2M Polytype Research Articles

Dioskouriite, CaCu4Cl6(OH)4∙4H2O: A New Mineral Description, Crystal Chemistry and Polytypism

A new mineral, dioskouriite, CaCu4Cl6(OH)4∙4H2O, represented by two polytypes, monoclinic (2M) and orthorhombic (2O), which occur together, was found in moderately hot zones of two active fumaroles, Glavnaya Tenoritovaya and Arsenatnaya, at the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. Dioskouriite seems to be a product of the interactions involving high-temperature sublimate minerals, fumarolic gas and atmospheric water vapor at temperatures not higher than 150 °C. It is associated with avdoninite, belloite, chlorothionite, eriochalcite, sylvite, halite, carnallite, mitscherlichite, chrysothallite, sanguite, romanorlovite, feodosiyite, mellizinkalite, flinteite, kainite, gypsum, sellaite and earlier hematite, tenorite and chalcocyanite in Glavnaya Tenoritovaya and with avdoninite and earlier hematite, tenorite, fluorophlogopite, diopside, clinoenstatite, sanidine, halite, aphthitalite-group sulfates, anhydrite, pseudobrookite, powellite and baryte in Arsenatnaya. Dioskouriite forms tabular, lamellar or flattened prismatic, typically sword-like crystals up to 0.01 mm × 0.04 mm × 0.1 mm combined in groups or crusts up to 1 × 2 mm2 in area. The mineral is transparent, bright green with vitreous luster. It is brittle; cleavage is distinct. The Mohs hardness is ca. 3. Dmeas is 2.75(1) and Dcalc is 2.765 for dioskouriite-2O and 2.820 g cm−3 for dioskouriite-2M. Dioskouriite-2O is optically biaxial (+), α = 1.695(4), β = 1.715(8), γ = 1.750(6) and 2Vmeas. = 70(10)°. The Raman spectrum is reported. The chemical composition (wt%, electron microprobe data, H2O calculated by total difference; dioskouriite-2O/dioskouriite-2M) is: K2O 0.03/0.21; MgO 0.08/0.47; CaO 8.99/8.60; CuO 49.24/49.06; Cl 32.53/32.66; H2O(calc.) 16.48/16.38; -O=Cl −7.35/−7.38; total 100/100. The empirical formulae based on 14 O + Cl apfu are: dioskouriite-2O: Ca1.04(Cu4.02Mg0.01)Σ4.03[Cl5.96(OH)3.90O0.14]Σ10∙4H2O; dioskouriite-2M: (Ca1.00K0.03)Σ4.03(Cu4.01Mg0.08)Σ4.09[Cl5.99(OH)3.83O0.18]Σ10∙4H2O. Dioskouriite-2M has the space group P21/c, a = 7.2792(8), b = 10.3000(7), c = 20.758(2) Å, β = 100.238(11)°, V = 1531.6(2) Å3 and Z = 4; dioskouriite-2O: P212121, a = 7.3193(7), b = 10.3710(10), c = 20.560(3) Å, V = 1560.6(3) Å3 and Z = 4. The crystal structure (solved from single-crystal XRD data, R = 0.104 and 0.081 for dioskouriite-2M and -2O, respectively) is unique. The structures of both polytypes are based upon identical BAB layers parallel to (001) and composed from Cu2+-centered polyhedra. The core of each layer is formed by a sheet A of edge-sharing mixed-ligand octahedra centered by Cu(1), Cu(2), Cu(3), Cu(5) and Cu(6) atoms, whereas distorted Cu(4)(OH)2Cl3 tetragonal pyramids are attached to the A sheet on both sides, along with the Ca(OH)2(H2O)4Cl2 eight-cornered polyhedra, which provide the linkage of the two adjacent layers via long Ca−Cl bonds. The Cu(4) and Ca polyhedra form the B sheet. The difference between the 2M and 2O polytypes arises as a result of different stacking of layers along the c axis. The cation array of the layer corresponds to the capped kagomé lattice that is also observed in several other natural Cu hydroxychlorides: atacamite, clinoatacamite, bobkingite and avdoninite. The mineral is named after Dioskouri, the famous inseparable twin brothers of ancient Greek mythology, Castor and Polydeuces, the same in face but different in exercises and achievements; the name is given in allusion to the existence of two polytypes that are indistinguishable in appearance but different in symmetry, unit cell configuration and XRD pattern.

