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Dessauite, (Sr,Pb)(Y,U)(Ti,Fe )20O38, a New Mineral of the Crichtonite Group from Buca Della Vena Mine, Tuscany, Italy

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Dessauite, (Sr,Pb)(Y,U)(Ti,Fe )20O38, a New Mineral of the Crichtonite Group from Buca Della Vena Mine, Tuscany, Italy American Mineralogist, Volume 82, pages 807±811, 1997 31 Dessauite, (Sr,Pb)(Y,U)(Ti,Fe )20O38, a new mineral of the crichtonite group from Buca della Vena mine, Tuscany, Italy PAOLO ORLANDI,1 MARCO PASERO,1 GIUSEPPE DUCHI,2 AND FILIPPO OLMI3 1Dipartimento di Scienze della Terra, UniversitaÁ di Pisa, Via S. Maria 53, I-56126 Pisa, Italy 2Via dei Lecci 77/i, I-55049 Viareggio (LU), Italy 3Consiglio Nazionale delle Ricerche, Centro di Studio per la Minerogenesi e la Geochimica Applicata, Via La Pira 4, I-50121 Firenze, Italy ABSTRACT Dessauite, a new mineral in the crichtonite group, occurs within cavities in calcite veins as tabular {001} rhombohedral, black, millimeter-sized crystals at Buca della Vena Mine, Apuan Alps (Tuscany, Italy). It is associated with derbylite, hematite, rutile, karelianite, siderite, and calcite. Dessauite is trigonal, space group R3, with a 5 9.197(1) AÊ , a5 68.75(2)8. Optically, dessauite is opaque and shows low bire¯ectance and very weak ple- ochroism. Electron microprobe analyses led to the following simpli®ed chemical formula: 31 (Sr,Pb)(Y,U)(Ti,Fe )20O38. The crystal structure of dessauite was re®ned from single-crys- tal X-ray diffraction data to R 5 0.065. Dessauite is isostructural with the others members of the crichtonite group; a peculiar structural feature is the presence of additional, partially occupied octahedral sites. A comparison of the crystal-chemical formulas of all the min- erals within the crichtonite group is presented. In view of the structural information, the analytical data have been re-arranged on the basis of the crystal-chemical formula ABC18T2O38, rather than AM21O38. INTRODUCTION erals and Mineral Names (Jambor and Puziewicz 1992) The crichtonite group includes a series of minerals, all and therefore cannot be regarded as a distinct mineral species. Dragila (1990) assigned ``romanite'' to the dav- characterized by the general formula AM21O38: crichton- ite, senaite, davidite-(La), davidite-(Ce), davidite-(Y), idite group, but such an assignment seems highly loveringite, landauite, lindsleyite, and mathiasite. The questionable. ®rst two minerals and the davidite species have been The aims of this paper are the description of a new known for more than 90 years, whereas the others have member of the crichtonite group, found in Tuscany, Italy, been described only very recently. However, little was and a review and comparison of the crystal-chemical for- known about the discriminating differences in their chem- mulas of the other minerals in the group. ical composition until crystal-structure analyses, for most The new mineral has been named dessauite in memory of them, allowed the unambiguous allocation of the dif- of the late Gabor Dessau (1907±1983), professor of ore ferent cations to the proper sites. mineralogy at the University of Pisa and devoted expert For completeness, three additions should be made to of the ore deposits