The Relationship Between Mineral Composition
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Mineralogical Magazine, April 2016, Vol. 80(2), pp. 291–310 The relationship between mineral composition, crystal structure and paragenetic sequence: the case of secondary Te mineralization at the Bird Nest drift, Otto Mountain, California, USA 1,* 2 3 4 5 ANDREW G. CHRISTY ,STUART J. MILLS ,ANTHONY R. KAMPF ,ROBERT M. HOUSLEY ,BRENT THORNE AND 6 JOE MARTY 1 Department of Applied Mathematics, Research School of Physics & Engineering, Mills Rd, Australian National University, Canberra, ACT 0200, Australia 2 Geosciences, Museum Victoria, GPO Box 666, Melbourne 3001, Victoria, Australia 3 Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA 4 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA 5 53898 S. Newport Circle, Bountiful, UT 84010, USA 6 65199 E. Silver Oak Road, Salt Lake City, UT 84108, USA [Received 1 February 2015; Accepted 19 May 2015; Associate Editor: Ian Graham] ABSTRACT An unusually diverse array of 25 secondary Te oxysalt minerals has been documented from Otto Mountain, California, and 18 of these from the Bird Nest drift sublocality. A paragenetic sequence for these minerals is proposed, using observed overgrowth relationships plus spatial association data and data from other localities. Apart from Te and O, the components Pb, Cu and H are essential in the majority of the minerals. The atomic Cu/Te ratio decreases through the paragenetic sequence. This, and the occurrence of minerals − 2À 2À 3+ with additional components such as Cl ,CO3 ,SO4 and Fe at an intermediate stage, suggests non- monotonic evolution of the parent fluids, reflecting differing access to or spatial distribution of various components. For the minerals with known crystal structures, two alternative ‘structural units’ were identified, one consisting only of the Te4+ or Te6+ oxyanion, while the other also included small, strongly-bound cations such as Cu2+. The degree of polymerization for the Te oxyanion correlated with the paragenetic sequence: the monomeric tellurate anions of early minerals were replaced progressively by dimers, chains and sheet structures, which may relate to a decreasing abundance of the ‘network modifying’ Cu2+ cation, analogous to Bowen’s discontinuous reaction series in igneous rock-forming silicates. No relationship was seen between paragenetic order and the larger type of structural unit, or structural complexity as defined by information content. This contrasts with results in the literature for evaporite sulfates and pegmatite phosphates. While structure–paragenesis relationships may be widespread, the exact nature of such relationships may be different for different chemical systems and different paragenetic environments. K EYWORDS: tellurate, tellurite, crystal structure, paragenetic sequence, complexity, secondary minerals, Otto Mountain, California. Introduction *E-mail: [email protected] THERE are many natural environments in which a DOI: 10.1180/minmag.2016.080.001 large number of mineral species is inferred to have © 2016 The Mineralogical Society ANDREW G. CHRISTY ET AL. crystallized sequentially. These paragenetic anion types, such as fluoroaluminates (Pabst, 1950) sequences are often described in mineralogical and borates (Christ, 1960). literature, based on direct observations of textural The vast majority of mineral species are not ‘rock relationships between species in mineral speci- forming’ in that they are not major constituents of mens. The comprehensiveness of such schemes has common igneous, metamorphic or sedimentary often been limited by lack of specimens that bulk compositions. Nevertheless, as noted above, preserve the appropriate contextual data. there are environments, such as strongly fractio- Paragenetic sequences are often recorded as line nated pegmatites and the oxidized zones of ore diagrams, where the paragenesis may be segmented bodies in which large numbers of rare species into categories such as ‘magmatic’, ‘hydrothermal’ occur, and in which distinctive paragenetic and ‘supergene’, as undertaken for the pegmatites sequences can be discerned. This is the case even of Bridger Mountains, Wyoming by McLaughlin for chemically complex and highly diverse meta- (1940), or into ‘stages’, as is the case for the morphosed and metasomatized ore bodies such as variscite nodules from Fairfield, Utah described by the Långban-type deposits of Sweden (Magnusson, Larsen (1942). Variations on these methods have 1930; Moore, 1970a; Jonsson and Broman, 2002). been used for over half a century, especially in Moore (1970b, 1973) made a pioneering attempt topological descriptions of mineral localities (cf. to correlate paragenetic order and crystal structure Keller, 