Molten salt synthesis of Ce doped zirconolite for the immobilisation of pyroprocessing wastes and separated plutonium

Molten salt mediated synthesis of zirconolite Ca0.9Zr0.9Ce0.2Ti2O7 was investigated, as a target ceramic matrix for the clean-up of waste molten salts from pyroprocessing of spent nuclear fuels and the immobilisation of separated plutonium. A systematic study of reaction variables, including reaction temperature, time, atmosphere, reagents and composition, was made to optimise the yield of the target zirconolite phase. Zirconolite 2M and 3T polytypes were formed as the major phase (with minor perovskite) between 1000 – 1400 °C, in air, with the relative proportion of 2M polytype increasing with temperature. Synthesis under 5% H2/N2 or Ar increased the proportion of minor perovskite phase and reduced the yield of the zirconolite phase. The yield of zirconolite polytypes was maximised with the addition of 10 wt.% TiO2 and 5 wt.% TiO2, yielding 91.7 ± 2.0 wt.% zirconolite, primarily as the 2M polytype, after reaction at 1200 °C for 2 h, in air. The particle size and morphology of the zirconolite product bears a close resemblance to that of the TiO2 precursor, demonstrating a dominant template growth mechanism. Although the molten salt mediated synthesis of zirconolite is effective at lower reaction temperature and time, compared to reactive sintering, this investigation has demonstrated that the approach does not offer any clear advantage with over conventional reactive sintering for the envisaged application.

The nature of Zn-phyllosilicates in the nonsulfide Mina Grande and Cristal zinc deposits (Bongará District, Northern Peru): The TEM-HRTEM and AEM perspective

Abstract Zn-phyllosilicates are common minerals in nonsulfide Zn deposits and can give crucial information about the genesis of these oxidized mineralizations. They seldom represent the prevailing economic species but might have a significant impact on mineral processing. This study has been carried out on the Mina Grande and Cristal Zn-sulfide/nonsulfide deposits, which occur in the Bongará district (Amazonas region, northern Peru). The Cristal and Mina Grande orebodies are hosted by the sedimentary (prevailingly carbonate) successions of the Pucará Group (Condorsinga formation, Lower Jurassic), in an area affected by Neogene tectonics and characterized by Late Miocene and Pliocene-Early Pleistocene uplift phases (Andean and Quechua tectonic pulses). The Cristal deposit consists of both sulfide (sphalerite with minor pyrite and galena) and nonsulfide concentrations. The nonsulfides consists of smithsonite, hemimorphite, hydrozincite, chalcophanite, goethite, and greenockite, locally associated with Zn-bearing phyllosilicates. The Mina Grande deposit consists almost exclusively of Zn-oxidized minerals in limestone host rocks. The nonsulfides association consists of hydrozincite, hemimorphite, smithsonite, fraipontite, and Fe-(hydr)oxides, also containing a clayey fraction. The study deals with TEM-HRTEM and AEM investigations on clayey materials, to determine their crystal-chemical features and the origin of the complex Zn-clays-bearing parageneses. In both deposits, Zn-bearing illites (1Md and 2M polytypes) and I/S clay minerals (I3) are the main detected phases, with few compositions close to (Zn-bearing) muscovite. In the clayey fraction at Mina Grande, fraipontite, a Zn-bearing mica called K-deficient hendricksite, and (Zn-bearing) kaolinite also occur. Zn-illites and smectites (always containing Zn in variable amounts) characterize the mineral association at Cristal. The investigated compositional gap between di- and tri-octahedral Zn-phyllosilicates gives indications on the genetic relationships between them and advances on the knowledge of these species. The present work gives an insight into the Zn-bearing phyllosilicates systems by determining the amount/mode of metal incorporation in their lattices and understanding the relationships of natural occurring clay-rich complex associations, which can act as models for possible synthetic counterparts.

