Laurentthomasite , Mg 2 K ( Be 2 Al ) Si 12 O 30 : a new milarite-group-type member from the Ihorombe region , Fianarantsoa Province , Madagascar

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 (+) 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 P6/mcc (no. 192), with a=9.95343(6) Å, c=14.15583(8) Å, V=1214.54(1) Å3
and Z=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 spectroscopies (inductively coupled plasma, ICP; optical emission, OES; mass, MS; and Mössbauer) give the
following empirical formula based on 30 anions per formula unit:
(Mg0.86 Sc0.54 Fe0.352+ Mn0.26)∑=2.01(K0.89 Na0.05 Y0.02 Ca0.01 Ba0.01)∑=0.98[(Be2.35 Al0.50 Mg0.11 Fe0.033+)∑=2.99(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 + 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 Fe2+ and Fe3+ in
octahedral A sites and tetrahedral T(2) sites, respectively.



Introduction
Besides economic reasons, Madagascar's pegmatites are amongst the best research fields for mineral collectors because of both the size and the aesthetic characteristics of many different mineral species. Although there are more than 370 valid mineral species from approximately 1000 different localities in Madagascar (Mindat, 2020a), including a relatively large number of new mineral species containing the light elements Li, Be or B (Table 1), no new species belonging to the milarite group have been described. Milarite and osumilite have been reported in the central and southern regions of Amoron'i Mania and Anosy (Pezzotta, 2005;Holder et al., 2018), but laurentthomasite, Mg 2 K(Be 2 Al)Si 12 O 30 , is the first mineral species belonging to the milarite group for which Madagascar is the type locality.
Laurentthomasite Mg 2 K(Be 2 Al)Si 12 O 30 is the anhydrous Mg-dominant analogue of milarite Ca 2 K(Be 2 Al)Si 12 O 30 and was approved as a new mineral species in April 2019 by the Commission on New Minerals, Nomenclature and Classification (CNMNC) of the International Mineralogical Association (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 to the milarite group (Forbes et al., 1972) is x with x = 0 − n (n < 2 pfu - Gagné and Hawthorne, 2016a), with the following known site occupancies: A = Al, Fe 3+ , Sn 4+ , Mg, Zr, Fe 2+ , Ca, Na, Y or Sc; B =, Na, K or H 2 O; C =, Na, K or Ba; T (1) = Al or Si; and T (2) = Li, Be, B, Mg, Al, Si, Mn, Fe 2+ or Zn. Milarite-group minerals are double-ring silicates having maximum topological symmetry corresponding to space group P 6/mcc, although cation ordering may lead to lower symmetry.
Together with the recent description of aluminosugilite (Nagashima et al., 2020), laurentthomasite attests to, once more, the great compositional flexibility of the milarite structure type; at the present time 25 members belong to this group, with the number constantly increasing from 15 in 1991 (Hawthorne et al., 1991) up to 23 in 2016 (Gagné and Hawthorne, 2016a).
Laurentthomasite captured our attention whilst analysing a set of gemstones coming from the south of Madagascar. Its dichroism going from deep blue to green yellow together with prismatic hexagonal crystals was a source of perplexity for collectors, since it could be confused with both corundum sapphire and/or cordierite, mineral species much more common in the Madagascan southern provinces. Both sapphire and cordierite were rejected based on hardness measurements: 6 vs. 7.5 and 9 (Mohs scale), respectively. On the other hand, the first results of chemical investigations showing the presence of elements such as Sc, K and Be stimulated our interest. Scandium-rich minerals are very rare: only 15 terrestrial mineral species have scandium as an important constituent, while another five are of extraterrestrial origin (Mindat, 2020b) The name laurentthomasite honours Laurent Thomas, born 1971 in Tours (Centre-Val de Loire, France). He has been a very active geologist, prospector and mineral dealer since the early 1990s, especially for African areas such as Madagascar. He is the one who first brought to the public knowledge some new species from Madagascar such as pezzottaite from Mandrosonoro, as well as new localities such those of grandidierite from Tranomaro, euclase from Itasy and chrysoberyl from Tsitondroina (see Lefevre and Thomas, 1998;Forner et al., 2001). The new mineral species and its name were approved by the Commission on New Minerals, Nomenclature and Classification, International Mineralogical Association (IMA 2018-157).
The laurentthomasite description is based on one holotype and two cotype specimens, which are deposited in the collections of the Muséum National d'Histoire Naturelle (MNHN) of Paris (France); catalogue numbers are MNHN_MIN_218.1_a for the holotype and MNHN_MIN_218.1_b and MNHN_MIN_218.1_c for cotypes.

