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Millsite CuTeO3·2H2O: a new polymorph of teineite from Gråurdfjellet, Oppdal kommune, Norway

DOI: 10.1180/minmag.2017.081.057 DOI Help

Authors: Michael Rumsey (Natural History Museum) , Mark D. Welch (Natural History Museum) , Frode Mo (Norwegian University of Science and) , Annette K. Kleppe (Diamond Light Source) , John Spratt (Natural History Museum) , Anthony R. Kampf (Natural History Museum) , Morten Raanes (Norwegian University of Science and)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Mineralogical Magazine

State: Published (Approved)
Published: August 2017

Abstract: Millsite, CuTeO3·2H2O, is a new mineral from Gråurdfjellet in Oppdal, Norway. It occurs as a minor secondary phase alongside teineite, other copper secondaries and relict primary tellurides in a boulder of quartz-rich granite, which is probably a glacial erratic. Millsite is bright cyan to royal blue in colour. The mineral is transparent to slightly translucent with a vitreous lustre and has a perfect (100) cleavage. It is brittle, has a conchoidal fracture and a pale green streak. Millsite is optically biaxial (+), α = 1.756(5), β = 1.794(5), γ = 1.925calc and 2Vmeas = 60(1)°; Millsite has monoclinic space group P21/c, with a = 7.4049(2) Å, b = 7.7873(2) Å, c = 8.5217(2) Å, β = 110.203(3)°, V = 461.17(2) Å3 and Z = 4. The empirical formula is Cu0.99(Te0.98Se0.02)O3(H2O)2. The five strongest reflections in the X-ray powder diffraction pattern are [dhkl in Å (hkl, Irel %)]: 6.954 (100, 100), 3.558 (012, 64), 2.838 (12¯2, 47), 2.675 (211, 43) and 3.175 (210, 39). The crystal structure has been determined to R1 = 0.016, wR2 = 0.036, GooF = 1.049. The diagnostic structural unit of millsite consists of a Cu2O6(H2O)4 dimer that is decorated with four TeO3 groups connecting adjacent dimers and defining (100) heteropolyhedral sheets. These heteropolyhedral sheets are only connected by layers of structurally significant hydrogen bonds and correlate with the (100) cleavage. Millsite is a polymorph of teineite with a unique configuration of the M2O6(H2O)4 dimer that leads to a sheet topology. No isostructural selenium or tellurium analogue exists. The monoclinic polymorph (P21/c) of chalcomenite “monoclinic-CuSeO3·2H2O” hereafter, ahlfeldite and MgSeO3·2H2O have M2O6(H2O)4 dimers, but their configuration differs significantly from that of millsite and leads to a framework topology rather than a sheet. Teineite does not have a dimeric structure and so is fundamentally different from millsite. The sheet topology of millsite appears to be unique among tellurites.

Journal Keywords: Millsite; new mineral; copper tellurite dihydrate; teineite; type mineral; Norway

Subject Areas: Earth Science

Facility: ESRF

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