Crystal structure and Mossbauer spectroscopy of the synthetic amphibole potassic-ferri-ferrorichterite at 298 K and low temperatures (80-110 K)

The crystal structure of synthetic potassic ferri-ferrorichterite, ideally K(Ca (sub 0.5) Na (sub 1.5) )Fe (sub 5) Si (sub 8) O (sub 22) (OH) (sub 2) , with a 10.145, b 18.184, c 5.296 Aa, beta 104.42 degrees , V 946.3 Aa (super 3) , Z = 2, space group C2/m, was refined for 298 and 110 K; there is no crystallographic phase transition down to 110 K. The two non-equivalent T sites and the octahedral M1 and M3 sites behave very rigidly on cooling. Bond lengths for the distorted M sites slightly decrease as T is lowered. The tetrahedral chain linking increases by 0.9 degrees between 298 and 110 K. Moessbauer spectra collected at 298 K yield rather high Fe (super 3+) values, up to 2.0 pfu; Fe (super 3+) prefers the M2 site. Three components detected in the ferrous quadrupole splitting distribution of the C-type sites were assigned to the M1, M2 and M3 sites, consistent with polyhedral distortion, and give the correct site occupancies.