Hydroxylclinohumite, a new member of the humite group: Twinning, crystal structure and crystal chemistry of the clinohumite subgroup

Hydroxylclinohumite Mg-9[SiO4](4)(OH, F)(2), a new mineral from Zelentsovskava mine near Magnitka (Zlatoust district, Southwestern Urals), is the OH-dominant equivalent of clinohumite Mg-9[SiO4](4)(F,OH)(2). The crystal structure of hydroxylclinohumite has been refined from single-crystal X-ray data to R = 0.026 for the 1912 observed unique reflections [F-o > 4 sigma(F-o): a = 4.7480(3) Angstrom, b = 10.2730(7) Angstrom, c = 13.6894(2) Angstrom, alpha = 100.72(1)degrees, V = 656.1(1) Angstrom(3), P2(1)/b (No. 14), Z = 2]. Twinning on {001}, which apparently doubles the c axis, was detected and a ratio of 0.94:0.06 between the volumes of the two twin components has been obtained by least-squares refinement. The crystal structure of hydroxylclinohumite corresponds to that of clinohumite except for the positions of the hydrogen atoms. The crystal chemistry of the clinohumite subgroup is discussed and a general formula M-8(2+) (M2+, M4+) (SiO4)(4)(OH, F, O)(2) (M2+ and M4+ mainly Mg and Ti, respectively) is proposed for it.