GENESIS AND PHYSICAL CHARACTERISTICS OF THE NEOGENE RED BEDS FROM THE CEDARHILS OF THESSALONIKI, MACEDONIA , GREECE

The genesis and the physical characteristics of the Neogene red beds of the cedar hills surrounding Thessaloniki are studied in this paper. The peri-urban forest, which covers these hills, has a 3,022 ha area. The topographic relief is smooth and is divided in eight small drainage basins, tapped through small creeks. The elevation of the surrounding hills varies between 85 and 560 m. The dominant land slopes vary between 20 and 55%. All the samples are coarse grained, poorly sorted and friable and present earthy lustre and red colour because of the extensive presence of iron oxides. Angular to sub-angular rock fragments derived from the metamorphic bedrock are very common. Petrographically, the studied red beds belong to the clayey sands. The extended presence (41-66%) of coarse silt and sand size grains (>20 pm) in the samples suggests a mild intensity of in situ weathering of the bedrock. X-ray diffraction analysis of the coarsest fractions 250-20 pm and 20-2 μιτι revealed in decreasing abundance the presence of quartz, feldspars, epidote, micas, chlorite, pyroxenes, amphiboles, and talc. These fractions contain the 2M polytype of mica, while in the fraction <2 pm the 1Μα polytype of illite predominates. In the clay fraction (<2 pm) illite, smectite, and chlorite predominate. The presence of mixed-layer minerals is limited, testifying the almost complete character of hydrolysis of the primary minerals. The formation of red beds took place on low relief land under alternating wet and dry seasons, which prevail in the eastern Mediterranean region since Neogene. The clay minerals are the in situ weathering products of the primary minerals of the greenschists, gneisses and gabbros predominating in the studied area. The extensive presence of clay size grains (11-26%) in the samples, their poor sorting, and their sub-angular morphology, indicate that the red beds are texturally immature. In addition, the abundance of feldspars and Fe-Mg minerals reflects mineralogical immaturity. The low relief and the long-lasting tectonic stability in the Thessaloniki district were essential for the significant thickness of the red beds. The oxygen isotope data of the <0.2 pm fraction (+18.2 to +18.8%o) confirm the pedogenic origin of the clay minerals present. The red beds studied present low plasticity with liquid limit (WÏ) 26.9 to 33.4% and plasticity index (lp) 9.1 to 17.3%. In addition, they have high consolidation index (lc) values (1.03 to 2.28). The swelling potential is low to medium and the activity varies between 0.5 and 1.0. The consolidation and induration degree of the samples analyzed is low, because of the great range of their mineralogical composition and the mild conditions of pressure and temperature to which they have been submitted. The studied red beds are not considered problematic for the foundation of various constructions on them.

A Review of the Structures of Vaterite: The Impossible, the Possible, and the Likely

The importance of the metastable vaterite form of CaCO3 is increasingly appreciated as an intermediate between amorphous calcium carbonate and more stable crystalline polymorphs in biogenic calcification pathways. Thus, there is a need to be able to identify nanocrystals of vaterite unambiguously, despite uncertainties about its exact crystal structure. The structures that have been proposed for vaterite are all based upon a common average structure with orientationally disordered carbonate groups. However, there is much ambiguity in the literature about the actual superstructure, and hence diffraction behavior of vaterite, some misinterpretation of data, and some claims that cannot be correct. This critical review of the literature aims to resolve the issue. The orientationally disordered (2H) structure is unlikely to occur, and the most plausible structures all share a √3 × √3 superstructure in the xy plane. Most experimental evidence is consistent with occurrence of 2M and 6H polytypes, but ab initio m...