1977; Birch and van der Heyden, 1988), for pegmatite phosphates, somewhat analogous to where this kind of information can assist in the Bowen sequence for igneous rocks, which was identification of species in hand specimens. later revisited in Moore (1982). This made use of a Probably the oldest and most widely applied of hierarchical classification of crystal structures that such paragenetic schemes is the ‘Discontinuous he was developing (Moore, 1970c). The possibility Reaction Series’ of Bowen (1922), which system- of understanding crystallization sequences in terms atizes the observation that different types of rock- of structure has been a major driving force for forming silicate minerals appear on the liquidus of a developing further structural taxonomies since then melt as it evolves in composition and decreases in (e.g. Hawthorne, 1983; Moore, 1984; Hawthorne, temperature. The solution of silicate crystal struc- 1985, 1990 and references therein). Advances in tures and their classification by Bragg (1930) made these classifications have led to several structure– it apparent that the silicate anions of the minerals paragenesis analyses for restricted compositional varied systematically in their degree of polymer- groups of minerals: evaporitic Mg sulfates ization as a function of liquidus temperature, (Hawthorne, 1992), vanadium minerals (Schindler ranging in topology from monomers and finite et al., 2000), borates (Schindler and Hawthorne, clusters, through infinite chains and sheets to three- 2001a,b,c) and uranyl oxyhydroxides (Schindler dimensional frameworks. It later became apparent and Hawthorne, 2004). that this gradation paralleled structural changes in In general, structural classifications have used the parent melt (cf. Mysen, 1983). Monomeric or empirical classification of basic principles. oligomeric silicate anions are associated with low Hawthorne (2014) summarizes the approach used silica concentration, but a high concentration of in his earlier works, where the bonded network of a cations such as Mg2+ that prefer six-fold coordin- structure is divided on the basis of bond valence ation by oxygen through bonds of moderately high (Brown, 1981) into a tightly bound ‘structural unit’ valence (∼0.3 valence units (vu), cf. Brown, 1981) and more weakly bound ‘interstitial complex’. and hence do not bond readily to bridging oxygen Explicit separation of octahedrally coordinated M atoms within silicate polymers. Conversely, highly cations, usually of valence 2–3, and tetrahedrally polymerized (alumino)silicate anions occur for coordinated T cations of higher valence (cf. high silica concentrations and counterions that Hawthorne, 1985, 1990), implies that the structural have less charge and larger coordination number unit itself can be divided further to give an (i.e. less Lewis acidic), such as Na+ and K+. Warren additional level of hierarchy. Note also that many and Pincus (1940) coined the descriptions ‘network large or infinitely extended structural units are forming’, for the former type of counterion and polymers of distinctive oligomeric clusters termed ‘network modifying’ for the latter in synthetic ‘fundamental building blocks’ (FBB) (Hawthorne, glasses; these terms were later applied to silicate 1983, 1990, 2014). It has been noted that only some melts by Bottinga and Weill (1972). The idea of of the geometrically possible FBB occur with any classifying minerals by structure, and particularly frequency, and that these particularly stable clusters degree of polymerization, was later applied to other may also be present in the parent fluid from which 292 BIRD NEST DRIFT Te PARAGENESIS the minerals grew (cf. Moore, 1970b,c; Hawthorne, The suite of Te oxysalt minerals from Otto 1979; Moore, 1984; Burns, 1995; Hawthorne, Mountain is large enough and diverse enough in 2014). Krivovichev et al. (2013) review structural structure to recommend investigation of the rela- hierarchies that are constructed using anion-centred tionship between structure and paragenetic coordination polyhedra rather than the more usual sequence. Therefore, we reviewed the published cation-centred polyhedra. This approach can be data on the locality and its minerals, in order to simpler when some of the cations are irregularly- collate textural observations which would allow us coordinated species with stereoactive lone pairs. to construct a paragenetic sequence for the Te More recently, methods for quantitatively calcu- oxysalt minerals. lating the complexity of crystal structures have been investigated by Estevez-Rams and González-Férez (2009), Steurer (2011) and Krivovichev (2013a,b, Occurrence and paragenesis of Te oxysalt 2014). The complexity parameter of Krivovichev is minerals at Otto Mountain based