The composition and structure of a new phase crystallizing in high-pressure experiments in the system CaO-Ca(OH)2-CaF2-SiO2 are described. The composition is on the join Ca2SiO3(OH)2-Ca2SiO3F2, with the relative amounts of OH and F near 50/50 from electron probe microanalysis, or near the ideal midpoint composition Ca2SiO3OHF. The symmetry is monoclinic, space group P21/c, with a = 5.8111(7), b = 10.6050(13), and c = 6.6968(8) Å, and β = 102.025(3)°, V = 403.65(8) Å3, Z = 4. In the structure isolated SiO3OH tetrahedral groups occupy narrow linear channels whose axes lie along a. The channels are lined by two types of eight-coordinated CaO5OHF2 polyhedra that share edges and faces. The axes of the channels lie on a nearly hexagonal array in the b-c plane, leading to a cp/b ratio of 0.618, close to the ideal ratio of 0.577 for a hexagonal array. However, there is no simple packing rule for either the cation or anion array. Although the channels are arranged in a nearly hexagonal fashion, the overall symmetry of the structure is not hexagonal, but instead has 5-rings of calcium polyhedra around the channels. The calcium atoms lie on a 5-3-5-3 net of triangles and pentagons that can be compared to the α-U3O8 net.

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