Clay mineralogy of the Boda Claystone Formation (Mecsek Mts., SW Hungary)

AbstractBoda Claystone Formation (BCF) is the host rock of the planned site for high level nuclear waste repository inHungary. Samples representing the dominant rock types of BCF were studied: albitic claystone, claystone with high illite content, and analcime bearing claystone. Clay minerals in these three rock types were characterized by Xray powder diffraction (XRD), transmission electron microscopy (TEM) and thermal analysis (DTA-TG), and the results were discussed from the point of view of the radionuclide sorption properties being studied in the future. Mineral compositions of bulk BCF samples vary in wide ranges. In the albitic sample, besides the dominant illite, few percent of chlorite represents the layer silicates in the clay fraction. Illite is the dominating phase in the illitic sample, with a few percent of chlorite. HRTEM study revealed that the thickness of illite particles rarely reaches 10 layers, usually are of 5-6 TOT layer thick. Illite crystals are generally thicker in the albitic sample than in the illitic one. The significant difference between the clay mineral characterisitics of the analcimous and the other two samples is that the former contains regularly interstratified chlorite/smectite beside the dominant illite.Based on the structural and chemical data two illite type minerals are present in the BCF samples: 1M polytype containing octahedral Fe and Mg besides Al, 2M polytype illite generally is free of Fe andMg. Close association of very thin illite plates and nanosized hematite crystals is typical textural feature for BCF.The goal of this study is to provide solid mineralogical basis for further studies focusing on radionuclide sorption properties.

Kermesite, Sb2S2O: crystal structure revision and order–disorder interpretation

Kermesite, Sb2S2O, is a desymmetrized order-disorder (OD) structure of layers. Two data sets were recorded using twinned crystals from Pezinok, Slovakia (named as Pz21, Pz24). The primitive unit cell is triclinic, P1, Z = 4, cell parameters are a = 8.1416 (3), b = 10.6968 (3), c = 5.7835 (2) Å, α = 102.758 (3), β = 110.657 (3), γ = 101.020 (3)°, R(obs) = 0.0243 (Pz21), and a = 8.1372 (2), b = 10.6969 (2), c = 5.7840 (1) Å, α = 102.787 (2), β = 110.606 (2), γ = 100.983 (2)°, R(obs) = 0.0321 (Pz24). The structure can also be described in the non-standard pseudo-monoclinic octuple (Z = 32), F-centered (Kupčík) cell with extra points in 1/4,1/4,0; 1/4,3/4,1/2; 3/4,1/4,1/2; 3/4,3/4,0, with parameters a = 21.6466 (9), b = 8.1416 (3), c = 20.3824 (9) Å, α = 90.079 (4), β = 101.985 (5), γ = 89.948 (4)° (Pz21), and a = 21.6558 (5), b = 8.1372 (2), c = 20.3859 (8) Å, α = 90.028 (3), β = 101.994 (3), γ = 89.986 (2)° (Pz24). The structure is built of layers parallel to the bc plane, stacked along the a vector of the octuple cell, composed of ribbons parallel to the b vector: (i) ribbon of two strips of SbO5 flattened quadrangular pyramids, sharing apical edges; (ii) ribbon of edge-sharing corrugated lozenges SbO3S. Basal S atoms of pyramids share corners of lozenges. Sb atoms are displaced out of coordination polyhedra into the inter-layer space. The OD layer comprises adjacent halves of the structure building layers. The layer group is A(1)2/m1, the protocell is defined by b, c, (a/4)sin β. The MDO1 (4A) polytype is generated by repetition of the t(1,1/4,0) [or alternatively t(1,-1/4,0)] translation. The co-existence of two kinds of domains give rise to the twinning. The twin operation is 2[010], twin index 2. The total continuation of [. a2 .] generates the MDO2 (2M) polytype, space group A12/a1. Simulated and real diffraction patterns are presented. The important values (edges, angles) and displacements of atoms due to the desymmetrization were evaluated. The comparison with structures of stibnite, bismuthinite and aikinite is added.

