615 The Canadian Mineralogist Vol. 49, pp. 615-621 (2011) DOI : 10.3749/canmin.49.2.615 THE CRYSTAL STRUCTURE OF A MICROLITE-GROUP MINERAL WITH A FORMULA NEAR NaCaTa2O6F FROM THE MORRO REDONDO MINE, CORONEL MURTA, MINAS GERAIS, BRAZIL MARCELO B. ANDRADE§ Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trab. São-carlense 400, 13560-970 São Carlos, SP, Brazil DANIEL ATENCIO Instituto de Geociências, Universidade de São Paulo, Rua do Lago, 562, 05508-080 São Paulo, SP, Brazil LUIZ A.D. MENEZES FILHO Instituto de Geociências, Universidade Federal de Minas Gerais, Avenida Antônio Carlos, 6627, 31270-901 Belo Horizonte, MG, Brazil JAVIER ELLENA Instituto de Física de São Carlos, Universidade de São Paulo, Av. Trab. São-carlense 400, 13560-970 São Carlos, SP, Brazil ABSTRACT We present electron-microprobe and single-crystal X-ray-diffraction data for a microlite-group mineral with a formula near NaCaTa2O6F from the Morro Redondo mine, Coronel Murta, Minas Gerais, Brazil. On the basis of these data, the formula A B X Y is (Na0.88Ca0.88Pb0.020.22)S2.00 (Ta1.70Nb0.14Si0.12As0.04)S2.00 [(O5.75(OH)0.25]S6.00 (F0.730.27)S1.00. According to the new nomenclature for the pyrochlore-supergroup minerals, it is intermediate between fluornatromicrolite and “fluorcalciomicrolite”. The crystal structure, Fd3m, a = 10.4396(12) Å, has been refined to an R1 value of 0.0258 (wR2 = 0.0715) for 107 reflections (MoKa radiation). There is a scarcity of crystal-chemical data for pyrochlore-supergroup minerals in the literature. A compilation of these data is presented here. Keywords: microlite group, “fluorcalciomicrolite”, fluornatromicrolite, crystal-structure refinement, solid solution. SOMMAIRE Nous présentons ici des données acquises avec une microsonde électronique et par diffraction X sur monocristal pour caractériser un minéral du groupe du microlite ayant une formule voisine de NaCaTa2O6F, provenant de la mine Morro A Redondo, Coronel Murta, Minas Gerais, Brésil. Compte tenu de ces données, la formule en serait (Na0.88Ca0.88Pb0.020.22)S2.00 B X Y (Ta1.70Nb0.14Si0.12As0.04)S2.00 [(O5.75(OH)0.25]S6.00 (F0.730.27)S1.00. Selon la nouvelle nomenclature des minéraux du super- groupe du pyrochlore, il s’agit d’une composition intermédiaire entre fluornatromicrolite et “fluorcalciomicrolite”. La structure du cristal, Fd3m, a = 10.4396(12) Å, a été affinée jusqu’à un résidu R1 égal à 0.0258 (wR2 = 0.0715) en utilisant 107 réflexions (rayonnement MoKa). Il y a une pénurie de données cristallochimiques pour les minéraux du supergroupe du pyrochlore dans la littérature. Nous présentons ici une compilation des données disponibles. (Traduit par la Rédaction) Mots-clés: groupe du microlite, “fluorcalciomicrolite”, fluornatromicrolite, affinement de la structure, solution solide. § E-mail address: [email protected] Downloaded from https://pubs.geoscienceworld.org/canmin/article-pdf/49/2/615/3457778/615_vol_49-2_art_14.pdf by USP Universidade de Sao Paulo user on 27 September 2018 616 thE CANADIAN MINERALOGIst INTRODUCTION granitic pegmatites (Lumpkin & Ewing 1992, Ercit et al. 1994, Zurevinski & Mitchell 2004). The pyrochlore Pyrochlore-supergroup minerals have been studied supergroup includes minerals of the pyrochlore, micro- in detail because of