The crystal structures of the humite minerals. IV. Clinohumite and titanoclinohumite

Abstract

The crystal structures of clinohumite [Mgs.rzFeo.ooMno.oeTio.ozCao.oor(SiOe)rFr.orHo.gaoo,osi a:4.jMt(2);b:10. 2501(5);c:t3.663 5(3)A; d:100.786(2);P2 t/blandtitanocli nohumite [Mgz a:Fer orMno o;Tio rrcao oor(Sior)aoHr.oroo.goi a : 4.7451(8); b : 10.288(2); c : 13.709(3)A; a = 101.00(2).; P2tlblhave been refined by three-dimensional least-squares methods to R : 0.051 and R = 0.042, respectively. Their structures are based on a hexagonal close-packed array of anions and are similar to those of other minerals in the humite series and to the olivines. The serrated chain of edge-sharing octahedra in clinohumite consists of five unique octahedra; of these' M(l)c and M(Z)aarelike those in the olivines; M(l)N, M(2)6 and M(3) are like those in humite. One-half the octahedral sites and one-ninth the tetrahedral sites are filled. The distribution of transition metal ions in clinohumite is consistent with that in humite and chondrodite, Fe2+ being ordered into the more distorted MOo olivine-like portion of the structure. However. the distribution in titanoclinohumite is not consistent with any of the humite minerals previously studied. The distribution of Ti appears to be random and the M(3) site, with one (OH) iigand, appears to be slightly enriched in Fe, the balance of the Fe being randomly distributed amongst the octahedral sites.