Parawollastonite (wollastonite-2M polytype) from the skarns in Zvezdel pluton, Eastern Rhodopes - a single crystal study

The wollastonite polytype in three samples from different zones of skarns in Zvesdel pluton was determined by single-crystal XRD structure refinement. The studied wollastonite crystals are nearly pure CaSiO 3 but containing a few tenths of FeO wt.% (determined using microprobe analysis). The structure refinement confirmed the monoclinic symmetry of the studied samples (space group P21/a and unit cell parameters a = 15.413(17) A, b = 7.336(5) A, c = 7.070(3) A, β = 95.24(5)° for the crystal from the plagioclase-pyroxene-wollastonite+garnet zone; a = 15.409(7) A, b = 7.320(3) A, c = 7.063(2) A, β = 95.36(3)° – wollastonite from the plagioclase-pyroxene-wollastonite zone, and a = 15.3770(11) A, b = 7.2990(4) A, c = 7.0491(5) A, β = 95.340(6)° – wollastonite from the plagioclase-pyroxenewollastonite+epidote zone). The latter structure was refined to R1 = 0.0521 and R(all) = 0.0754 using 1970 unique reflections. The Si–O bond lengths vary between 1.580(3) and 1.667(3) A (mean 1.624 A), Ca–O bond lengths between 2.251(3) and 2.662(3) A (mean 2.390 A), and the Si–O–Si angles are 140.03(17), 139.39(18), and 150.81(18)° (mean 143.31°). The proved monoclinic wollastonite (wollastonite-2M) is regarded as a higher temperature polytype with respect to the triclinic one. The presence only of the wollastonite-2M polytype in the studied skarns indicates longer duration of the thermal influence at the contact of carbonate xenoliths with monzonite magma or could be a result of an external stress, which is probably the stretching of the crystals due to differences in the thermal expansion coefficient of wol lastonite with respect to the surrounding matrix.