Hydroxycalciomicrolite, Ca1.5Ta2O6(OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to 1.5 mm in size. The crystals are yellow and translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 56. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm(3). Hydroxycalciomicrolite is isotropic, n(calc). = 2.010. The infrared and Raman spectra exhibit bands due to OH stretching vibrations. The chemical composition determined from electron microprobe analysis (n = 13) is (wt.%): Na2O 0.36(8), CaO 15.64(13), SnO2 0.26(3), Nb2O5 2.82(30), Ta2O5 78.39(22), MnO 0.12(2), F 0.72(12) and H2O 1.30 (from the crystal structure data), O = F -0.30, total 99.31(32), yielding an empirical formula, (Ca1.48Na0.06Mn0.01)(Sigma 1.55)(Ta1.88Nb0.11Sn0.01)S2.00O6.00[(OH)(0.76)F0.20O0.04]. Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) angstrom, V = 1131.53(2) angstrom(3) and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P4(3)32 symmetry, instead of Fd (3) over barm. The reduction in symmetry is due to long-range ordering of Ca and vacancies on the A sites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species with P4(3)32 or lower-symmetry space group can be discovered and characterized.

Abstract Mineralogical-mineral chemical characterisation of Nb-Ta-bearingassemblages from the Stripåsen granitic pegmatite, NorbergJens Henriksson The occurrence of Nb-Ta-bearing minerals has earlier been briefly documented inthe Stripåsen granitic pegmatite, Norberg, central Bergslagen. This report describesthe background, preparation and basic characterisation of sampled Nb-Ta-bearingmineral assemblages from this pegmatite. These comprise (Y,REE,U,Th)-(Nb,Ta,Ti)-oxides of mainly a tantalum-dominated A 2-m B 2 X 6-w Y 1-n -type, belonging to the pyrochloresupergroup. The major primary mineral is suggested to have been afluorcalciomicrolite, ideally (Ca,Na,☐) 2 Ta 2 O 6 F. In association with abundant fractureformation related to metamictisation of the microlite minerals, they are interpreted tohave been variably altered by late-stage fluids. Thus, the system has beenextensively modified after the primary crystallisation of the pegmatite melt. Besidesthese (Y,REE,U,Th)-(Nb,Ta,Ti)-oxides, monazite-(Ce) ((Ce, LREE)PO 4 ), nativebismuth (Bi) and presumably radiogenic galena (PbS) were identified. The sampleshave mainly been studied by means of scanning-electron-microscopy with energydispersive spectroscopy (SEM-EDS), utilising reflected polarised light microscopy asa complementary tool. The study was done within the framework of the EU-supported FRAME-project which is focused on a selection of metals and minerals,among others niobium (Nb) and tantalum (Ta). / Sammanfattning Mineralogisk-mineralkemisk karakterisering av Nb-Ta-förande associationerfrån Stripåsenpegmatiten, NorbergJens Henriksson Förekomsten av Nb-Ta-förande mineral har tidigare dokumenteras iStripåsenpegmatiten, Norberg, centrala Bergslagen. Här redogörs för bakgrund,provförberedelse och grundläggande karakterisering av provtagna Nb-Ta-förandemineralassociationer från denna pegmatit. Proven som undersökts innehåller(Y,REE,U,Th)-(Nb,Ta,Ti)-oxider av främst tantaldominerad A 2-m B 2 X 6-w Y 1-n -typtillhörande pyroklorsupergruppen. Den huvudsakliga primära fasen tolkas ha varit enfluorkalciomikrolit, idealiskt (Ca,Na,☐) 2 Ta 2 O 6 F. I anslutning till rikligt förekommandesprickor som är relaterade till omfattande metamiktisering har mikroliternaomvandlats i varierande grad av sena fluider. Systemet har alltså förändrats kraftigtefter den primära kristallisationen av pegmatitsmältan. Förutom (Y,REE,U,Th)-(Nb,Ta,Ti)-oxider identifierades monazit-(Ce) ((Ce,LREE)PO 4 ), gedigen vismut (Bi)och vad som sannolikt är radiogen blyglans (PbS). Provmaterialet har huvudsakligenundersökts med genom svepelektronmikroskopi med energidispersiv spektroskopi(SEM-EDS), varvid malmmikroskopi har använts som ett kompletterande verktyg.Studien gjordes inom ramverket för det EU-stödda FRAME-projektet som blandannat fokuserar på förekomster av de kritiska och konfliktrelaterade metallerna niob(Nb) och tantal (Ta).