Original paper
Lasnierite, (Ca,Sr)(Mg,Fe)2Al(PO4)3, a new phosphate accompanying lazulite from Mt. Ibity, Madagascar: an example of structural characterization from dynamical refinement of precession electron diffraction data on submicrometre sample
Rondeau, Benjamin; Devouard, Bertrand; Jacob, Damien; Roussel, Pascal; Stephant, Nicolas; Boulet, Constance; Mollé, Valentin; Corre, Marianna; Fritsch, Emmanuel; Ferraris, Cristiano; Parodi, Gian Carlo
European Journal of Mineralogy Volume 31 Number 2 (2019), p. 379 - 388
44 references
published: Jun 7, 2019
published online: Nov 28, 2018
manuscript accepted: Oct 11, 2018
final revised version received: Oct 11, 2018
manuscript received: Jul 5, 2018
Abstract
Lasnierite, (Ca,Sr)(Mg,Fe)2Al(PO4)3, is a new phosphate mineral from Mt. Ibity, close to Itremo, central Madagascar. The mineral was found as inclusions accompanying blue lazulite and many other minerals in blue metaquartzite occasionally used as a gemstone or ornamental stone. The crystals were very small and embedded in quartz so that they could not be extracted mechanically, precluding any “classical” X-ray diffraction on either single-crystal or powder sample. Hence, Focused Ion Beam (FIB) lamellae were extracted and the structure was successfully solved and refined using the recently proposed method of precession electron diffraction tomography (PEDT) associated to dynamical refinement. This underlines the efficiency of this new method for defining mineral species on volumes too small to be manipulated by classical means of X-ray diffraction. Accompanying minerals include various phosphates, sulfates, oxides and silicates. Lasnierite occurs as transparent, near-colourless bladed crystals up to 120 × 60 μm in section. Only five crystals have been identified yet in our samples. The empirical formula, based on wavelength dispersive spectrometry (WDS) electron microprobe analyses and calculated on 12 (O+F) atoms per formula unit, is (Ca0.59Sr0.37)∑0.96(Mg1.42Fe0.54)∑1.96Al0.87(P2.99Si0.01)∑3.00(O11.41F0.59)∑12. LA-ICPMS shows that concentrations of Li, Be and B are below the detection limit. The calculated density based on the empirical formula is 3.162 g cm−3. Lasnierite is orthorhombic, space group Pbcn, with unit-cell parameters: a = 6.2771(3) Å, b = 17.684(3) Å, c = 8.1631(4) Å, Z = 4 and V = 906.1(2) Å3. The structure is new. The Raman spectra show characteristic peaks at 99, 122, 278, 418, 510, 569, 590, 621, 661, 985, 1028, 1040, 1063, 1100, 1126 and 1148 cm−1 (most intense bands are italicised). Raman spectra also show the absence of H2O and CO2.
Keywords
lasnierite • new mineral • crystal structure • anhydrous phosphate • calcium magnesium aluminium phosphate • transmission electron microscopy • dynamical refinement • precession electron diffraction tomography