their interesting magnetic properties lite, roméite, betafite and elsmoreite groups. and advanced technological applications. The “geomet- There is a scarcity of crystal-chemical data in the rical frustration” concept is related to the incapacity of literature. There are crystal-structure determinations magnetic systems to satisfy simultaneously all the inter- published for only nine pyrochlore-supergroup minerals actions (ferromagnetic and antiferromagnetic) between (Table 1). In this paper, we present results of electron- spins in order to minimize the total energy. The study microprobe and single-crystal diffraction studies of a of frustration among spins in the corner-sharing tetra- microlite-group mineral from the Morro Redondo mine. hedra of pyrochlore-like structures of synthetic origin has contributed to a better understanding of new exotic OCCURRENCE low-temperature states, magnetic ordering (Wang et al. 2006, Normand 2009), and superconductivity, such as in The Morro Redondo pegmatite is located in the RbOs2O6 (Brühwiler et al. 2004). Titanate (CaZrTi2O7) northern part of the Oriental Pegmatite Province of and zirconate (Gd2Zr2O7, Er2Zr2O7) pyrochlores play Minas Gerais (that extends 320 km east to west and 720 an important role in the development of long-term km north to south, reaching the southern part of Bahia geological depositories of high-level radioactive waste and the northwestern part of Espírito Santo state. It such as plutonium (Laverov et al. 2010). Minerals of the was formed during the Brasiliano orogeny (also known pyrochlore supergroup have a formula A2–mB2X6–wY1–n as Araçuaí orogeny), dated from 600 to 450 million (m = 0 to 2, w = 0 to 0.7, n = 0 to 1), in which the A site years ago. The pegmatite is located inside the Coronel is normally occupied by Na, Ca, Ag, Mn, Sr, Ba, Fe2+, Murta pegmatite field, which comprises tens of gem- Pb2+, Sn2+, Sb3+, Bi3+, Y, Ce (and other REE), Sc, U, bearing granitic pegmatites that have been extensively Th, , or H2O. The B site typically accommodates Ta, mined for the production of gem-grade elbaite (green, Nb, Ti, Sb5+, W, and also V5+, Sn4+, Zr, Hf, Fe3+, Mg, blue, red and multi-colored), gem-grade beryl (mostly Al, Si. The X site typically is occupied by O, but can “morganite”), ornamental quartz and perthitic K-feld- include subordinate OH and F. The Y site is occupied spar (raw material for ceramic and glass industries). by OH, F, O, , H2O, K, Cs, Rb. These minerals are Morro Redondo is the largest pegmatite found commonly found in carbonatites, alkaline rocks and in this field: it has a length of 400 meters and an TABLE 1. REFINED CRYSTAL-STRUCTURE DATA: CELL PARAMETER a AND OCCUPANCY OF THE SITES A, B, X AND Y _______________________________________________________________________________________________________________ a (Å) ABX Y Ref. _______________________________________________________________________________________________________________ fluorcalciomicrolite* – 10.4396(12) Na0.88Ca0.88Pb0.02G0.22 Ta1.70Nb0.14Si0.12 O5.75 F0.73G0.27 (1) fluornatromicrolite As0.04 (OH)0.25 intermediate member 3+ hydroxykenomicrolite 10.515(2) G1.09Sb 0.57Bi0.01Na0.31 Ta1.88Nb0.12 O5.69 (OH,F)0.69 (2) (from Vasin–Myl’k Mtn.) Pb0.02 (OH,F)0.31 Cs0.31 3+ hydroxykenomicrolite 10.496(1) G0.93Na0.45Sb 0.39Bi0.02 Ta1.95Nb0.05 O5.78 (OH,F)0.55 (2) (from Tanco, Manitoba) Pb0.14Ca0.06 (OH,F)0.22 Cs0.22K0.01 kenoplumbomicrolite* 10.571(1) Pb1.30Ca0.29G0.30Na0.08 Ta0.82Nb0.62Si0.23 O6 G0.52(OH)0.25 (3) 4+ 3+ U 0.03 Sn0.15Ti0.07Fe 0.10Al0.01 O0.23 fluorcalciopyrochlore* 10.4200(7) Ca0.73Sr0.07Mn0.04Pb0.01 Nb1.72Ta0.02Ti0.16 O5.03 F0.62(OH)0.38 (4) 3+ G0.69Na0.37K0.02Ce0.03 Si0.07Fe 0.03 (OH)0.97 4+ La0.01Nd0.01U 0.02 hydroxycalciopyrochlore* 10.3738(7) Ca0.72Sr0.08Mn0.04Pb0.01 Nb1.48Ta0.04Ti0.16 O4.63 (OH)0.54F0.46 (4) 3+ G0.84Na0.18K0.05Ce0.04 Si0.24Fe 0.08 (OH)1.37 4+ La0.01Nd0.01U 0.02 hydropyrochlore 10.604(1) (H2O)0.99G0.95Sr0.05Ca0.01 Nb1.80Ti0.20 O4.06(OH)0.94 (H2O)0.86K0.14 (5) 5+ fluornatroroméite* 10.265(2) Na1.00Ca0.80Mn0.01G0.19 Sb 2 O5.69(OH)0.31 F0.89(OH)0.05G0.06 (6) 3+ 5+ 3+ hydroxycalcioroméite 10.277(1) Ca1.04Mn0.07Sb 0.65 Sb 0.99Ti0.76Fe 0.19 O6 (OH)0.91G0.09 (7) G0.23Na0.01 Al0.06 3+ 5+ hydroxycalcioroméite 10.311(7) Ca0.91Mn0.06G0.37Sb 0.27 Sb 1.28Ti0.72 O6 OH (8) 3+ Fe 0.19Al0.10Na0.10 4+ oxycalciobetafite* 10.2637(13) Ca1.29U 0.50Na0.18REE0.03 Ti1.09Nb0.79Ta0.01 O6 O0.98F0.02 (9) 3+ Zr0.14Fe 0.04 3+ hydrokenoelsmoreite 10.352(1) G1.33(H2O)0.59Ca0.06Na0.02 W1.46Fe 0.54 O4.70(OH)1.30 (H2O)0.80K0.20 (10) _______________________________________________________________________________________________________________ * Need to be completely described in order to be approved as valid species. References: (1) this work, (2) Ercit et al. (1993), (3) Bindi et al. (2006), (4) Bonazzi et al. (2006), (5) Ercit et al. (1994), (6) Matsubara et al. (1996), (7) Rouse et al. (1998), (8) Zubkova et al. (2000), (9) Cámara et al. (2004), (10) Ercit & Robinson (1994). Downloaded from https://pubs.geoscienceworld.org/canmin/article-pdf/49/2/615/3457778/615_vol_49-2_art_14.pdf by USP Universidade de Sao Paulo user on 27 September 2018 thE stRUCTURE OF A MICROLITE-GROUP MINERAL, MINAS GERAIS 617 average thickness of 30 meters, has a N–S strike and Since then, all attempts at finding more mineralized dips 70°W. It shows a well-developed zoning, with cavities in the deeper parts of the pegmatite were unsuc- a wall zone about 3 m thick, constituted of perthitic cessful. Two subsequent prospecting rotary-drilling feldspar and quartz showing a graphic texture, with campaigns failed to find more mineralized areas, and smaller amounts of dark muscovite plates, schorl and confirmed that the pegmatite has an atypical cross- almandine. The intermediate zone is thick (about 10 section of a parallelogram measuring 30 3 40 meters. m) and is constituted mostly of centimetric microcline After the mining operations were discontinued in and quartz grains, lighter brown centimetric muscovite 1997, intermittent and risky digging has been conducted plates, schorl, albite, montebrasite and beryl crystals. by individual miners, who take the risk of digging The inner zone consists of a discontinuous quartz core, into the safety pillars. One of these miners found the that in some areas can be up to 10 m thick, surrounded microlite crystal that is the object of this study in the by albite-dominant replacement bodies with decimetric vicinity of the cavity of the mined-out tourmaline zone, crystals of schorl.