A single-crystal neutron and X-ray diffraction study of a Li, Be-bearing brittle mica

Abstract

AbstractThe crystal chemistry of a meso-octahedral Li,Be-bearing mica from the Harding pegmatite (Dixon, Taos County, New Mexico, USA) has been investigated by constant-wavelength single-crystal neutron diffraction at 20 K, single-crystal X-ray diffraction at 100 K and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The chemical composition based on ICP-AES analysis leads to the following chemical formula (calculated on the basis of 12 oxygen atoms):Ca(Na0.26K0.04Ca0.69)∑0.99M(Li0.29Mg0.03Fe0.023+Al1.78)∑2.12T(Al1.73Be0.16Si2.11)S4.00O12H2.53. The apparent excess of H is probably due to the fact that the fraction of H2O was assumed by difference to 100 wt.%, and slightly overestimated. On the basis of the previous experimental findings on Li,Be-bearing mica, X-ray (at 100 K) and neutron (at 20 K) structure refinements were performed in the space groupsCcandC2/c. The neutron structure refinement in the space groupCcoffers a view about the (Al,Be,Si)-tetrahedral ordering: the best fit of the refinement was reached with theT1 andT4 sites occupied by (Be + Al) andT2 andT3 fully occupied by Si. This leads to a final population ofT(Al1.88Be0.12Si2.00)∑4.00p.f.u., in reasonable agreement with the chemical analysis. The neutron refinement provides unambigous evidence of the occurrence of Li at theM1 site. The refined fraction of Li at theM1 site ranges between 0.27 and 0.29 a.p.f.u., in excellent agreement with the chemical analysis. The presence of Li, at least at a significant level, at theM2 (andM3) site can be ruled out, as a full site occupancy with the scattering length of Al was obtained. The location of the H sites and the complex hydrogen-bonding scheme are described. A comparison between the structure features of this Li,Be-mica and other brittle micas is carried out.