Contribution to the mineralogy of wollastonite from the contact aureole near Xanthi and Kimmeria (N. Greece)

The studied area belongs to the Rhodope Massif and is located approximately 1km north of town of Xanthi, where a plutonic body of mainly granodioritic composition and Oligocene age intrudes into marble. It is generally medium-grained, composed mainly of plagioclase, orthoclase, quartz, biotite and hornblende.
   Three samples of the skarn formation were collected; two from the aureole near Xanthi (samples WXT1 and WXT2) and one from the aureole near Kimmeria (sample WXB1).Thin sections of the samples were prepared in order to determine their mineralogical and textural characteristics. Furthermore, X-ray powder diffraction (XRPD) study was performed using a Philips PW1710 diffractometer with Ni-filtered CuKa radiation. Representative quantity of the samples was treated chemically. In this way the organic matter, fine carbonates and iron oxides (COI) were removed. The abundance and semi-quantitative estimates of the mineral phases present was determined from the untreated samples, whereas the form of the wollastonite present along with its unit cell properties were established from the treated ones (31 lines each). Finally, chemical analyses of the wollastonite were carried out using a JEOL JSM-840A Scanning Electron Microscope (SEM) equipped with attached Energy Dispersive Spectrometer.
   All the samples are in general of massive fabric, forming fibrous aggregates with no distinct spatial orientation. Samples WXT1 and WXT2 are coarser grained compared to WXB1. The samples from Xanthi reveal elongated crystals of wollastonite with fractures parallel to the secondary cleavage (001), mainly filled with fine micritic calcite. The sample from Kimmeria reveals finer and elongated crystals of wollastonite, as a sample being also richer in calcite in aggregate form.
   The samples are mostly composed of wollastonite (73-80%), along with considerable amounts of calcite (3-13%). Andradite is found in considerable amounts (up to 18%) only in the samples from Xanthi, while quartz is present only in Kimmeria samples. Clinopyroxene is not always found, as well as feldspars. The COI amount is greater in Kimmeria samples, showing an increasing tendency with the calcite content present in the samples.
   From the unit cell data obtained from the chemically treated samples it is shown that all the samples are of triclinic structure, being in general very close to the wollastonite nominal structure. These from Xanthi show generally a slightly lower (a) angle. The wollastonite from Kimmeria demonstrates a more complex chemical content, which does not affect its crystal structure. The mean chemical formula of wollastonite in sample WXT1 is (Ca_5.895Mn_0.100Fe_0.017)Si_5.994O₁₈, in sample WXT2 is (Ca_5.846Mn_0.152)Si_6.001O₁₈ and in WXB1 is (Ca_5.833Mn_0.120Mg_0.045)Si_6.001O₁₈. The wollastonite from Kimmeria incorporates Mg²⁺ in its structure, whereas wollastonite from Xanthi Fe²⁺. Both demonstrate substitution of Mn²⁺ for Ca²⁺. The absence of vesuvianite and plagioclase, along with the presence of clinopyroxene, garnet, minor calcite and traces of quartz, indicates 0.05<XCO2<0.2 and temperature range of approximately 650-700ºC at 3 Kbar (corresponding to 10-20 km depth). This also implies a volumetric H₂O wollastonite ratio of greater than 7:1. The skarn formation was not the same around the granodiorite, with reaction CaCO₃+SiO₂↔CaSiO₃+CO₂ reaching almost completion to its western margin, rather than its northern one, possibly due to insufficient amount of time and the type of marble permeability. The magmatic fluids interacting with the marble wall rock were gradually depleted in silica content and subsequently enriched in Al, Fe and Mg, forming andradite garnet and clinopyroxene.