Abstract
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 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 P6/mcc (no. 192), with a=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 Fe0.352+ Mn0.26)∑=2.01(K0.89 Na0.05 Y0.02 Ca0.01 Ba0.01)∑=0.98[(Be2.35 Al0.50 Mg0.11 Fe0.033+)∑=2.99(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 > 2σ(I) collected on a four-circle diffractometer with CuKα radiation. By comparison with the general formula of the milarite group, A2B2C[T(2)3T(1)12O30](H2O) x (0<x<n, with n<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 (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.
Highlights
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
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-group minerals are double-ring silicates having maximum topological symmetry corresponding to space group P 6/mcc, cation ordering may lead to lower symmetry
Summary
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. 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. 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. 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. 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
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
