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, Mg2K(Be2Al)Si12O30, is the first mineral species belonging to the milarite group for which Madagascar is the type locality.

Laurentthomasite Mg2K(Be2Al)Si12O30 is the anhydrous Mg-dominant analogue of milarite Ca2K(Be2Al)Si12O30 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 VIA2IXB2XIIC[IVT(2)3IVT(1)12O30](H2O)x with x=0-n (n<2 pfu – Gagné and Hawthorne, 2016a), with the following known site occupancies: A = Al, Fe3+, Sn4+, Mg, Zr, Fe2+, Ca, Na, Y or Sc; B=, Na, K or H2O; C=, Na, K or Ba; T(1) = Al or Si; and T(2) = Li, Be, B, Mg, Al, Si, Mn, Fe2+ or Zn. Milarite-group minerals are double-ring silicates having maximum topological symmetry corresponding to space group P6/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.

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 et al., 2000; 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.

The sample material consists of tabular euhedral {0001} hexagonal crystals with maximum width and thickness values of 15 and 5 mm, respectively (Fig. 1a–b); all observed laurentthomasite crystals are characterized by dark micrometric inclusions of elongated crystals of an Fe2+-rich phase as confirmed by Mössbauer spectroscopy. Observed forms are {1010} and {0001}. Crystals show a very strong dichroism with cobalt blue and green-yellow colours when observed along [0001] and [1000], respectively (Fig. 2a–b). The origin of dichroism lies in the presence of the transition metal ions Fe2+ and Fe3+, 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.

In our 23 µm thick thin section, the mineral is colourless and pleochroism is not visible; it is optically uniaxial (+), with ω=1.540(2) and ε=1.545(3) (λ=589 nm) measured by refractometry; and dispersion was not observed. The compatibility index (1 – KP/KC)=0.028 is rated as excellent (Mandarino, 1981).

For elements heavier than carbon, wavelength-dispersive X-ray 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 Sc2O3); 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 Fe3+ 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 (39.6µm×28.7µm×16.9µm) was analysed using an X-ray diffraction four-circle Agilent SuperNova 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 CrysAlisPro; empirical absorption correction was applied.

As discussed by Gagné and Hawthorne (2016a), the structure of the milarite-group minerals is a beryllo-alumino-silicate framework consisting of a four-connected three-dimensional net.

The majority of the minerals belonging to the milarite group can be described with space-group symmetry P6/mcc (Gagné and Hawthorne, 2016a), which is reduced to P62c and Pnc2 because of cation ordering in respectively roedderite and armenite (Armbruster, 1989, 1999).

The main feature of the laurentthomasite structure is a [T(1)12O30] unit consisting of double six-membered rings of corner-linked SiO4 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)O4 tetrahedra. These obtained structural blocks are arranged in groups of three around one centrally located AO6 octahedron. The C site is situated in the centre of the channel between two [T(1)12O30] units (Fig. 7).

Laurentthomasite presents the charge arrangement [10] of Gagné and Hawthorne (2016b). It is related to oftedalite (ScCaKBe3Si12O30; Cooper et al., 2006) by the (AScT(2)Be)-1(AMgT(2)Al) and ACa-1AMg substitutions, to milarite (Ca2KBe2AlSi12O30) by the ACa-2AMg2 substitution, and to friedrichbeckeite (Mg2NaKBe3Si12O30; Lengauer et al., 2009) by the (BNaT(2)Be)-1(B□T(2)Al) substitution.

Hawthorne et al. (1991) observed <T-O> distances of 1.609–1.612 Å for several milarite crystals with T(1)= (Si11.89-Si12): mean value of 1.611 Å. Anyway, the observed <T(1)–O > distance, 1.608(2) Å (Table 8), is compatible with the full occupancy of Si at the T(1) site, and it compares well, within experimental error, with the distances observed in other T(1)Si members of the milarite group, as for instance 1.609 Å in sugilite (Armbruster and Oberhänsli, 1988) or 1.610 Å in aluminosugilite (Nagashima et al., 2020). Therefore, even if the empirical formula of laurentthomasite shows Si = 11.90 apfu vs. Al = 0.10 apfu, the scattering curve of Al was not taken into account during refinement. The tetrahedral quadratic elongation value (TQE) (Table 8), as defined in Robinson et al. (1971), shows that the T(1) site is rather regular (7.39∘2), whereas the T(2) sites is much more distorted [see the tetrahedral angle variance (TAV = 196.13∘2)], in agreement with a mixed occupancy site.

The observed <T(2)–O > distance of 1.674(1) Å (Table 8) is slightly larger than the predicted value of 1.637 Å (Gagné and Hawthorne, 2016b) for a pure [4]Be-O bond length. As, according to the chemical analysis, the T(2) occupancy, besides the presence of Be2+ (2.35 apfu) and Al (0.50 apfu), also shows the presence of Mg (0.11 apfu) and a very small amount of Fe3+ determined by Mössbauer spectroscopy. Considering thus a [4]Mg-O of 1.939 Å (Gagné and Hawthorne, 2016b), and the larger ionic radius of Al3+ with respect to Be2+, one can easily justify the slightly higher value (1.674 vs. 1.637 Å). The refined occupancy for Be is 0.74(1) atoms per site (i.e. 2.22 apfu) (Table 7), which is comparable to the one resulting from chemical analyses (2.35 apfu), and the refined occupancy of Al (inclusive of Mg and Fe), at T(2) of 0.26(1) (i.e. 0.78 apfu). The refined site scattering at T(2) is therefore 18.82 epfu (electrons per formula unit), and compared to the scattering calculated from chemistry (18.0 epfu), this indicates that, in the crystal studied by single crystal X-ray diffraction, Mg2+ at T(2) might be lower (or even [4]Fe3+ higher up to 0.06 apfu). Better agreement could be obtained with only Al and Be at T(2), although this is not supported by chemical analyses nor supported by the mean bond length at T(2). Allocation of 0.03 apfu Fe3+ at this site seems to be in agreement with the larger size observed. On the basis of the assumed mixed occupancy of T(2), the calculated value [using bond-distances from Shannon, 1976 and Gagnè and Hawthorne, 2016a] is 1.665 Å, in reasonable agreement with the observed value of 1.674 Å.

The refinement yields a C site fully occupied by K (19.1 epfu), in good agreement with the chemical analysis that shows 0.89 K + 0.05 Na + 0.02 Y + 0.01 Ca + 0.01 Ba = 0.98 apfu, corresponding to a calculated site scattering of 19.0 epfu. The <C–O > distance, 3.086(1) Å (Table 8), is in good agreement with the [12]K–O bond length (3.095 Å) predicted by Gagné and Hawthorne (2016b) of 3.095 Å, considering the presence of about 10 % smaller cations (Na, Ca and Ba).

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 (∑A2+=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 Sc2O3). Nevertheless, the presence of 0.61 apfu of Fe2+ and Mn2+ (0.35 Fe2+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 Fe3+). 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 Si4+ at the T(1) site, and the O2 site is threefold bonded to two Si4+ 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.