Crystal Structure vs. Vibrational Behavior of Wollastonite-1A from Băița Bihor, Bihor Mountains, Romania

Wollastonite-1A from Băița Bihor occurs in distal calcic skarns developed in the contact zone of a mainly granodioritic batholith, of Upper Cretaceous age, with Mesozoic limestones and dolostones. Wollastonite generally occurs in the inner part of metasomatic columns, in monomineralic skarns or associated with grossular and molybdenite-2H as ore mineral. The physical properties (i.e., refraction indices α = 1.616, β = 1.629, and γ = 1.631, 2Vα = 39° and density Dm = 2.922(3) g/cm3) are typical for a term close to the stoichiometry, which is confirmed by the chemical analysis. The chemical structural formula of the analyzed wollastonite-1A is (Ca1.000Mg0.002Mn0.001Fe0.001)(Al0.004Ti0.001Si0.994)O3, which closely approximates the ideal CaSiO3. The Gladstone–Dale compatibility indices account for an excellent agreement between physical and chemical data. The mineral can be satisfactorily refined as triclinic, space group P1¯, with R1 = 0.0678 and cell parameters a = 7.9233(3) Å, b = 7.3203(3) Å, c = 7.0651(3) Å, α = 90.053(3)°, β = 95.208(3)°, γ = 103.384(3)°. Both the IR and Raman spectra principally reveal bands related to vibrations of bridged and non-bridged oxygens pertaining to SiO4 structural tetrahedra. At Băița Bihor, wollastonite-1A is part of the prograde paragenesis, marked by a peak temperature of 550–600 °C.