodochrosite, and sérandite.  In the pegmatites it is found proximal to rhodochrosite, often nucleating on the {001} faces.  Micheelsenite has been found in the south-east corner of the quarry, near the "Poudrette pegmatite".  Here it is associated with calcite, pyrite (some oxidized with an unidentified, fine-grained reddish material), natrolite, fluorite and titanite.

     Micheelsenite is hexagonal, space group P6₃; a 10.828(3), c 10.516(4)Å, V 1067.8(5)ų, Z = 2.  It is found as acicular to fibrous crystals in loosely packed spherical aggregates (max. 3mm diameter) and in bundles of randomly oriented, hair-like matted fibres.  The crystals are heavily striated along [001] and terminated by pedion {001}.  The hexagonal cross-sections are generally < 5µm, rarely to 0.3mm.  The mineral is white to colourless, translucent to transparent, with a vitreous lustre and a white streak.  Micheelsenite from the mineralized seam in the hornfels occurs as small (0.6 mm x 0.1 mm thick) rounded plates. These plates are opaque while viewed along [001] but transparent viewed on edge.  The mineral is brittle with a splintery fracture, and exhibits good {100} and {001} cleavage.  It has a hardness of 3.5 - 4, a measured and calculated density of 2.15(1) and 2.17(1) g/cm³ respectively, and it does not fluoresce in LW or SW ultraviolet light.  Micheelsenite is uniaxial negative, non-pleochroic with w 1.532(1) and e 1.503(1).

         The structure has been determined and will be the subject of another paper. The basic structure is however consistent with that of the ettringite group of minerals.

© 2001  Neues Jahrbuch fur Mineralogie