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

Ferraris, Cristiano; Pignatelli, Isabella; Cámara, Fernando; Parodi, Giancarlo; Pont, Sylvain; Schreyer, Martin; Wei, Fengxia

Laurentthomasite, ideally inline-formulaMg2K(inline-formulaBe2Al)inline-formulaSi12inline-formulaO30, 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 (inline-formula+) 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 cminline-formula−3. Laurentthomasite is hexagonal, space group inline-formulaP6/mcc (no. 192), with inline-formulaa=9.95343(6) Å, inline-formulac=14.15583(8) Å, inline-formulaV=1214.54(1) Åinline-formula3 and inline-formulaZ=2. The strongest nine lines in the X-ray powder diffraction pattern [inline-formulad 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: (inline-formulaMg0.86inline-formulaSc0.54inline-formula M24inlinescrollmathml chem normal Fe normal 0.35 normal 2 + 30pt17ptsvg-formulamathimg145cda4999dea87b487f108d9132fef9 ejm-32-355-2020-ie00001.svg30pt17ptejm-32-355-2020-ie00001.png  Mninline-formula M25inlinescrollmathml normal 0.26 ) = normal 2.01 50pt15ptsvg-formulamathimgbfb15aaeafb8ead61c20010fbd16e8d0 ejm-32-355-2020-ie00002.svg50pt15ptejm-32-355-2020-ie00002.png (inline-formulaK0.89inline-formulaNa0.05inline-formulaY0.02inline-formulaCa0.01 Bainline-formula M30inlinescrollmathml normal 0.01 ) = normal 0.98 50pt15ptsvg-formulamathimgf241fb4212c2e68fd93548b4bdd3e263 ejm-32-355-2020-ie00003.svg50pt15ptejm-32-355-2020-ie00003.png [(inline-formulaBe2.35inline-formulaAl0.50inline-formulaMg0.11 Feinline-formula M34inlinescrollmathml normal 0.03 normal 3 + ) = normal 2.99 50pt18ptsvg-formulamathimgd4f52d6e952c3eb02e345673b38d81b6 ejm-32-355-2020-ie00004.svg50pt18ptejm-32-355-2020-ie00004.png (inline-formulaSi11.90inline-formulaAl0.10)inline-formulaO30]; the simplified formula is (Mg, Sc)inline-formula2(K, Na)[(Be, Al, Mg)inline-formula3(Si, Al)inline-formula12inline-formulaO30]. The crystal structure was refined to an inline-formulaR index of 1.89 % based on 430 reflections with inline-formulaIo inline-formula> 2inline-formulaσ(I) collected on a four-circle diffractometer with Cuinline-formulaKα radiation. By comparison with the general formula of the milarite group, inline-formulaA2B2C[inline-formulaT(2)inline-formula3T(1)inline-formula12inline-formulaO30](inline-formulaH2O) inline-formulax (inline-formula M54inlinescrollmathml normal 0 < x < n 46pt10ptsvg-formulamathimgef44310ef0a77d9a56fc910a51b62dd2 ejm-32-355-2020-ie00005.svg46pt10ptejm-32-355-2020-ie00005.png , with inline-formulan<2 pfu, per formula unit), the laurentthomasite structure consists of a beryllo-alumino-silicate framework in which the inline-formulaT(1) site is occupied by Si and minor Al and forms [inline-formulaSi12inline-formulaO30] cages linked by the inline-formulaT(2) site mainly occupied by (Be inline-formula+ Al). The inline-formulaA and inline-formulaC sites occur in the interstices of the framework while the inline-formulaB site is vacant. The origin of the strong dichroism is related to a charge transfer process between inline-formulaFe2+ and inline-formulaFe3+ in octahedral inline-formulaA sites and tetrahedral inline-formulaT(2) sites, respectively.

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Ferraris, Cristiano / Pignatelli, Isabella / Cámara, Fernando / et al: Laurentthomasite, Mg2K(Be2Al)Si12O30: a new milarite-group-type member from the Ihorombe region, Fianarantsoa Province, Madagascar. 2020. Copernicus Publications.

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