Polytypism, Disorder, and Anion Exchange Properties of Divalent Ion (Zn, Co) Containing Bayerite-Derived Layered Double Hydroxides

Incorporation of Zn(2+) into bayerite results in the formation of a cation-ordered layered double hydroxide (LDH) of monoclinic symmetry in which about half the vacancies of Al(OH)(3) are occupied by Zn(2+) giving rise to positively charged layers. Charge compensation takes place by the incorporation of sulfate ions in the interlayer region. Structure refinement reveals that the adjacent layers in the crystal are related by a 2(1) axis (we call it 2M(1) polytype) with sulfate coordinating in the D(2d) symmetry in the interlayer region. Another polytype in which adjacent layers are related by a 2-fold axis (2M(2) polytype) can also be envisaged. Faulted crystals arising from intergrowths of the 2M(2) polytype within the 2M(1) structure were also obtained. These bayerite-based LDHs have a distinctly different interlayer chemistry when compared to the better known brucite-based LDHs, in that they have a strong affinity for tetrahedral ions such as SO(4)(2-), CrO(4)(2-), and MoO(4)(2-) and a poor affinity for CO(3)(2-) ions. These observations have implications for the use of LDHs in applications related to chromate sorption.

Synthesis and characterisation of Pu-doped zirconolites –(Ca1−xPux)Zr(Ti2-2xFe2x)O7

Substituting Pu(IV) onto the Ca site and using Fe(III) as a charge balancing species, a series of 6 compositions of 239Pu doped zirconolite (of general formula (Ca1−xPux)Zr(Ti2-2xFe2x)O7) have been fabricated, where x = 0.1, 0.2, 0.3, 0.4, 0.6 and 0.7. Sintered pellets from each of these compositions have been characterised by XRD, SEM, EDX and Raman techniques in order to analyse the change in microstructure and determine the polytype boundaries as the doping level increases, showing that the zirconolite is stable in the natural 2M polytype up until x = 0.3, then adopting the 3T polytype from x = 0.3 to x = 0.4, but from there onwards an increasing degree of phase separation occurs.

Neutron and Resonant X-ray Diffraction Studies of Zirconolite-2M

AbstractZirconolite (nominally CaZrTi2O7) is a constituent phase of potential waste forms for the safe immobilisation of actinide wastes. Structural studies of such materials provide important information about cation ordering, lattice parameters, and strain effects, and provide input into the modelling of alpha decay damage and the development of future wasteform designs. A suite of zirconolites based on the replacement of Ti with Nb and Fe has been studied using high resolution neutron diffraction and resonant X-ray diffraction to determine the degree of disorder across the available cation sites. Resonant X-ray diffraction is a unique method which allows the location of certain cations to be determined accurately by taking advantage of the change in scattering power close to an absorption edge (e.g., Nb-K and Zr-K). Using standard X-ray diffraction alone this is not possible and there is little scattering difference between Nb and Zr.Raman spectroscopy and measured lattice parameters have shown that the exchange of Ti with Nb and Fe has a non-linear effect on the unit cell dimensions and Raman peak positions while retaining the 2M polytype. Mössbauer spectroscopy has shown that the Fe preferentially fills the Ti split (C2) site. The results from this study provide a more complete picture of the cation order-disorder problem and are generally consistent with the behaviour of lattice parameters across the series.

Stacking structures in pyrophyllite revealed by high-resolution transmission electron microscopy (HRTEM)

Stacking structures in pyrophyllite, Al2Si4O10(OH)2, were investigated mainly by using high-resolution transmission electron microscopy (HRTEM). The specimens examined were large lath-shaped crystals (Berosovska, Urals, Russia) and massive aggregates of fine platy crystals (Nohwa, southwest Korea). Both specimens showed powder X-ray diffraction (XRD) patterns similar to those reported previously as the 2M polytype. The common stacking sequence in the two specimens is not monoclinic with two-layer periodicity as previously reported, but a uniform orientation of the 2:1 layers and near complete disorder of two alternative directions of interlayer displacement, i.e., lateral displacement between the two tetrahedral sheets across an interlayer region. The directions of interlayer displacement are about ± 2π/3 from that of the intralayer shift (lateral displacement between the two tetrahedral sheets within a 2:1 layer). Simulation of powder XRD patterns by this stacking model closely approximates the experimental pattern. Elongation of the lath-shaped Berosovska crystals corresponds to the direction of the intralayer shift, as seen in illite-1M.

