Refinement of the crystal structure of cronstedtite-1T

The crystal structure of cronstedtite-1T was refined in space group P31m, u sing two crystals: a triangular tabular crystal from Herja, Romania and a c onical crystal from Lostwithiel, Cornwall, England. The Herja sample has th e composition of (Fe2.202+Fe0.803+)(Si1.20Fe0.783+Al0.02)O-5(OH)(4) and the Lostwithiel sample has the composition of (Fe(2.32)(2+)Fe(0.68)(3+)Bs)(Si1 .32Fe0.663+Al0.02)O-5(OH)(4). The results of refinements are as follows: a = 5.512(1) Angstrom, c = 7.106(1) Angstrom, R = 3.07%, and 342 independent reflections; and a = 5.503(1), c = 7.104(1) Angstrom, R = 2.24%, and 335 in dependent reflections for the Herja and Lostwithiel samples, respectively. The structure consists of one tetrahedral and one octahedral sheet. There i s one octahedral site, M1, occupied by Fe only, and one tetrahedral site, T 1, occupied by Si and Fe in the ratio of 0.617(8):0.383 (Herja) and 0.699(6 ):0.301 (Lostwithiel). Positions of two hydrogen atoms were determined from a difference map for the Lostwithiel data. The ditrigonalization angle of the tetrahedral sheet is alpha = -11.5 degrees (Herja) and alpha = -11.1 de grees (Lostwithiel), and the structures have a Franzini-layer type of B. Th e crystals studied are affected by +/-b/3 stacking faults which produced sl ight streaking of h - k not equal 3n reflections.