A contribution to the mineralogy of the Larderello geothermal field. X-ray crystallographic studies on borate minerals “bechilite” and “lagonite” and crystal structure determination of ginorite

This work reports the results of mineralogical studies on some borate minerals from the Larderello geothermal field. XRD patterns of “bechilite” and “lagonite” confirmed they actually are respectively admixtures of sassolite and ammonioborite, and of sassolite with minor santite, gypsum and larderellite. Single crystal structural study of ginorite from the type locality, Sasso Pisano (Castelnuovo val di Cecina, Pisa) confirmed its isotypism with strontioginorite. Ginorite is monoclinic, space group P21/a, with unit cell parameters a =12.7673(1) Å, b = 14.3112(11), c = 12.7298(9), β= 101.055(5)° V=2282.8(2) Å3. The refinement of its crystal structure converged to R1 = 0.058 on the basis of 3387 reflections with Fo > 4σ (Fo). Analogously to strontioginorite, ginorite crystal structure can be described in terms of complex “sheets” parallel to (010), made up by borate groups and Ca coordination polyhedra, with interlayer linkage assured by Ca cations and hydrogen bonds. The cell volume contraction of ginorite respect to strontioginorite is related to the shrinkage of Ca coordination polyhedra present in the complex Me-borate sheets.