Crystal chemistry and polytypism of tyrolite

The crystal structures of the 1M and 2M polytypes of tyrolite have been solved from single-crystal X-ray diffraction data. The structure of tyrolite-1M [monoclinic, P2/c, a = 27.562(3), b = 5.5682(7), c = 10.4662(15) Å, β = 98.074(11)°, V = 1590.3(3) Å 3 ] has been refined to R 1 = 0.086 on the basis of 2522 unique observed reflections collected using synchrotron radiation at the Swiss-Norwegian beamline BM01 of the European Synchrotron Research Facility (SNBL at the ESRF). The structure of tyrolite-2M [monoclinic, C2/c, a = 54.520(6), b = 5.5638(6), c = 10.4647(10) Å, β = 96.432(9)°, V = 3154.4(6) Å 3 ] has been refined to R 1 = 0.144 on the basis of 2666 unique observed reflections obtained from a non-merohedrally twinned crystal using in-house X-ray radiation and a STOE IPDS II imageplate diffractometer. The structures are based upon complex nanolayers consisting of Cu, As, and Ca coordination polyhedra. The core of the nanolayer is a copper arsenate substructure consisting of A and B sublayers. The B sublayer consists of chains of edge-sharing Cu octahedra running along the b axis. The A sublayer contains trimeric units of Cu octahedra sharing corners with AsO 4 tetrahedra. Two adjacent A sublayers are linked by the octahedral chains of the B sublayer resulting in formation of the 18 Å thick ABA slab. The ABA slab is sandwiched between sublayers of Ca 2+ cations and H 2 O molecules. Adjacent nanolayers are connected by hydrogen bonds to the interlayer species (carbonate anions and H 2 O molecules). The structures of tyrolite-1M and tyrolite-2M differ by the stacking sequence of the nanolayers only. The adjacent nanolayers in tyrolite-2M are shifted by b/2 = 2.8 Å in comparison to the relative position of the nanolayers in tyrolite-1M. The structural formula of tyrolite can be written as [Ca 2 Cu 9 (AsO 4 ) 4 (OH) 8 (CO 3 )(H 2 O) 11 ](H 2 O) x where x = 0-1.

Clinopyroxene exsolution in wollastonite from Namaqualand granulite, South Africa

Chemical and crystallographic properties of clinopyroxene exsolution in wollastonite are described from metamorphosed calc-silicate granulite, Namaqualand, South Africa. The wollastonite is Ca1.96Fe0.01Al0.01Si2.01-O6 belonging to space group P21/a (2M polytype) and the clinopyroxene is Ca0.99Mg0.75-0.80Fe0.17-0.21Na0.02Al0.03Si1.99-2.00O6 belonging to C2/c. An electron backscattered diffraction investigation suggests that the clinopyroxene lamellae elongated along [11̅1] lie on (120) and (100) of the wollastonite-2M, and [11̅0] of both lamellae are parallel to [001] of the wollastonite-2M. The formation of the exsolution probably results from the relatively high peak metamorphic temperature (800-860ºC) of the Namaqualand granulite and its slow cooling rate.

