Abstract

Carlosturanite is a new rock-forming silicate occurring in a network of veins crosscutting the antigorite serpentinite of Sampeyre in the Monviso ophiolite, Italy. It is light-brown, asbestiform, and the [010] fibers are paralleled by fibrous diopside and chrysotile. The mineral is monoclinic Cm, with a = 36.70, b = 9.41, c = 7.291 Å, β = 101.1°. The strongest lines in the X-ray powder diffraction pattern are: 18.02(25)(200), 7.17(100)(001,201), 3.595(45)(10.00, 002), 3.397(55)(202), 2.562(40)(802), 2.280(35)(14.01,16.01). Similar refractive indices are measured along (1.605) and across (1.600) the fiber axis. Carlosturanite dehydrates upon heating, forming chrysotile and hematite (400°C) and finally forsterite (770°C). The infrared pattern shows absorption bands due to hydroxyl anions and silicate tetrahedra. Chemical data lead to the empirical chemical formula  
(Mg37.77Fe2.172+Ti1.094+Mn0.392+Cr0.123+)Σ41.54(Si22.92Al0.81)Σ23.73H72.51O126

(Dcalc = 2.606, Dobs = 2.63g/cm3) or, ideally and according to a structural model, M21[T12O28(OH)4](OH)30 · H2O (Z = 2). Carlosturanite or carlosturanite-like phases may be expected to develop in serpentinite compositions under low grade matamorphic conditions.

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