American Mineralogist, Volume 98, pages 236–241, 2013 Rongibbsite, Pb2(Si4Al)O11(OH), a new zeolitic aluminosilicate mineral with an interrupted framework from Maricopa County, Arizona, U.S.A. HEXIONG YANG,* ROBERT T. DOWNS, STANLEY H. EVANS, ROBERT A. JENKINS, AND ELIAS M. BLOCH Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721-0077, U.S.A. ABSTRACT A new zeolitic aluminosilicate mineral species, rongibbsite, ideally Pb2(Si4Al)O11(OH), has been found in a quartz vein in the Proterozoic gneiss of the Big Horn Mountains, Maricopa County, Arizona, U.S.A. The mineral is of secondary origin and is associated with wickenburgite, fornacite, mimetite, murdochite, and creaseyite. Rongibbsite crystals are bladed (elongated along the c axis, up to 0.70 × 0.20 × 0.05 mm), often in tufts. Dominant forms are {100}, {010}, {001}, and {101}. Twinning is common across (100). The mineral is colorless, transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of ∼5; cleavage is perfect on {100} and no parting was observed. 3 The calculated density is 4.43 g/cm . Optically, rongibbsite is biaxial (+), with nα = 1.690, nβ = 1.694, Z nγ = 1.700, c = 26°, 2Vmeas = 65(2)°. It is insoluble in water, acetone, or hydrochloric acid. Electron microprobe analysis yielded an empirical formula Pb2.05(Si3.89Al1.11)O11(OH). Rongibbsite is monoclinic, with space group I2/m and unit-cell parameters a = 7.8356(6), b = 13.913(1), c = 10.278(1) Å, β = 92.925(4)°, and V = 1119.0(2) Å3. Its structure features an interrupted framework made of three symmetrically distinct TO4 tetrahedra (T = Si + Al). The framework density is 17.9 T per 1000 Å3. Unlike many known interrupted frameworks in zeolite-type materials, which are usually broken up by OH or F, the framework in rongibbsite is interrupted by O atoms. There are various corner-shared tetrahedral rings in the framework of rongibbsite, including two types of 4-membered, three 6-membered, and one 8-membered rings. The extraframework Pb and OH reside alternately in the channels formed by the 8-membered rings. The Pb cations are disordered over two split sites, Pb and Pb′, with site occupancies of 0.8 and 0.2, respectively, and a Pb-Pb′ distance of 0.229 Å, providing a structural explanation for the two strong Raman bands (at 3527 and 3444 cm–1) attributable to the O-H stretching vibrations. The average bond lengths for the T1, T2, and T3 tetra- hedra are 1.620, 1.648, and 1.681 Å, respectively, indicating that the preference of Al for the three tetrahedral sites is T3 >> T2 > T1. Rongibbsite represents the first natural aluminosilicate with Pb as the only extraframework cation. Keywords: Rongibbsite, zeolitic aluminosilicate, Pb-bearing, interrupted framework, crystal structure, X-ray diffraction, Raman spectra INTRODUCTION work structures. While maricopaite is the only natural zeolite A new zeolitic aluminosilicate mineral species, rongibbsite, having Pb as a dominant extraframework cation, rongibbsite represents the first natural aluminosilicate with Pb as the only ideally Pb2(Si4Al)O11(OH), has been found in the Big Horn Mountains, Maricopa County, Arizona, U.S.A. It is named extraframework cation. after its finder, Ronald Bradford Gibbs, a mineral collector SAMPLE DESCRIPTION AND EXPERIMENTAL METHODS and a mining engineer in Tucson, Arizona. The new mineral and its name have been approved by the Commission on New Occurrence, physical and chemical properties, and Raman Minerals, Nomenclature and Classification (CNMNC) of the spectra International Mineralogical Association (IMA2010-055). Part Rongibbsite was found in material collected from a small unnamed prospect of the co-type sample has been deposited at the University of in the Big Horn District, Big Horn Mountains, Maricopa County, Arizona, U.S.A. (lat. 33°69′ N and long. 113°22′). Rongibbsite occurs with other secondary lead Arizona Mineral Museum (catalog no. 19292) and the RRUFF and copper minerals in a quartz vein in Proterozoic gneiss. Mineral occurrences in Project (deposition no. R100031). In this paper, we describe the the Big Horn district are gold-rich, basement hosted narrow quartz pods and veins physical and chemical properties of rongibbsite and its struc- associated with late Cretaceous intrusives (Allen 1985). Associated minerals are tural features determined from single-crystal X-ray diffraction wickenburgite, fornacite, mimetite, murdochite, and creaseyite. Other minerals found in the quartz veins include: anglesite, cerussite, chrysocolla, iranite, gold, and Raman spectroscopy. Along with the zeolite maricopaite, mottramite, willemite, phoenicochroite, planchéite, iron oxides, the sulfides ga- Ca2Pb7(Si36Al12)O99⋅n(H2O,OH), found in the same region (Rouse lena and chalcopyrite, and zeolites including stilbite, heulandite, and