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Laurentthomasite, Mg K(Be2al)Si12o30

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Laurentthomasite, Mg K(Be2al)Si12o30 Eur. J. Mineral., 32, 355–365, 2020 https://doi.org/10.5194/ejm-32-355-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Laurentthomasite, Mg2K(Be2Al)Si12O30: a new milarite-group-type member from the Ihorombe region, Fianarantsoa Province, Madagascar Cristiano Ferraris1, Isabella Pignatelli2, Fernando Cámara3, Giancarlo Parodi1, Sylvain Pont1, Martin Schreyer4, and Fengxia Wei5 1Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, UMR 7590, Muséum National d’Histoire Naturelle, CP 52, 61 rue Buffon, 75005 Paris, France 2Laboratoire GeoRessources, Université de Lorraine, Faculté des Sciences et Technologies, Rue Jacques Callot, BP 70239, 54506 Vandœuvre-lès-Nancy, France 3Dipartimento di Scienze della Terra “A. Desio”, Università degli Studi di Milano, Via Mangiagalli, 34 – 20133 Milan, Italy 4Malvern Panalytical B.V., Lelyweg 1 (7602 EA), P.O. Box 13, 7600 AA Almelo, the Netherlands 5Institute of Materials Research and Engineering (IMRE), 2 Fusionopolis Way, Innovis, #08-03, 138634 Singapore Correspondence: Cristiano Ferraris ([email protected]) Received: 12 February 2020 – Revised: 6 May 2020 – Accepted: 14 May 2020 – Published: 17 June 2020 Abstract. Laurentthomasite, ideally Mg2K(Be2Al)Si12O30, is a new milarite-group member found within quartz-syenite pegmatites from the Ihorombe region, Fianarantsoa Province, Madagascar. It occurs as euhedral {0001} hexagonal crystals, maximum 15 mm large and 5 mm thick. The crystals show a very strong dichroism with cobalt blue and green-yellow colours when observed along [0001] and [1000], respectively. The mineral is transparent, uniaxial (C) and its lustre is vitreous. The hardness is about 6 (Mohs scale), showing a poor {0001} cleavage, irregular to conchoidal fracture, and a measured density of 2.67(8) g cm−3. Laurentthomasite is hexagonal, space group P 6/mcc (no. 192), with a D 9:95343.6/ Å, c D 14:15583.8/ Å, V D 1214:54.1/ Å3 and Z D 2. The strongest nine lines in the X-ray powder diffraction pattern [d in Å – (I) – hkl] are 3.171 – (10) – 211, 4.064 – (8) – 112, 2.732 – (8) – 204, 4.965 – (6) – 110, 2.732 – (4) – 204, 3.533 – (3) – 004, 7.055 – (2) – 002, 4.302 – (2) – 200 and 3.675 – (2) – 202. Chemical analyses by electron microprobe and several spectro- scopies (inductively coupled plasma, ICP; optical emission, OES; mass, MS; and Mössbauer) give the following C 2 P empirical formula based on 30 anions per formula unit: (Mg0:86 Sc0:54 Fe0:35 Mn0:26/ D2:01(K0:89 Na0:05 Y0:02 C P 3 P Ca0:01 Ba0:01/ D0:98[(Be2:35 Al0:50 Mg0:11 Fe0:03/ D2:99(Si11:90 Al0:10)O30]; the simplified formula is (Mg, Sc)2(K, Na)[(Be, Al, Mg)3(Si, Al)12O30]. The crystal structure was refined to an R index of 1.89 % based on 430 reflections with Io > 2σ .I/ collected on a four-circle diffractometer with CuKα radiation. By comparison with the general formula of the milarite group, A2B2C[T (2)3T (1)12O30](H2O) x (0 < x < n, with n < 2 pfu, per formula unit), the laurentthomasite structure consists of a beryllo-alumino-silicate framework in which the T (1) site is occupied by Si and minor Al and forms [Si12O30] cages linked by the T (2) site mainly occupied by (Be C Al). The A and C sites occur in the interstices of the framework while the B site is vacant. The origin of the strong dichroism is related to a charge transfer process between Fe2C and Fe3C in octahedral A sites and tetrahedral T (2) sites, respectively. Published by Copernicus Publications on behalf of the European mineralogical societies DMG, SEM, SIMP & SFMC. 356 C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 1 Introduction constituent, while another five are of extraterrestrial origin (Mindat, 2020b) The name laurentthomasite honours Laurent Thomas, born Besides economic reasons, Madagascar’s pegmatites are 1971 in Tours (Centre-Val de Loire, France). He has been a amongst the best research fields for mineral collectors be- very active geologist, prospector and mineral dealer since the cause of both the size and the aesthetic characteristics of early 1990s, especially for African areas such as Madagas- many different mineral species. Although there are more than car. He is the one who first brought to the public knowledge 370 valid mineral species from approximately 1000 different some new species from Madagascar such as pezzottaite from localities in Madagascar (Mindat, 2020a), including a rela- Mandrosonoro, as well as new localities such those of grandi- tively large number of new mineral species containing the dierite from Tranomaro, euclase from Itasy and chrysoberyl light elements Li, Be or B (Table 1), no new species belong- from Tsitondroina (see Lefevre and Thomas, 1998; Forner et ing to the milarite group have been described. Milarite and al., 2001). The new mineral species and its name were ap- osumilite have been reported