Crystal chemistry of the serandite-pectolite series and related minerals

The serandite structure has been refined to R = 3.0 percent, using a crystal from Tanohata mine, Japan (approximately 39.9°N, 141.9°E), P1, a 7.683(1), b 6.889(1), c 6.747(1) Å, α 90.53(5)°, β 94.12(2)°, γ 102.75(2)°, Z = 2 × [(Mn_1.88Ca_0.17Mg_0.01)Na_1.00HSi_2.97O₉]. The noteworthy features of the structure of serandite which is isotypic to pectolite include: (1) Almost all Si–O and all Na–O lengths are smaller than the corresponding lengths in pectolite; the mean values are 1.623(2)[1.630(2)] Å and 2.485(1)[2.580(1)] Å respectively (values in brackets are those for pectolite). (2) A comparison of the serandite and pectolite structures suggests that the shrinkage of octahedral bands due to substitution of Mn for Ca gives rise to distortions of individual silicate tetrahedra rather than a change in configuration of silicate chains. (3) Ca atoms are preferably located at the M(1) position which corresponds to Ca(1) in the pectolite structure; the occupany at M(1) is 0.84(1) Mn and 0.16(1)Ca.

The specific mode of cation ordering has been interpreted based on the following geometrical considerations: (1) the relations between the configuration of Si₃O₉ chains and the size of cation octahedra; and (2) the location of silicate chains relative to octahedral bands. Rules brought out from these considerations may be used to explain the modes of cation ordering in the wollastonite-group minerals in general.