American Mineralogist, Volume 97, pages 1874–1880, 2012 In situ dehydration behavior of zeolite-like cavansite: A single-crystal X-ray study ROSA MICAELA DANISI,* THOMAS ARMBRUSTER, AND BILJANA LAZIC Mineralogical Crystallography, Institute of Geological Sciences, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland ABSTRACT To track dehydration behavior of cavansite, Ca(VO)(Si4O10)·4H2O [space group Pnma, a = 9.6329(2), b = 13.6606(2), c = 9.7949(2) Å, V = 1288.92(4) Å3] single-crystal X-ray diffraction data on a crystal from Wagholi quarry, Poona district (India) were collected up to 400 °C in steps of 25 °C up to 250 °C and in steps of 50 °C between 250 and 400 °C. The structure of cavansite is characterized 4+ by layers of silicate tetrahedra connected by V O5 square pyramids. This way a porous framework structure is formed with Ca and H2O as extraframework occupants. At room temperature, the hydrogen bond system was analyzed. Ca is eightfold coordinated by four bonds to O of the framework structure and four bonds to H2O molecules. H2O linked to Ca is hydrogen bonded to the framework and also to adjacent H2O molecules. The dehydration in cavansite proceeds in four steps. At 75 °C, H2O at O9 was completely expelled leading to 3 H2O pfu with only minor impact on framework distortion and contraction [V = 1282.73(3) Å3]. The Ca coordination declined from origi- nally eightfold to sevenfold and H2O at O7 displayed positional disorder. At 175 °C, the split O7 sites approached the former O9 position. In addition, the sum of the three split positions O7, O7a, and O7b decreased to 50% occupancy yielding 2 H2O pfu accompanied by a strong decrease in volume [V = 1206.89(8) Å3]. The Ca coordination was further reduced from sevenfold to sixfold. At 350 °C, H2O at O8 was released leading to a formula with 1 H2O pfu causing additional struc- tural contraction (V = 1156(11) Å3). At this temperature, Ca adopted fivefold coordination and O7 rearranged to disordered positions closer to the original O9 H2O site. At 400 °C, cavansite lost crystallinity but the VO2+ characteristic blue color was preserved. Stepwise removal of water is discussed on the basis of literature data reporting differential thermal analyses, differential thermo-gravimetry experiments and temperature dependent IR spectra in the range of OH stretching vibrations. Keywords: Cavansite, dehydration, crystal structure, hydrogen bonding INTRODUCTION but crystal quality and experimental setup did not allow the determination of hydrogen positions. However, Solov’ev et al. Cavansite, Ca(VO)(Si4O10)·4H2O, is a porous layer silicate, dimorphous with pentagonite. The mineral’s name is derived (1993) reported the location of the hydrogen atoms attached to from the chemical elements of which it is composed: calcium H2O molecules. Corresponding identifications of hydrogen-bond (ca-), vanadium (-van-), and silicon (-si). Discovered in 1960 at systems have recently been repeated by Hughes et al. (2011). Owyhee Dam, Malheur County (Oregon, U.S.A.), cavansite is a The atomic arrangement in cavansite consists of undulat- relatively rare mineral (Staples et al. 1973). It occurs in cavities ing pyroxenoid-like (SiO3)n chains joined laterally into sheets and veinlets in basalt and vugs in tuff as radiating greenish-blue parallel to the a-c plane to form a network of four-and eight- prismatic crystals associated with calcite, analcime, thomsonite, membered rings in (010). Adjacent chains have the tetrahedral heulandite, stilbite, and apophyllite (Staples et al. 1973). How- apices pointing alternately up and down along the b axes (Evans ever, the largest and best samples come from Poona district in 1973). The sheets are cross-linked via the basal square of VO5 India where cavansite occurs in tholeiitic basalts of the Deccan pyramids, which are completed by one short apical V-O bond Volcanic Province (Phadke and Apte 1994), and in pores of of ca. 1.6 Å (e.g., Evans 1973; Solov’ev et al. 1993; Hughes et 2+ altered basalt breccias and tuffaceous andesite in association al. 2011) named vanadyl group. This VO ion is responsible with calcite, heulandite, stilbite, and rare apophyllite (Powar and for the brilliant sky blue color of the crystals (Rossman 2011). Byrappa 2001). More recently, cavansite has also been reported A porous three-dimensional framework is formed by the linkage from basalts in Rio Grande do Sul in southern Brazil (Frank et of tetrahedral sheets with VO5 square pyramids. Large cavities al. 2005) and from Aranga quarry, Kaipara district, North Island in the structure are occupied in zeolite-like fashion by Ca and New Zealand (Thornton 2004). coordinating H2O molecules. Calcium has four bonds to oxygen The structure of cavansite was solved by Evans (1973) atoms of silicate tetrahedra and four bonds to H2O molecules completing an eightfold coordination. The cavansite framework shows strong similarities to that of gis- * E-mail: [email