NaNa2(MgMn2TiLi)Si8O22O2, a new anhydrous amphibole from the Kajlidongri Manganese Mine, Jhabua District, Madhya Pradesh, India

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, Mnrich 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/cm, has a pale pink streak, vitreous luster, and does not ß uoresce 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) Å, 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 reÞ nement 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 Ni0.02Li0.90Fe 0.71Mn 0.85Ti 0.60Al0.10)(Si7.96Al0.04)O22[(OH)0.80 O1.20]; the ideal end-member composition NaNa2(MgMn2LiTi)Si8O22O2. The crystal structure of dellaventuraite was reÞ ned to an R index of 3.8% using MoKα X-ray intensity data. The M1 site is occupied by Ti, Mn, and Mg in approximately equal amounts, the M2 site is occupied primarily by Mg and Fe, and M3 is occupied by Li with minor Mg and Mn. Local bond-valence considerations suggest that O at O3 is linked to TiMg or MnMn at the adjacent M1 sites, and that OH at O3 is linked to MgMg at the adjacent M1 sites. TAIT ET AL.: DELLAVENTURAITE 305 quartzose rocks, interbedded with phyllites, quartz schist, and gondites (metamorphosed, non-calcareous magniferous argillites and arenites) of the Aravalli Supergroup. Subsequent to their formation, post-tectonic epigenetic activity introduced veins cutting across the above-mentioned rocks, with later supergene alteration. The presence of feldspars in quartz veins indicates their pegmatitic origin (Nayak 1966). Dellaventuraite was found in a rock consisting of 20% amphibole, 20% aegirine-augite, 50% quartz, 10% albite (Ab100), and trace apatite. The amphiboles are anhedral to subhedral with an average grain size of 0.4 mm and a maximum size of 1.5 mm. Dellaventuraite occurs together with leakeite and kornite, commonly overgrowing aegirine-augite (Fig. 1), and is the least abundant of these three amphiboles. The pyroxene has an average grainsize of 0.5 mm and a maximum size of 0.9 mm. Quartz forms the matrix of the rock, being andedral with an average grain size of 1.2 mm and a mosaic texture. The feldspar is subhedral to anhedral with an average grain size of 0.3 mm and a maximum grain size of 1.5 mm, and shows poorly developed albite twinning in some grains. The trace apatite is very Þ ne-grained, acicular, and occurs within the quartz and feldspar. Physical and optical properties Dellaventuraite is pink to red with a pale-pink streak and transparent with a vitreous luster; it shows no ß uorescence under long-wave or short-wave ultraviolet light. Crystals are anhedral, but the amphibole in the rock ranges widely in composition, from dellaventuraite to leakeite to kornite, and dellaventuraite can only be distinguished by electron-microprobe analysis. Dellaventuraite has perfect {110} cleavage intersecting at ~56°, is brittle, has conchoidal fracture, and has a Mohs ̓hardness of 5. The calculated density is 3.184 g/cm. A spindle stage was used to orient a crystal for refractive-index measurement and determination of 2V by extinction curves. The optic orientation was determined by transferring the crystal from the spindle stage to a precession camera and determining the relative axial relations by X-ray diffraction. 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, measured with gel-Þ ltered Na light (λ = 589.9 nm); 2V(obs) = 49 ± 3°, 2V(calc) = 41°. Chemical composition Dellaventuraite was analyzed by electron-microprobe using a Cameca SX-50 at the University of Manitoba operating in the wavelength-dispersive mode with excitation voltage 15 kV, specimen current 20 nA, a peak-count time of 20 s, and a background-count time of 10 s. The following standards and crystals were used for Ka X-ray lines: diopside (Si and Ca–PET); titanite (Ti-LiF); arfvedsonite (Fe-LiF); spessartine (Mn-LiF); olivine albite (Na-TAP); Kyanite (Al-TAP); orthoclase (K-PET); ß uororiebeckite (F-TAP); gahnite (Zn-Lif); and pentlandite (NiLiF). Data reduction was done using the φ(ρZ) procedure of Pouchou and Pichoir (1984, 1985). The average of 10 analyses on a single grain is given in Table 1 [analysis(1)]. Analysis (2) (Table 1) is the mean of 10 analyses on the crystal used to collect X-ray intensity data for crystal-structure reÞ nement. Ion-microprobe analysis of the crystal used for X-ray data collection was done on a Cameca IMS 4f at Pavia, Italy. Due to the very small dimensions of the amphibole crystal, an O primary beam ≤5 micrometers in diameter, corresponding to a beam current of ~1.2–2.0 nA, was used. The sample was left in the ion-microprobe sample chamber to degas for two days, together with H standards that were used in the calibration procedure. The energy-Þ ltering technique was used to eliminate any possible molecular interference and to reduce matrix effects affecting light-element ionization. Secondary positive-ion currents were measured at masses 1 (H), 7 (Li), and 30 (Si was used as the reference element) and corrected for isotopic abundances. The results were put on a quantitative basis using empirical calibration curves based on standard silicate samples. In particular, for H quantiÞ cation, we used a sample of tremolite (Hawthorne and Grundy 1976) (SiO2 = 56.57 wt%; H2O = 1.46 wt%) and high-Fe amphibole standards, and corrections were done to take into account the variation of the ion-yield relative to Si for H [=IY(H/Si)] vs. (Fe + Mn) (at. content) in the sample (Ottolini and Hawthorne 2001; Ottolini et al. 2002). Lithium quantiÞ cation was done according to the procedure of Ottolini et al. (1993). The accuracy of H and Li analysis is estimated to be in the order of 10% relative. The unit formula was calculated on the basis of 24(O,OH,F). When calculating the unit formulae of amphiboles characterized by SREF (Site-scattering REFinement) from microprobe data, we derive the Fe/Fe ratio from stereochemical criteria (Hawthorne 1983). When there is signiÞ cant Mn present, it is usually assumed that Mn is in the divalent state (e.g., Hawthorne et al. 1993). However, the situation in dellaventuraite is somewhat different as the O3 site is occupied predominantly by O. Hawthorne et al. (1995) described another amphibole, ungarettiite, in which O is dominant at the O3 site. In ungarettiite, the M1A site is occupied by Mn, and the unusual pattern of bond lengths (involving a very short bond to O3) is promoted by the Jahn-Teller distortion typical TABLE 1. Average chemical composition (wt%) and unit formula (apfu) of dellaventuraite