in the central and southern re- proved by the Commission on New Minerals, Nomenclature gions of Amoron’i Mania and Anosy (Pezzotta, 2005; Holder and Classification, International Mineralogical Association et al., 2018), but laurentthomasite, Mg2K(Be2Al)Si12O30, is (IMA 2018–157). the first mineral species belonging to the milarite group for The laurentthomasite description is based on one holo- which Madagascar is the type locality. type and two cotype specimens, which are deposited Laurentthomasite Mg2K(Be2Al)Si12O30 is the anhydrous in the collections of the Muséum National d’Histoire Mg-dominant analogue of milarite Ca2K(Be2Al)Si12O30 and Naturelle (MNHN) of Paris (France); catalogue num- was approved as a new mineral species in April 2019 by the bers are MNHN_MIN_218.1_a for the holotype and Commission on New Minerals, Nomenclature and Classifi- MNHN_MIN_218.1_b and MNHN_MIN_218.1_c for co- cation (CNMNC) of the International Mineralogical Associ- types. ation (IMA) (2018–157) as a member of the milarite group (9.CM.05 in the classification of Strunz and Nickel, 2001). The general formula of minerals belonging 2 Geological settings to the milarite group (Forbes et al., 1972) is VI IX XII IV IV D − A2 B2 C[ T (2)3 T (1)12O30](H2O)x with x 0 n The studied samples were collected by Laurent Thomas (n < 2 pfu – Gagné and Hawthorne, 2016a), with the follow- in the Ihorombe region (Fianarantsoa Province) at about ing known site occupancies: A = Al, Fe3C, Sn4C, Mg, Zr, 80 km north-east of the village of Betroka (23◦1600300 S, 2C ◦ 0 00 Fe , Ca, Na, Y or Sc; B D, Na, K or H2O; C D, Na, K or 46 05 49 E), in southern Madagascar. The area is located Ba; T (1) D Al or Si; and T (2) D Li, Be, B, Mg, Al, Si, Mn, in the north-eastern part of the Ranotsara shear zone in the Fe2C or Zn. Milarite-group minerals are double-ring silicates Antananarivo block, one of the five tectonic units into which having maximum topological symmetry corresponding to Madagascar is divided (Collins et al., 2000; Collins and space group P 6/mcc, although cation ordering may lead to Windley, 2002). The Antananarivo block consists of 2550– lower symmetry. 2500 Myr granitoids tectonically interlayered with granites, Together with the recent description of aluminosugilite syenites and gabbros (Tucker et al., 1999; Kröner et al., (Nagashima et al., 2020), laurentthomasite attests to, once 2000). The whole of the Antananarivo block was ther- more, the great compositional flexibility of the milarite struc- mally and structurally reworked between ∼ 750 and 500 Myr ture type; at the present time 25 members belong to this (Collins and Windley, 2002), with pre-existing rocks being group, with the number constantly increasing from 15 in metamorphosed to granulite facies and with the development 1991 (Hawthorne et al., 1991) up to 23 in 2016 (Gagné and of gneissic fabrics. Magmatism at circa 550 Myr produced Hawthorne, 2016a). granitoids bodies metres to kilometres across that are char- Laurentthomasite captured our attention whilst analysing acteristic of this tectonic unit. Laurentthomasite was found a set of gemstones coming from the south of Madagascar. within a pegmatite (coeval with these granitoid bodies) in Its dichroism going from deep blue to green yellow together a series of biotite- and amphibole-rich gneisses as well as with prismatic hexagonal crystals was a source of perplexity migmatites and a set of pyroxene-rich alkali syenites (Be- for collectors, since it could be confused with both corun- sairie, 1959; Tucker et al., 1999; Kröner et al., 2000). dum sapphire and/or cordierite, mineral species much more Laurentthomasite is associated with orthoclase, massive common in the Madagascan southern provinces. Both sap- quartz, rare pale green apatite crystals, phenakite, beryl, al- phire and cordierite were rejected based on hardness mea- bite, magnetite, thortveitite and cheralite (Fig. 1): at the mi- surements: 6 vs. 7.5 and 9 (Mohs scale), respectively. On the croscopic scale unknown phases (currently under investiga- other hand, the first results of chemical investigations show- tion) containing different ratios of Nb, Ta and W are also ing the presence of elements such as Sc, K and Be stimu- present. Due to the extremely weathered conditions of the lated our interest. Scandium-rich minerals are very rare: only few outcrops, pegmatite samples containing laurentthoma- 15 terrestrial mineral species have scandium as an important site are rare and deeply weathered into laterite; so far only Eur. J. Mineral., 32, 355–365, 2020 https://doi.org/10.5194/ejm-32-355-2020 C. Ferraris et al.: Laurentthomasite, Mg2K(Be2Al)Si12O30 357 Table 1. Approved mineral species containing Li, Be and/or B from Madagascar. Mineral Formula Reference∗ 5C 5C Béhierite (Ta , Nb )(BO4) Fleischer (1962) Bityite LiCaAl2(AlBeSi2O10).OH/2 Lacroix (1908) Fluor-liddicoatite Ca(Li2Al)Al6(Si6O18)(BO3)3.OH/3F Dunn et al.
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