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Poppiite, the V3+ End-Member of the Pumpellyite Group: Description and Crystal Structure

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Poppiite, the V3+ End-Member of the Pumpellyite Group: Description and Crystal Structure American Mineralogist, Volume 91, pages 584–588, 2006 Poppiite, the V3+ end-member of the pumpellyite group: Description and crystal structure MARIA FRANCA BRIGATTI,1,* ENRICO CAPRILLI,1 AND MARCO MARCHESINI2 1Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Largo S. Eufemia 19, I-41100 Modena, Italy 2ENI-AGIP Towers, I-20097 San Donato Milanese, Italy ABSTRACT Poppiite, a new mineral from Gambatesa mine (Val Graveglia, Genova, Northern Italy), is the V3+ [VII] [VI] 3+ 3+ 2+ end-member of the pumpellyite group [ (Ca7.68 Na0.27 K0.03 Rb0.02)Σ8.00 (V1.26 Fe1.02 Mg0.78 Mn0.59 Al0.31 2+ [VI] 3+ [IV] Cu0.04)Σ4.00 (V6.89 Al1.07 Ti0.04)Σ8.00 (Si11.69Al0.31)Σ12.00 O42 (OH)14; C2/m, a = 19.2889(6), b = 6.0444(2), β 3 3 3 c = 8.8783(3) Å, = 97.328(2)°, V = 1026.66(6) Å , Dmeas = 3.36(2) g/cm , and Dcalc = 3.44 g/cm ]. Poppiite crystals, with size varying from 0.1 to 0.6 mm, are minute, greenish-brown, and prismatic, and are associated with roscoelite, ganophyllite, manganaxinite, goldmanite, and calcite. The strongest lines in the X-ray powder diffraction pattern [d (Å), I , (hkl)] are: 2.930, 100, – obs – rel (511); 3.817, 70, (202); 2.548, 65, (313); 2.551, 62, (420); 1.612, 57, (731,424); and 2.367, 51, (222, 403). Poppiite is optically negative, with 2Vcalc = 44°, nα = 1.768(9), nβ 1.804(8), nγ 1.810(9). The pleochroic scheme is α = light yellowish brown, β = deep greenish brown, and γ = brown to reddish brown. The crystal structure was reÞ ned using 1918 unique reß ections to R = 0.0307. Like the other pumpellyite-group minerals the crystal structure of poppiite consists of chains of edge-sharing octa- hedra linked by SiO4, Si2O7, and CaO7 polyhedra. Keywords: New mineral, analysis chemical (mineral), crystal structure, optical properties, XRD data INTRODUCTION structure of julgoldite was described by Allmann and Donnay The general formula of pumpellyite-group minerals, char- (1973), and later revised by Artioli et al. (2003), who suggested acterized by a ß exible crystal structure that can accommodate that Fe on X and Y sites only occurs in the ferric state, based on synchrotron X-ray powder diffraction and Mössbauer spec- various cations, is W8X4Y8T12O56–n(OH)n. Natural end-members were described with Al, Fe2+, Fe3+, Mg, and Mn2+ as dominating troscopy evidence. 3+ cations in the octahedral X sites, and with Al, Fe3+, Mn3+, and Pan and Fleet (1992) described some V -rich pumpellyite 3+ Cr3+ as dominating cations in the octahedral Y sites. Alkali and crystals, with V ranging from 0.43 to 7.39 apfu, occurring in alkaline earths metals, mostly Ca, occupy the seven-coordinated green mica schist from Hemlo gold Deposit (Ontario). According W sites and Si, together with a usually small Al amount, occupies to Pan and Fleet (1992), this occurrence represents a possible tetrahedral T positions. new mineral, with V as dominant cation in both Y and X sites. Pumpellyite group minerals are classified based on the However, the limited amount of material available prevented a dominating cation in Y position. In pumpellyite (Palache and more detailed characterization. Vassar 1925; Galli and Alberti 1969; Allmann and Donnay 1971; This work provides a mineralogical and structural description 3+ 3+ Yoshiasa and Matsumoto 1985) Al is the dominating cation. of the V -pumpellyite group end-member, where V was dem- Julgoldite (Moore 1971; Allmann and Donnay 1973; Artioli et onstrated to be the dominating cation in both octahedral X and Y al. 2003), shuiskite (Ivanov et al. 1981), and okotskite (Togari sites. The mineral occurs in the Mn-mine area of Gambatesa, near and Akasaka 1987; Artioli et al. 1996; Akasaka et al. 1997) are the village of Reppia (Val Graveglia, Northern Apennines, Italy) isomorphous of pumpellyite with Fe3+, Cr3+, and Mn3+ partly or and was named poppiite in memory of Luciano Poppi, Professor completely replacing Al3+. Different species are then identiÞ ed of Mineralogy at Modena and Reggio Emilia University, and all based on the dominating cation in X position. A sufÞ x indicat- his generous and fruitful efforts devoted to mineral science. The ing the dominating cation in X site should thus be added to the new mineral and its name were approved by the Commission subgroup name (Passaglia and Gottardi 1973). on New Minerals and Mineral Names of IMA (vote 2005-018). Some structural data on pumpellyite and its isomorphs were The holotype material is deposited at the “Museo Mineralogico derived from a limited number of single-crystal X-ray reÞ ne- e Geologico Estense, GEMMA1786” at Modena and Reggio ments available in the literature, despite the usual poor quality Emilia University, Italy (sample