Inesite, Ca2Mn7Si10O28(OH)2 · 5H2O, from the Hale Creek Mine, Trinity County, California, is triclinic, space group Pl, with cell dimensions a = 8.889(2), b = 9.247(2), c = 11.975(3)A; α = 88.15(2), β = 132.07(2) and γ 96.64(2)°; Z = 1. The crystal structure has been determined through a combination of the three-dimensional Patterson and the symbolic addition methods. The structure has been refined by the method of least squares to an R factor of 0.032 for 4243 reflections.

The crystal structure of inesite consists of two components: (a) a polyhedral band, consisting of a sequence of seven edge-sharing Mn octahedra and two Ca pentagonal bipyramids, connected to two similar sequences on either side by edge-sharing; and (b) double silicate chains with a five-tetrahedral-repeat period, which contain alternating six- and eight-mem-bered rings. These silicate double chains knit the adjacent Ca,Mn polyhedral bands into a three-dimensional framework. The average Ca–O distance is 2.431A. The average Mn–O distances within the four different Mn octahedra are 2.243, 2.217, 2.218 and 2.210A. The three crystallographically independent water molecules serve as apical ligands to the Ca and Mn atoms. The site of one of the water molecules is statistically occupied half the time, accounting for five water molecules in the unit cell. All seven hydrogen atoms are involved in hydrogen bonding. The recipient of a hydrogen bond is either a water molecule or a bridging oxygen atom bonded to two sfficon only. The average Si–O bond lengths within the five different Si tetrahedra are 1.621, 1.626, 1.623, 1.627 and 1.630A. The Si–O–Si angles range from 130.0° to 143.5°, the smaller Si–O–Si angles being associated with longer Si–O bonds and vice versa. The six- and eight-membered silicate rings are nearly planar.

The oriented thermal transformation of inesite to a high-calcium rhodonite at ~800°C involves considerable cation migration and breakage of Si–O bonds and is far from a simple dehydration reaction, as has been postulated.

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