Geological settings
The studied samples were collected by Laurent Thomas in the Ihorombe region (Fianarantsoa Province) at about 80 km north-east of the village of Betroka (23 • 16 03 S, 46 • 05 49 E), in southern Madagascar. The area is located in the north-eastern part of the Ranotsara shear zone in the Antananarivo block, one of the five tectonic units into which Madagascar is divided Collins and Windley, 2002). The Antananarivo block consists of 2550-2500 Myr granitoids tectonically interlayered with granites, syenites and gabbros (Tucker et al., 1999;Kröner et al., 2000). The whole of the Antananarivo block was thermally and structurally reworked between ∼ 750 and 500 Myr (Collins and Windley, 2002), with pre-existing rocks being metamorphosed to granulite facies and with the development of gneissic fabrics. Magmatism at circa 550 Myr produced granitoids bodies metres to kilometres across that are characteristic of this tectonic unit. Laurentthomasite was found within a pegmatite (coeval with these granitoid bodies) in a series of biotite-and amphibole-rich gneisses as well as migmatites and a set of pyroxene-rich alkali syenites (Besairie, 1959;Tucker et al., 1999;Kröner et al., 2000).
Laurentthomasite is associated with orthoclase, massive quartz, rare pale green apatite crystals, phenakite, beryl, albite, magnetite, thortveitite and cheralite ( Fig. 1): at the microscopic scale unknown phases (currently under investigation) containing different ratios of Nb, Ta and W are also present. Due to the extremely weathered conditions of the few outcrops, pegmatite samples containing laurentthomasite are rare and deeply weathered into laterite; so far only  one crystal in a matrix has been found, whilst isolated specimens are more common. The genesis of the new mineral laurentthomasite is related to crystallization of Sc-enriched pegmatites like those occurring at the Tørdal pegmatite field in southern Norway (Steffenssen et al., 2019). In contrast to the Tørdal bodies, the Madagascan pegmatite where laurentthomasite was found does not have any garnet that could be a major host for scandium.

Appearance and physical properties
The sample material consists of tabular euhedral {0001} hexagonal crystals with maximum width and thickness values of 15 and 5 mm, respectively ( Fig. 1a-  The origin of dichroism lies in the presence of the transition metal ions Fe 2+ and Fe 3+ , located in octahedral A sites and tetrahedral T (2) sites, respectively, and inter-valence charge transfer between them. Twinning was not observed. The mineral has a light blue streak and is transparent and non-fluorescent. Its lustre is vitreous. The hardness is about 6 (Mohs scale) based on scratching tests, with poor cleavage parallel to {0001}. The tenacity is brittle with irregular to conchoidal fracture. Laurentthomasite has a measured density of 2.67(8) g cm −3 (hydrostatic balance) and a calculated density of 2.66(4) g cm −3 using the empirical formula.

