Laurentthomasite, Mg2K(Be2Al)Si12O30: 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 15mm large and 5mm 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 P6/mcc (no. 192), with a D 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 Fe2C 0:35 Mn0:26/P = 2.01(K0:89 Na0:05 Y0:02 Ca0:01 Ba0:01/P D0:98[(Be2:35 Al0:50 Mg0:11 Fe3C 0:03/P 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>2.I/collected on a four-circle diffractometer with CuK radiation. By comparison with the general formula of the milarite group, A2B2C[T (2)3T (1)12O30](H2O) x (0 < x < 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 (BeCAl). 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.