Abstract

Dellaventuraite is a new amphibole species from the Kajlidongri manganese mine, Jhabua District, Madhya Pradesh, India. It occurs with leakeite, kornite, albite, braunite, and bixbyite associated with cross-cutting epigenetic veins that have reacted with regionally metamorphosed rocks containing Mn-rich minerals (braunite, bixbyite, jacobsite, spessartine) to produce Mn-rich amphiboles, Mn-rich pyroxenes, Mn-rich mica, piemontite, and manganophyllite. Dellaventuraite occurs as anhedral grains, the color of which varies from pink to red, depending on Mn content. It is brittle, H = 5, Dcalc = 3.184 g/cm3, has a pale pink streak, vitreous luster, and does not fluoresce in ultraviolet light; it has perfect cleavage on {110} and conchoidal fracture. In transmitted plane-polarized light, dellaventuraite is strongly pleochroic, X = pale mauve-brown, Y ~ Z = dark red-brown; Y ^ a = 20° (in β obtuse), Z = b, with absorption X < Y ~ Z. It is biaxial positive, ηα = 1.688 ± 0.003, ηβ = 1.692 ± 0.005, ηγ = 1.721 ± 0.003, 2V(obs) = 49 ± 3°, 2V(calc) = 41°. Dellaventuraite is monoclinic, space group C2/m, a = 9.808(1), b = 17.840(2), c = 5.2848(5) Å, γ= 104.653(1)°, V = 894.6(2) Å3, Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I,hkl)]: 2.697(10,151), 2.542(9,–202), 3.127(8,310), 3.378(7,131), 2.154(7,261), 1.434 (7,–661), 4.450(6,021), 8.459(5,110), 2.727(5,–331), 2.328(5,–351). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO2 = 54.22, Al2O3 = 0.81, TiO2 = 5.45, Fe2O3 = 6.44, Mn2O3 = 7.57, ZnO = 0.12, NiO = 0.16, MgO = 8.26, Li2O = 1.53, CaO = 1.85, Na2O = 8.12, K2O = 2.12, H2O = 0.80, Cr, V, F, Cl not detected, sum 97.41 wt%. The formula unit, calculated on the basis of 24(O,OH,F) is (K0.40Na0.61)(Na1.71Ca0.29) (Mg1.81Zn0.01 Ni0.02Li0.90Fe3+0.71Mn3+0.85Ti4+0.60Al0.10)(Si7.96Al0.04)O22[(OH)0.80 O1.20]; the ideal end-member composition NaNa2(MgMn3+2LiTi4+ )Si8O22O2.

The crystal structure of dellaventuraite was refined to an R index of 3.8% using MoKα X-ray intensity data. The M1 site is occupied by Ti4+, Mn3+, and Mg in approximately equal amounts, the M2 site is occupied primarily by Mg and Fe3+, and M3 is occupied by Li with minor Mg and Mn2+. Local bond-valence considerations suggest that O2− at O3 is linked to Ti4+Mg or Mn3+Mn3+ at the adjacent M1 sites, and that OH at O3 is linked to MgMg at the adjacent M1 sites.

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