Devitoite, a new heterophyllosilicate mineral with astrophyllite-like layers from eastern Fresno County, California

Abstract

Devitoite, [Ba_6(PO_4)_2(CO_3)] [Fe^(2+)_7Fe^(3+)_2(Si_4O_(12))_2O_2(OH)_4], is a new mineral species from the Esquire #8 claim along Big Creek in eastern Fresno County, California, U.S.A. It is also found at the nearby Esquire #7 claim and at Trumbull Peak in Mariposa County. The mineral is named for Alfred (Fred) DeVito (1937–2004). Devitoite crystallized very late in a sequence of minerals resulting from fluids interacting with a quartz–sanbornite vein along its margin with the country rock. The mineral occurs in subparallel intergrowths of very thin brown blades, flattened on {001} and elongate and striated parallel to [100]. The mineral has a cream to pale brown streak, a silky luster, a Mohs hardness of approximately 4, and two cleavages: {001} perfect and {010} good. The calculated density is 4.044 g/cm^3. It is optically biaxial (+), α 1.730(3), β 1.735(6), γ 1.755(3); 2V_(calc) = 53.6°; orientation: X ≈ b, Y ≈ c, Z ≈ α; pleochroism: brown, Y >>> X > Z. Normalized electron-microprobe analyses provided: BaO 38.83, CaO 0.50, MgO 0.75, FeO 22.02, Fe_2O_3 6.08, Al_2O_3 1.50, SiO_2 20.97, TiO_2 0.80, P_2O_5 4.84, H_2O 1.67, CO_2 2.04, total 100.00 wt%, with FeO and Fe_2O_3 assignments and H_2O and CO_2 based on the structure. The empirical formula is [(Ba_(5.45) Ca_(0.19))_(∑5.64)(PO_4)_(1.47)O_(0.30)(CO_3)] [(Fe^(2+)_(6.60)Mg_(0.40))_(∑7)(Fe^(3+)_(1.64)Ti^(4+)_(0.22)Al^(3+)0.14)_(∑2)(Si_(7.51)Al_(0.49))_(∑8)O_(26)(OH)_4]. Devitoite is triclinic, Pl, α 5.3437(7), b 11.6726(15), c 14.680(2) Å, α91.337(4), β 96.757(4), y 103.233(4)°, V 884.0(2) Å^3 and Z = 1. The crystal-structure determination (R_1 = 9.56% for 1370 F_o > 4σF) shows the mineral to be a heterophyllosilicate with astrophyllite-type HOH layers in which five-coordinated Fe^(3+) takes the place of Ti^(4+). The interlayer region contains Ba atoms, PO_4 groups and CO_3 groups. The configuration of the Ba and PO_4 in the interlayer region is similar to that found in the structure of yoshimuraite.

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