Sinhalite (MgAlB0₄), a new mineral. (With Plate XXVII)

Abstract

The investigation which has led to this description of a new mineral began because of a suggestion by Dr. W. F. Foshag, when on a Visit in 1951 to the Mineral Gallery of the British Museum (Natural History), that a brown cut gemstone exhibited as olivine had perhaps been incorrectly determined. More recently, Dr. Foshag has stated that the idea came from Dr. George Switzer, who as a result of an X-ray powder photograph taken in June 1950 of a similar specimen in the collection of the United States National Museum in Washington concluded that his material was not olivine and was likely to be a new species. The present work would not have been pursued had it been realized at the time that Dr. Switzer intended to continue the study when he had suitable material for analysis.

Crystal Structure of Sinhalite MgAlBO4 under High Pressure

“…These values are consistent with fit of experimental data to a second-order BM EOS, i.e., with B 0 ′ = 4 (fixed), which yields V 0 = 242.7(1) Å 3 and B 0 = 173(1) GPa. Notably, these experimental results compare very well with those obtained from theoretical calculations: V 0 = 244.16(1) Å 3 , B 0 = 167.64(1) GPa, and B 0 ′ = 4.51(1).…”

“…For this reason and taking into account the good agreement found between experimental and theoretical data in (i) lattice parameters and atomic positions at ambient conditions (see Table 1) and (ii) the unit-cell compressibility data, we use data from our ab initio total-energy simulations to study the variation of bond distances and polyhedral compressibilities with pressure. Polyhedral volumes for MgAlBO 4 vary smoothly with pressure (see Figure 5 and Table 2) and give the following bulk moduli: 130 (1), and 409(1) GPa for the M(1-Al), M(2-Mg), and T(B) sites, respectively, by using a second-order BM EOS. Table 2 also summarizes the quadratic elongation and bond angle variance 28 in the different polyhedra of sinhalite at different pressures since these two parameters can provide a rough idea of the distortion and compressibility mechanisms of the different polyhedral units as a function of pressure.…”

“…The Final chemical composition for MgAlBO 4 was obtained by assuming stoichiometric borate and charge balancing against BO 4 5− , as reported elsewhere. 1,13 No traces of other elements apart from Mg, Al, B, and O were found.…”

“…Ca 2+ or Mn 2+ , and also non-silicate compounds such as chrysoberyl, Al2BeO4, or sinhalite, MgAlBO4. The olivine-type M 1 The structure of mineral sinhalite, MgAlBO4, was reported from a natural sample 1,2 , but it was also synthesized under hydrothermal conditions 3 containing B atoms 4 . Both AlO6 and MgO6 octahedra are distorted and the BO4 tetrahedron has a skewed appearance with an abnormally long B -O distance.…”

“…, 130(1) and 409(1) GPa for the M(1-Al), M(2-Mg) and T(B) sites, respectively, by using a 2 nd order BM EOS.Table 2 alsosummarizes the quadratic elongation and bond angle variance28 in the different polyhedra of sinhalite at different pressures, since these two parameters can provide a rough idea of the distortion and compressibility mechanisms of the different polyhedral units as a function of pressure. It can be observed that both parameters decrease slightly with pressure in all polyhedra, in particular at a greater rate for MgO6; thus showing a progressive reduction of the distortion of all polyhedral units with increasing pressure.…”