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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.

A single garnet crystal of kimzeyite from Anguillara Sabazia, Sabatini Volcanic District (Rome), has been studied by electron probe microanalysis (EPMA), single crystal X-ray diffraction (XRD) and Raman spectroscopy (RS). X-ray intensities for the structure refinement were collected by means of both a conventional four circle diffractometer (CD) and an area detector (AD) diffractometer. Salient crystallographic data from AD measurements are: a = 12.397(4) A, V = 1905.24 A 3 , μ= 25 cm −1 , crystal dimension ∼ 40 μm, D x =3.83 g cm −3 . The chemical analyses allowed to ascertain that Ca, Ba, Fe, Al, Mg, REE, Zr, Si and Ti are present in the structure. The transition element distribution is: Fe 3+ (Y) ≪ Fe 3+ (Z), and Ti as Ti 4+ occurs only at Y. No hydrogen atoms were detected. The problem of the correct distribution of Ba and REE (essentially Eu and Gd) is dealt with, but not fully solved in the present study. Because of the very small amount of sample available and of the strong dilution of Ba and REE (∼0.03 and 0.02 atoms per formula unit respectively) the correct distribution of these atoms over the X and Y sites is problematical. The radius ratio rule and electrostatic requirements suggest Ba at X and REE at Y. From these principles, the following site distribution can be deduced: Ca 2+ and Ba 2+ at X; Mg 2 +, REE 3+ , Zr 4+ , Ti 4+ , Fe 3+ at Y; Si 4+ , Al 3+ , Fe 3+ at Z. On the other hand, structural details and the best fit between EPMA and XRD site populations indicate REE at X and Ba at Y. Bond strength theory arguments support this hypothesis. The main substitution mechanisms can be summed up as follows: 1) Ti 4+ (Y) + Fe 3+ (Z) ↔ Si 4+ (Z) + M 3+ (Y) [schorlomite substitution]; 2) Zr 4+ (Y) + M 3+ (Z) ↔ Si 4+ (Z) + M 3+ (Y) [kimzeyite substitution]; 3) Mg 2+ (Y) + Zr 4+ (Y) ↔ 2M 3+ (Y) [morimotoite-like substitution], where M 3+ = (Al, Fe). In addition, following the bond strength theory indications, another mechanism which possibly involves the X and Y sites is also taken into account: 4) REE 3+ (X) + Ba 2+ (Y) ↔ Ca 2+ (X) + M 3+ (Y).