On the crystal-chemistry of meyerhofferite, CaB3O3(OH)5·H2O

The crystal structure and crystal chemistry of meyerhofferite, ideally CaB 3 O 3 (OH) 5 ·H 2 O, was investigated by a multi-methodological approach based on titrimetric determination of boron, gravimetric determination of calcium, determination of fluorine by ion selective electrode, determination of water content by heating, other minor elements by inductively coupled plasma atomic emission spectroscopy, along with single-crystal synchrotron X-ray and neutron diffraction. The concentration of more than 50 chemical elements was measured. The combination of these techniques proves that the composition of meyerhofferite approaches the ideal one (i.e., (Ca 1.012 Mg 0.003 ) (B 2.984 Si 0.001 )O 3 (OH) 5 ·1.018H 2 O), with only a modest fraction of Mg (with MgO ≈ 0.03 wt%) replacing Ca, and with Si the only potential substituent of tetrahedral B (with SiO 2 ≈ 0.02 wt%). The content of REE and other minor elements is, overall, not significant, including that of fluorine as a potential OH − substituent (i.e.,