laumontite. and Peacor 1994), rongibbsite joins a small group of natural and Rongibbsite crystals are bladed (elongated along the c axis) (up to 0.70 × 0.20 × synthetic compounds that possess interrupted tetrahedral frame- 0.05 mm), often in tufts (Fig. 1). Dominant forms are {100}, {010}, {001}, and {101}. Twinning is common on (100). The mineral is colorless, transparent with white streak and vitreous luster. It is brittle and has a Mohs hardness of ∼5; cleav- * E-mail: [email protected] age is perfect on {100} and no parting was observed. The calculated density is 0003-004X/13/0001–236$05.00/DOI: http://dx.doi.org/10.2138/am.2013.4252 236 YANG ET AL.: RONGIBBSITE, A NEW ZEOLITIC ALUMINOSILICATE MINERAL 237 4.43 g/cm3 using the empirical formula. Optically, rongibbsite is biaxial (+), with Z nα = 1.690, nβ = 1.694, nγ = 1.700, c = 26°, 2Vmeas = 65(2)°, and 2Vcalc = 66°. The dispersion is strong (r > v). The compatibility index (1 – Kp/Kc) is 0.019 (superior). It is insoluble in water, acetone, or hydrochloric acid. The chemical composition was determined with a CAMECA SX50 electron microprobe at 15 kV and 5 nA with a beam diameter of 20 µm. The standards used include diopside for Si, anorthite for Al, and Pb-glass (NIST-K0229) for Pb, yield- ing an average composition (wt%) (11 points) of SiO2 30.64(15), Al2O3 7.44(19), + PbO 59.80(40), H2O 1.18 (estimated for charge balance), and total = 99.06(47). The resultant chemical formula, calculated on the basis of 12 O atoms (from the structure determination), is Pb2.05(Si3.89Al1.11)O11(OH), which can be simplified as Pb2(Si4Al)O11(OH). The Raman spectrum of rongibbsite was collected on a randomly oriented crystal from 15 scans at 30 s and 100% power per scan on a Thermo-Almega microRaman system, using a solid-state laser with a frequency of 532 nm and a thermoelectric cooled CCD detector. The laser is partially polarized with 4 cm–1 resolution and a spot size of 1 µm. X‑ray crystallography Because of the limited amount of available material, no powder X-ray diffrac- FIGURE 1. Photograph of rongibbsite crystals. tion data were measured for rongibbsite. Listed in Table 1 are the powder X-ray diffraction data calculated from the determined structure using the program XPOW (Downs et al. 1993). Single-crystal X-ray diffraction data were collected from a sharing the vertex O atoms between T2O4 tetrahedra. There are nearly equi-dimensional, untwinned crystal (0.03 × 0.04 × 0.05 mm) on a Bruker several kinds of symmetrically distinct tetrahedral rings in the X8 APEX2 CCD X-ray diffractometer equipped with graphite-monochromatized framework, including one 8-membered, three 6-membered, and MoKα radiation with frame widths of 0.5° in ω and 30 s counting time per frame. All reflections were indexed on the basis of a monoclinic unit cell (Table 2). The two 4-membered rings (Fig. 4). The intricate arrangements of intensity data were corrected for X-ray absorption using the Bruker program these rings are illustrated in Figure 5. The extraframework Pb SADABS. The systematic absences of reflections suggest possible space group C2, cations are situated in the channels formed by the 8-membered Cm, or C2/m. The crystal structure was solved and refined using SHELX97 (Shel- rings and distributed over two split sites, Pb and Pb′, with site drick 2008) based on the space group C2/m, because it yielded the best refinement statistics in terms of bond lengths and angles, atomic displacement parameters, and occupancies of 0.8 and 0.2, respectively, and a Pb–Pb′ distance R factors. However, to avoid the large β angle (125.463°) in the C-lattice setting, of 0.229 Å. Site-splitting for Pb is quite common, especially in we adopted space group I2/m (β = 92.925°) in this study. A preliminary structure materials constructed of framework structures (e.g., Szymanski refinement based on the ideal chemical formula revealed an outstanding residual 1988; Moore et al. 1989, 1991; Gunter et al. 1994; Holtstam et peak in the proximity of the Pb site on the difference Fourier maps. A site-split al. 1995; Downs et al. 1995; Tribaudino et al. 1998; Siidra et model for Pb was, therefore, introduced in the subsequent refinements, with the occupancies of Pb at the two sites allowed to vary. No site occupancies were refined al. 2009). It is worth noting that the Si/Al ratio in the structure between Si and Al among the three tetrahedral sites (T1, T2, and T3), due to their is also about 0.8/0.2, the same as the Pb occupancies between similar X-ray scattering power. For simplicity, all Al atoms were assigned to the the two split sites. Perhaps the Pb site-splitting is a requirement T3 site during the refinement because the average bond distance for this site (1.681 of the (Si4Al)O11 network configuration.