Analytical methods
For elements heavier than carbon, wavelength-dispersive Xray spectroscopy (WDS) 40-point analyses were performed with an electron microprobe CAMECA SX100 (Service d'Analyse Microsonde Camparis -Université Pierre et Marie Curie, Paris), at an accelerating voltage of 15 kV, a probe current of 10 nÅ and a beam diameter of 5 µm. Analysed elements and standards were Si, Mg and Ca (diopside); Al and K (orthoclase); Sc (synthetic Sc 2 O 3 ); Ti and Mn (pyrophanite); Fe (hematite); Zn (sphalerite); Ba (baryte); and Na (albite). The polished section of the analysed crystal was free of inclusions, at least to the depth reached by the electron beam. Both beryllium and trace elements (including rare earth elements, REEs) were analysed using about 0.1 g of powdered sample by inductively coupled plasma optical emission spectrometry (ICP-OES) and mass spectrometry (ICP-MS) at the SARM laboratory of CRPG-CNRS. The amount of Fe 3+ within the structure was deduced from Mössbauer spectroscopy (Fig. 3). The miniaturized Mössbauer spectrometer (MIMOS II) developed by Klingelhöfer et al. (1996) at the LCPME laboratory (Université de Lorraine, France) was used to investigate the oxidation state of iron in laurentthomasite samples.
Raman spectra (Fig. 4) were obtained using a modified Princeton Instruments spectrometer at the IMPMC (Institut de minéralogie, de physique des matériaux et de cosmochimie, Université Pierre et Marie Curie, Paris, France). The instrument is synchronized with a nanosecond pulsed diode pumped solid state (DPSS) laser operating at 532 nm with a 1.5 ns duration for the pulse, a 10 to 2000 Hz repetition rate and up to 1 mJ output energy per pulse. The spectrometer was synchronized and optimized to collect the signal only during the laser pulse in order to minimize the luminescence background. Powder diffraction X-ray data were collected between 4 and 100 • (CuKα1-2-circle θ-θ goniometer) using a Bragg-Brentano X'pert Pro MPD diffractometer at the IMPMC.
A single-crystal (39.6 µm × 28.7 µm × 16.9 µm) was analysed using an X-ray diffraction four-circle Agilent Super-Nova instrument at the Institute of Materials Research and Engineering of Singapore (CuKα radiation); the data were collected in the 2θ range 0.279 to 113.785 • at 10 s exposure for each frame (0.029 • width) and integrated by the software CrysAlis Pro ; empirical absorption correction was applied. Tables 2 and 3 show the chemical composition of laurentthomasite. The deconvoluted experimental Mössbauer spectrum shows the presence of two types of octahedral Fe 2+ : the blue and the green curves belong to the laurentthomasite and the inclusions mentioned above, respectively (Fig. 3). As shown in Table 4 the Fe 2+ and Fe 3+ relative contents of laurentthomasite are 92.0 % and 5.8 %, respectively. Thus, after normalizing to 100 %, Fe 2+ constitutes 94 % of the Fe in laurentthomasite itself. The violet doublet represents the tetrahedral Fe 3+ contained within laurentthomasite; the centre shifts (CS), quadrupole splitting (QS) and relative areas are reported in Table 4.

Chemical analyses
In Fig. 4 the Raman spectrum of laurentthomasite is compared to those of milarite and osumilite (Lafuente et al., 2015). The spectra are very similar in having the most intense peak around 480 cm −1 and comparable to the known spectra of other milarite-group minerals like brannockite, chayesite, or poudretteite (Lafuente et al., 2015) or friedrichbeckeite, and almarudite (Lengauer et al., 2009). Differences in Raman shifts of individual bands are due to the chemical variations, bond geometries and bond strengths for different mineral species. Figure 4 clearly shows that the laurentthomasite spectrum shares more affinities with the osumilite than with milarite; the most pronounced difference between the three spectra is the splitting of bands around 280 cm −1 , with an evident separation into two components with maxima at 263 and 288 cm −1 for laurentthomasite. The

Crystallographic data
The cell parameters obtained from Rietveld refinement of the X-ray powder diffraction data (HighScore suite program, Degen et al., 2014) (Fig. 5; Table 5) are a = 9.95343(6) Å, c = 14.15583(8) Å, V = 1214.54(1) Å 3 and Z = 2. Parameters used for the Rietveld refinement were (a) the 0 shift fixed at 0, (b) the background modelled using a flat and a 1/X background as well as five parameters of a Chebyshev function, (c) isotropic displacement parameters for each atomic positions, (d) atomic occupancies for the non-oxygen positions, (e) the peak shape modelled using a pseudo-Voigt func- tion with Finger-Cox-Jephcoat (FCJ) asymmetry (Finger et al., 1994) rather than a fundamental parameter approach so the microstructural parameters like size and strain were not refined, and (f) the preferred orientation was not refined.
Single crystal X-ray diffraction investigations show that laurentthomasite belongs to the hexagonal crystal system with space group P 6/mcc (no. 192) and unit cell parameters a = 9.95800(7) Å, c = 14.114916(11) Å, V = 1215.081(15) Å 3 and Z = 2 ( Table 6). The structure of laurentthomasite was refined using SHELXL-2012 (Sheldrick,  2015) starting from the atom coordinates of oftedalite (Cooper et al., 2006). Scattering curves of fully ionized species were used at cation sites (Rossi et al., 1983;Hawthorne et al., 1995): for the T (1) site ionized Si was refined vs. neutral Si; according to information from chemical analyses and also the fact that Fe and Mn have very similar scattering factors, Fe 2+ + Mn 2+ (0.61 apfu, atoms per formula unit) was constrained to be equal to Sc 3+ (0.54 apfu) and then refined versus Mg 2+ at the A site. Neutral vs. ionized scattering curves were used at oxygen sites (Rossi et al., 1983;Hawthorne et al., 1995). The Fourier difference map did not reveal any maximum above 0.29 e − Å 3 . Anisotropic full-matrix least-squares refinement on F 2 o yielded R 1 = 1.89 % [430 reflections with I o > 2σ I ] and R all = 1.89 % (431 reflections). Experimental details are reported in Table 6. Refined atom coordinates and equivalent isotropic displacement parameters are reported in Table 7. Selected interatomic distances and bond angles are given in Table 8. A Crystallographic Information File (CIF) and list of observed and calculated structure factors are available as electronic material. The structure is isotypic with minerals of the milarite group (Gagné and Hawthorne, 2016a).