protected] mondine, CaAl2Si2O8⋅4H2O. However, in gismondine the exposed 0003-004X/12/1112–1874$05.00/DOI: http://dx.doi.org/10.2138/am.2012.4228 1874 DANISI ET AL.: DEHYDRATION BEHAVIOR OF CAVANSITE 1875 tetrahedral apices in adjacent layers are linked directly to each other et al. 2009), in situ FTIR and Raman spectroscopy (Prasad and to form a three-dimensional aluminosilicate network (Fischer 1963) Prasad 2007) were done. and not via vanadyl-type VO5 square pyramids as in cavansite. The aim of this study is to further characterize the hydrogen- The dimorphism of cavansite and pentagonite is based on the bond system and the in situ dehydration behavior of cavansite difference in linkages in the silicate layer. Undulating (SiO3)n using single-cystal X-ray diffraction on crystals from Wagholi chains are discernible in both, but in cavansite these chains are quarry, Poona district (India). This investigation was initiated by joined laterally into sheets made up of fourfold and eightfold a supposed contradiction: Solov’ev et al. (1993) report H2O III rings, while in pentagonite they are differently joined so that with the highest atomic displacement parameter and the longest only sixfold rings are formed (Evans 1973). bond to Ca, also confirmed by Hughes et al. (2011). At first Cavansite has not yet been synthesized in the laboratory glance, one should assume that, upon heating, H2O III with the but other open-framework silicates containing vanadium are weakest bond is expelled first. However, the structural study of reported. Wang et al. (2002) describe two open-framework partly dehydrated cavansite by Rinaldi et al. (1975) indicates that vanadosilicates, VSH-1K and VSH-2Cs that contain a similar H2O (their W1) close to H2O III (room temperature) is the most structural motif found in the natural minerals cavansite and stable one before complete dehydration occurs accompanied pentagonite. Moreover, a series of novel vanadium silicates with structural breakdown. with open-framework and microporous structures has been EXPERIMENTAL METHODS synthesized under mild hydrothermal conditions with free chan- A cavansite single crystal from Wagholi, Poona district, Maharashtra, India of nel diameters as large as 6.5 Å (Wang et al. 2002). Ten distinct approximate dimensions 0.1 × 0.1 × 0.5 mm was selected for our structure study framework types containing vanadyl groups have been identified and mounted in an open 0.1 mm diameter quartz-glass capillary. and show a great potential to be used as molecular sieves or in Single-crystal X-ray diffraction data were collected with a Bruker APEX II catalysis due to their high thermal stability and ion-exchange diffractometer using MoKα (λ = 0.71073 Å) X-ray radiation with 50 kV and 30 mA X-ray power. To study in situ dehydration, complete data sets were collected properties (Wang et al. 2002). in steps of 25 °C up to 250 °C and in steps of 50 °C up to 400 °C using a self- Previous structural studies of the dehydration behavior of constructed temperature controlled hot nitrogen blower. Before data collections the cavansite are limited to a room-temperature single-cystal X-ray crystal was kept at least 30 min at the next measuring temperature. diffraction experiment of a crystal previously partly dehydrated CCD area-detector data were integrated and an empirical absorption correction ° was applied using the Apex2 v. 2011.4-1 software package. Data-collection param- under vacuum at 220 C (Rinaldi et al. 1975). In addition, differ- eters and refinement parameters are given in Table 1. Neutral atom scattering-factors ential thermal analysis (DTA) and differential thermo-gravimetry were used for structure refinement with SHELXL-97 (Sheldrick 2008). Hydrogen (DTG) (Phadke and Apte 1994; Powar and Byrappa 2001; Ishida positions were extracted from difference-Fourier maps applying the restraint H-O TABLE 1. Parameters for X-ray data collection and crystal-structure refinement of cavansite Cavansite (RT) Cavansite (75 °C) Cavansite (175 °C) Cavansite (350 °C) Crystal data Unit-cell dimensions (Å) a = 9.6329(2) a = 9.60900(10) a = 9.4746(4) a = 9.39(5) b = 13.6606(2) b = 13.6833(2) b = 13.2620(5) b = 13.11(8) c = 9.7949(2) c = 9.7559(2) c = 9.6050(4) c = 9.39(5) Volume (Å3) 1288.92(4) 1282.73(3) 1206.89(8) 1156(11) Space group Pnma (No. 62) Pnma (No. 62) Pnma (No. 62) Pnma (No. 62) Z 4 4 4 4 Chemical formula Ca(VO)(Si4O10)·4H2O Ca(VO)(Si4O10)·3H2O Ca(VO)(Si4O10)·2H2O Ca(VO)(Si4O10)·1H2O Intensity measurement Crystal shape prismatic prismatic prismatic prismatic Crystal size (mm) 0.1 × 0.1 × 0.5 0.1 × 0.1 × 0.5 0.1 × 0.1 × 0.5 0.1 × 0.1 × 0.5 Diffractometer APEX II SMART APEX II SMART APEX II SMART APEX II SMART X-ray radiation MoKα λ = 0.71073 Å MoKα λ = 0.71073 Å MoKα λ = 0.71073 Å MoKα λ = 0.71073 Å X-ray power 50 kV, 30 mA 50 kV, 30 mA 50 kV, 30 mA 50 kV, 30 mA Monochromator graphite graphite graphite graphite Temperature 293 K 348 K 448 K 623 K Time per frame 10 s 10 s 10 s 10 s Max.