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 Mg2 K( Be2 Al) Si12 O30 , 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 P 6/mcc (no. 192), with a=9.95343(6)  A, c=14.15583(8)  A, V=1214.54(1)  A 3 and Z=2 . The strongest nine lines in the X-ray powder diffraction pattern [ d in A – (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 Mossbauer) give the following empirical formula based on 30 anions per formula unit: ( Mg0.86 Sc0.54 Fe 0.35 2 +  Mn 0.26 ) ∑ = 2.01 ( K0.89 Na0.05 Y0.02 Ca0.01 Ba 0.01 ) ∑ = 0.98 [( Be2.35 Al0.50 Mg0.11 Fe 0.03 3 + ) ∑ = 2.99 ( Si11.90 Al0.10 ) O30 ]; the simplified formula is (Mg, Sc) 2 (K, Na)[(Be, Al, Mg) 3 (Si, Al) 12 O30 ]. The crystal structure was refined to an R index of 1.89 % based on 430 reflections with I o  >  2 σ(I) collected on a four-circle diffractometer with Cu Kα radiation. By comparison with the general formula of the milarite group, A2B2C [ T (2) 3T (1) 12 O30 ]( H2O ) x ( 0 x n , with n  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 [ Si12 O30 ] 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.