of Tuscany. Both the mineral and its the above list. (1) A mineral from Italy referred to as name have been approved by the I.M.A. Commission on ``davidite from Pizzo Cervandone'' by Stalder and BuÈhler New Minerals and Mineral Names. The type material is (1987) probably deserves the status of a distinct mineral deposited in the Museo di Storia Naturale e del Territorio, species because of its peculiar chemical composition. (2) University of Pisa (catalog no. 16385). A Re-rich unnamed mineral of uncertain origin preserved OCCURRENCE in the Natural History Museum, Geneva (Switzerland), was described by Sarp et al. (1981). That mineral, how- Dessauite was found in the Buca della Vena Mine, Ap- ever, despite having the same cell constants and space uan Alps, near the town of Stazzema (LU), northern Tus- group as the other crichtonite-group minerals, could be cany, Italy. It occurs in calcite veins hosted within dolo- somewhat different since it has excess cations (more than mite (``grezzoni'') and is associated with calcite, rutile, 23 cations per 38 O atoms, instead of 22) and several hematite, siderite, and derbylite. Dessauite is the latest in differences exist in the X-ray diffraction pattern. (3) A a long series of rare minerals identi®ed within the Buca poorly characterized mineral from Romania has been re- della Vena mine in the last decade, such as apuanite, ver- ferred to as ``romanite'' (Dragila 1990), but it has not siliaite, stibivanite22O, karelianite, robinsonite, tintinaite, been submitted to the I.M.A. Commission on New Min- and andorite. All these minerals originated from hydro- 0003±004X/97/0708±0807$05.00 807 808 ORLANDI ET AL.: DESSAUITE TABLE 2. X-ray powder pattern of dessauite dobs dcalc hkl I 3.412 3.414 220 m 3.064 3.068 131 vw 2.998 2.997 211 w 2.902 2.901 231 m 2.846 2.845 232 mw 2.758 2.755 030 vw 2.624 2.641 230 vw 2.499 2.495 331 mw 2.431 2.433 131 vw 2.250 2.249 241 w 2.140 2.144 140 w 1.916 1.917 340 mw 1.805 1.808 251 mw 1.712 1.712 150 vw 1.603 1.606 451 m 1.548 1.551 521 w FIGURE 1. SEM photograph of tabular {001} dessauite crys- 1.511 1.511 363 w tals on calcite. Scale bar: 0.1 mm. 1.441 1.441 431 m Notes: Gandol® camera, diameter 114.6 mm, FeKa radiation, l5 1.93728 AÊ . Intensities were estimated by eye and are given as follows: m thermal ¯uids circulating through a small hematite-barite 5 medium; mw 5 medium weak, w 5 weak, vw 5 very weak. ore deposit within dolomite, during an alpine metamor- phic event (Orlandi and Checchi 1986). PHYSICAL AND OPTICAL PROPERTIES tored on a Faraday cup), counting time 40 s for peak and Dessauite occurs as small, ¯attened rhombohedral crys- 20 s for both left and right backgrounds. The following tals, tabular {001} with hexagonal outline (Fig. 1). Di- standards were used: metallic vanadium (V), rutile (Ti), mensions: diameter up to 1 mm, thickness up to 0.2 mm. monazite (Th, La, Ce, Pr, Nd), metallic uranium (U), al- The mineral is black and opaque with a metallic luster, and mandine (Al, Si, Mg), chromite (Cr), ilmenite (Fe), YAG (Y), diopside (Ca), rhodonite (Mn), willemite (Zn), ce- it is brittle. The Vickers hardness, VHN100, is 1105 to 1782. Cleavage was not observed, and the fracture is conchoidal. lestine (Sr), benitoite (Ba), and crocoite (Pb). The pres- The density could not be measured because of the small ence of Si, Pr, and Nd was always tested but these ele- ments were never detected. The raw data were reduced grain size; the calculated density is 4.68 g/cm3. In re¯ected plane-polarized light the color is ash-grey with pale bluish using a Bence-Albee correction routine (Bence and Albee tones. Internal re¯ections were not observed. The anisot- 1968). ropy is weak in shades of very dark blue and tan. The Four of the 21 analyses were rejected because of major bire¯ectance is low, and the pleochroism is very weak. The inhomogeneities. The analytical data and the resulting re¯ectance values R1 and R2 were measured on a digital crystal-chemical formula are given in Table 3. On this microphotometer EEL-165 in air and in oil with refractive index 1.5150(2) using SiC as a standard. The relevant val- TABLE 3. Electron microprobe analysis (average of 17 points) ues are listed in Table 1. The X-ray powder pattern of of dessauite from Buca della Vena Mine dessauite is reported in Table 2. Oxide wt% Range CHEMICAL COMPOSITION V2O5 0.89 0.83±1.06 A total of 21 chemical analyses were made using a TiO2 55.71 52.25±59.62 ThO 0.11 0.02±0.19 JEOL JXA-8600 electron microprobe equipped with a 2 UO2 4.52 2.63±8.74 Tracor Northern Series II system (at CNR-CSMGA, Fi- Al2O3 0.08 0.04±0.20 Cr O 0.21 0.05±0.36 renze). No elements (with Z . 9) other than those re- 2 3 Fe O 27.24 26.26±28.18 ported here were detected by a preliminary 300 s energy- 2 3 Y2O3 2.02 1.37±2.64 dispersive scan. The working conditions were: La2O3 1.14 0.89±1.40 Ce O 0.68 0.48±0.84 accelerating voltage 20 kV, beam current 30 nA (moni- 2 3 MgO 0.03 0.00±0.16 CaO 0.12 0.07±0.22 MnO 0.56 0.31±0.97 TABLE 1. Re¯ectance values (%) for dessauite ZnO 0.42 0.36±0.47 SrO 2.34 2.19±2.45 Wave- Air Oil BaO 0.83 0.61±1.04 length PbO 3.18 2.90±3.39 (nm) R1 R2 R1 R2 Total 100.08 470 17.3 17.7 5.6 6.0 Notes: Empirical formula (calculated on the basis O 5 38): (Sr0.41Pb0.26 546 16.6 17.1 5.0 5.5 Ba0.10 Ca0.04 Th0.01)S50.82 (Y0.33 U0.31 Mn0.14 La0.13 Ce0.08)S50.99(Ti12.78Fe6.25V0.189 31 589 16.1 16.6 4.8 5.3 Zn0.0Cr0.05Al0.03Mg0.01)S519.39O38. Simpli®ed formula: (Sr,Pb)(Y,U)(Ti,Fe )20 650 16.0 16.5 4.6 5.1 O38. ORLANDI ET AL.: DESSAUITE 809 TABLE 5. Positional and displacement parameters for dessauite Site Occupancy xyzU* M0 Sr0.42Pb0.24Ba0.12M0.22 0 0 0 0.0186(6) M1 Y0.30U0.30Mn0.12La0.12Ce0.06M0.10 ½ ½ ½ 0.0063(4) M2 Fe0.93M0.07 0.3095(2) 0.3095(2) 0.3095(2) 0.0089(7) M3 Fe0.77Ti0.19M0.04 0.3479(2) 0.1263(2) 0.0207(2) 0.0063(4) M4 Ti0.96M0.04 0.3088(3) 0.7208(3) 0.1464(3) 0.0068(5) M5 Ti0.96M0.04 0.4740(3) 0.0791(3) 0.6426(3) 0.0063(5) M6 Fe0.07M0.93 0.396(2) 0.396(2) 0.396(2) 0.010(7) M7 Fe0.12M0.88 0.264(2) 0.423(2) 0.806(2) 0.007(2) M8 Fe0.06M0.94 0.226(4) 0.138(4) 0.456(4) 0.009(6) M9 Fe0.06M0.94 0.926(4) 0.237(3) 0.158(3) 0.005(5) O1 0.304(1) 0.628(1) 0.380(1) 0.006(2) O2 0.153(1) 0.238(1) 0.938(1) 0.004(2) O3 0.918(1) 0.457(1) 0.303(1) 0.005(1) O4 0.144(1) 0.519(1) 0.990(1) 0.006(1) O5 0.391(1) 0.480(1) 0.138(1) 0.009(2) O6 0.7062(9) 0.2379(9) 0.0714(9) 0.001(1) O7 0.2150(8) 0.2150(8) 0.2150(8) 0.011(3) * Ueq for cations M0±M5; Uiso for cations M6±M9 and O atoms.
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