Discussion
As discussed by Gagné and Hawthorne (2016a), the structure of the milarite-group minerals is a beryllo-aluminosilicate framework consisting of a four-connected threedimensional net.
The majority of the minerals belonging to the milarite group can be described with space-group symmetry P 6/mcc (Gagné and Hawthorne, 2016a), which is reduced to P 62c and Pnc2 because of cation ordering in respectively roedderite and armenite (Armbruster, 1989(Armbruster, , 1999. The main feature of the laurentthomasite structure is a [T (1) 12 O 30 ] unit consisting of double six-membered rings of corner-linked SiO 4 tetrahedra (Fig. 6). These rings are stacked along the c axis, thus establishing a channel system along [0001]. These units are interconnected by sharing corners with T (2)O 4 tetrahedra. These obtained structural blocks are arranged in groups of three around one centrally located AO 6 octahedron. The C site is situated in the centre of the channel between two [T (1) 12 O 30 ] units (Fig. 7).
The A site is a strongly distorted octahedron as shown by the octahedral angle variance (OAV = 166.92 • , Table 8); the observed < A-O > distance of 2.136(1) Å (Table 8) is slightly longer than predicted for a pure [6] Mg-O bond length (2.089 Å) (Gagné and Hawthorne, 2016b). Considering the A site population obtained by chemical analyses ( A 2+ = 1.47 apfu) and taking into account the quite significant presence of Sc (0.54 apfu), one could justify the derived longer value, given the [6] Sc-O bond length (2.121 Å: Cong et al., 2010; bixbyite-type Sc 2 O 3 ). Nevertheless, the presence of 0.61 apfu of Fe 2+ and Mn 2+ (0.35 Fe 2+ 0.26 Mn) also contributes to the lengthening of the < A-O > mean bond distance, in closer agreement with the observed value. However, this is not yet enough because the calculated mean bond length is 2.123(1) Å (using the ionic radii of Shannon, 1976) ( Table 8). The A site is therefore larger than expected; this is probably due to (i) the A octahedron sharing three edges with three T (2) tetrahedra as well as to (b) an increased charge at the T (2) site due to some replacement of Be by Al (and minor Fe 3+ ). All these features induce an increase of the A-O(3) bond distance. Incidentally, the refined ionization of O3 shows almost completely ionized oxygen at that site (92 %), compared with ionization refined at O1 and O2 sites (ca. 65 %, Table 7). This should be ascribed to the strong incidence of charge at the O3 site, which is threefold coordinated with one T (2) site, one T (1) site and one A site, whereas the O1 site is twofold bonded just to two Si 4+ at the T (1) site, and the O2 site is threefold bonded to two Si 4+ at the T (1) site and has a long bond to K + at the C sites. It is therefore more plausible to have a more covalent bond at the O1 and O2 sites, in full agreement with refinement. The refined scattering (37.12 epfu - Table 7) of the A site is in very good agreement with the chemical analysis (37.2 epfu). The Fourier difference map clearly indicates that the B sites are vacant in laurentthomasite.
The milarite group has yet again demonstrated an extraordinary versatility in its crystal chemistry.
Data availability. The data used in this paper can be found in the Supplement.