ST 005898). of naturally occurring crystals. The structural model of pumpel- OCCURRENCE, PARAGENESIS, PHYSICAL, AND lyite (space group A2/m), Þ rst proposed by Gottardi (1965), was OPTICAL PROPERTIES subsequently investigated by Galli and Alberti (1969), Yoshiasa and Matsumoto (1985), and Artioli and Geiger (1994). The Poppiite was collected in the Mn-mine area of Gambatesa, near the village of Reppia (Val Graveglia, Northern Apennines, Italy). Mn-ores were deposited, as sedimentary layers (one * E-mail: [email protected] centimeter to several meters thick), during the Middle Callovian 0003-004X/06/0004–584$05.00/DOI: 10.2138/am.2006.2033 584 BRIGATTI ET AL.: THE NEW MINERAL POPPIITE 585 stage (Abbate et al. 1986) at the base of the cherts (M. Alpe can be observed among different fragments. Formation). During Alpine recrystallization, sedimentary-dia- The empirical formula, computed from chemical data, was genetic phases were replaced by massive braunite + quartz + based on 98 anion charges, namely O42(OH)14, thus following hematite aggregates under prehnite-pumpellyite facies condi- the example of most of the authors who characterized pumpel- tions (Cortesogno et al. 1979). During this metamorphic event, lyite-group minerals and the recommendations of Passaglia and a main tectonic phase induced mobilization of Mn and Si and Gottardi (1973). The resulting chemical formula is: thickening of the mineralized layers (up to some tens of meters), 3+ 3+ 2+ mainly at the fold hinges. Mn-silicate and carbonate assemblages (Ca7.68 Na0.27 K0.03 Rb0.02)Σ8.00 (V8.15 Al1.38 Fe1.02 Mg0.78 Mn0.59 2+ thus developed extensively in the thickened lenses, mainly as Ti0.04Cu0.04)Σ12.00(Si11.69Al0.31)Σ12.00O42(OH)14 bands, replacing the massive braunite, or as fracture-Þ lling veins. A later extensional stage induced brittle deformation. The simpliÞ ed formula, based on (O,OH)14 is: These mineral assemblages were thus altered and progressively 3+ 3+ 2+ 3+ replaced. Caryopilite, inesite, parsettensite, neotocite, ranciéite, Ca2 (V , Fe , Mg, Mn ) (V , Al)2 (Si, Al)3 (O, OH)14. manganite, rhodochrosite, and manganoan calcite are the main replacing phases whereas pyrolusite, barian phillipsite, K-feld- TABLE 1. Physical and optical properties of poppiite 3 spar, calcite, aragonite, calcian kutnohorite, barite, chalcocite, Densitymeas* 3.36(2) g/cm 3 pyrite, octahedrite, volborthite, and native copper are the most Densitycalc † 3.44 g/cm Color brown to greenish brown common fracture Þ lling phases (Cortesogno et al. 1979). Streak greenish white Poppiite, identiÞ ed during an extensive investigation of sili- Luster vitreous, transparent, non-fl uorescent (UV) cates from Gambatesa mine area (Caprilli 2004), is restricted to Tenacity Brittle Fracture sub-conchoidal a few crystals associated with roscoelite (Brigatti et al. 2003), Cleavage {100} and {001} ganophyllite, manganaxinite, goldmanite, and calcite. The Twinning Sometime polysynthetic mineral forms radiating aggregates (about 2 mm in diameter) Optical class biaxial (-) Pleochroism α = light yellowish brown of minute, acicular prismatic platy crystals, up to 0.6 mm long, β = deep greenish brown elongated, and frequently striated parallel to [010] (Fig. 1). The γ = brown to reddish brown physical and optical properties of poppiite are summarized in 2Vcalc 44° Table 1. nα‡ 1.768(9) nβ‡ 1.804(8) CHEMICAL COMPOSITION nγ‡ 1.810(9) Orientation OAP ⊥ [010] and β = γ The chemical composition of poppiite (Table 2) was de- * Measured by torsion microbalance VDF "United" (Bermann 1939). termined using a wavelength-dispersive ARL-SEMQ electron † Calculated from electron microprobe chemical analysis, with a cell volume of 1026.66(6) Å3. microprobe (operating conditions: 15 kV accelerating voltage, ‡ Immersion method (Cargille immersion liquids). 15 nA sample current, and beam diameter approximately 5 μm). Analyses and data reductions were performed using the Probe TABLE 2. Chemical data of poppiite from Gambatesa mine (Northern software package of Donovan (1995). The following standards Italy) compared to vanadian pumpellyite from Golden Giant were used: microcline (K, Al), albite (Na), spessartine (Mn), mine (Hemlo Gold deposit, Ontario) (Pan and Fleet 1994, ilmenite (Fe, Ti), clinopyroxene (Si, Ca), olivine (Mg), chromite analysis No. 1, Table 2) (Cr), pollucite (Cs), metallic vanadium (V), ß uorite (F), Cu94Sn6 wt% Atoms pfu‡ (Cu), synthetic Rb glass (Rb). Minor elements (0.01 to 0.2 wt% Gambatesa Golden Giant Gambatesa Golden Giant oxide) are Ti, Cu, K, Rb, and F. No additional element with SiO2 33.19 (32.63–33.33) 33.23 Si 11.69 11.94 TiO2 0.14 (0.09–0.18) 1.13 Al 0.31 0.06 atomic number ≥9 was detected. The point analyses from a same Al2O3 4.08 (4.00–4.17) 4.55 T sites 12.0 12.0 fragment are homogeneous and only limited chemical variations V2O3 28.86 (28.40–28.92) 25.67 Ti 0.04 0.30 Cr2O3 b.d.t.
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