CLINOMETABORITE, NATURAL β-METABORIC ACID, FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Clinometaborite beta-HBO2, the monoclinic modification of metaboric acid, which has long been known as a synthetic product only, was found as a sublimate in an active medium-temperature intracrater fumarole at La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. It forms aggregates of stout crystals up to 2 mm long in an altered pyroclastic breccia, associated with metaborite, sassolite and adranosite. The unaltered mineral is colorless and translucent, with a vitreous luster; it becomes chalky white after some months of exposure to open air. The crystal habit is prismatic; twinning was not observed. The strongest six lines in the X-ray powder-diffraction pattern [d(obs) in angstrom(I)hkl] are: 3.078(100)201, 4.193(20)111, 6.773(15)001, 2.550(10)(1) over bar 22, 3.224(8)(2) over bar 01, and 2.702(8)031. The unit cell is monoclinic, space group P2(1)/a, with a 7.127(2), b 8.842(3), c 6.773(2) angstrom, beta 93.21(1)degrees, V 426.1(2) angstrom(3), Z = 12, values that virtually coincide with those of the synthetic compound. The calculated density is 2.049 g/cm(3). The structure was refined to a final R of 0.043 for 977 observed reflections [I > 2 sigma(I)]. Both the mineral and the mineral name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA 2010-022).