Microchemical and crystallographic characterisation of fluorite-based ceramic wasteforms

ABSTRACTA number of possible options have been proposed for the encapsulation and immobilisation of long lived actinide (Act) fractions in nuclear waste. Ceramics offer superior durability against chemical migration and the ability to be tailored to accommodate a variety ofdifferent waste streams. Research on the fabrication of dense, durable crystalline matrices for the safe disposal of fissile plutonium is ongoing and this study reports quantitative chemical, structural and spectroscopic analysis on fluorite based host phases.Ceramics based on the fluorite structure are known to be able to incorporate a variety of actinides and in this work two candidate ceramic matrices were investigated: a pyrochlore, Gd2Zr1.60Ce0.20Hf0.20O7; and a zirconolite, (Ca0.90Gd0.10)(Zr0.50Ce0.20Hf0.20Gd0.10)Ti2O7. The chemical compositions of the two major phases observed in the ‘zirconolite’ sample were consistent with the 2M and 4M zirconolite polytypes and the presence of the 4M structure was confirmed by Electron Diffraction (ED). The major phase in the ‘pyrochlore’ ceramic was confirmed by ED to have the pyrochlore structure. Electron Energy Loss Spectroscopy (EELS) data indicated the presence of both Ce3+ and Ce4+ in all the samples.

Incorporation of Uranium in Zirconolite (CaZrTi2O7)

Up to 0.15 formula unit (f.u.) of U4+ incorporated in the Zr site of zirconolite by firing CaUxZr(1−x)Ti2O7 compositions in argon at 1400°C allows retention of the 2M polytype. Further U4+ substitution for Zr, up to 0.4 f.u., produces the 4M polytype (containing ∼0.4 f.u. of U) plus the 2M polytype containing 0.15 f.u. of U. The pyrochlore structure (containing 0.6 f.u. of U) forms in conjunction with the 4M polytype at U contents of 0.4 f.u. up to 0.7 f.u. Higher U contents give the pyrochlore structure, but the solid‐state reactivity of even alkoxide‐based preparations becomes increasingly poor for x > ∼0.7 and hot pressing in graphite dies at ∼1250°C is necessary to achieve near single‐phase pyrochlore structures for x= 1. When samples of CaUxZr1−xTi2O7 stoichiometry (x= 0.1 and 0.2) are oxidized at 1400°C in air, diffuse reflectance spectroscopy (DRS) shows evidence for U5+ formation at the expense of U4+ via enhanced absorption bands, ∼50 nm in half‐width, near 970 and 1500 nm and correspondingly weakened absorption at 1150 and 1660 nm. Weight gains consistent with complete oxidation of U4+ to U5+ are observed when finely powdered argon‐fired samples with x= 0.1 and 0.4 are heated in air to 1200°C. Evidence for U valence states higher than +4 in both argon‐ and air‐heated materials containing charge compensators to encourage U5+ or U6+ formation was also derived from DRS (showing U5+ in particular), and XANES. DRS shows weak absorption bands attributable to U4+ in zirconolites containing ∼0.2 f.u. of U incorporated in the Ca site via Mg or Al substitutions in the Ti sites, with the spectrum being closely but not exactly similar to that attributed to U4+ in the Zr site. Zirconolite and pyrochlore compositions which are melted at 1500° or 1550°C in argon and furnace‐cooled yield broadly similar phase assemblages to the corresponding sintered materials, but there is evidence of incongruent melting in all materials.

Clay Mineralogy of Phyllic and Intermediate Argillic Alteration at Bingham, Utah

The earliest alteration mineral zones in the porphyry Cu-Mo-Au ore deposit at Bingham, Utah, include the outermost propylitic and inner potassic alteration of igneous rocks. Innermost phyllic and intermediate argillic alteration is superimposed on propylitic and potassic alteration. Petrographic, X-ray diffraction, and electron microprobe analyses are used to characterize the clay mineralogy of phyllic and intermediate argillic alteration. Phyllic alteration affects monzonite near the contact with the quartz monzonite porphyry in the southwest and northeast portions of the deposit and consists primarily of veins and vein selvages of illite interstratified with an average of 7 percent smectite (R >3 ordering). The veins and selvages also contain small amounts of smectite and kaolinite. The illite/smectite interstratification is a mixture of 1M and 2M polytypes. Structural formulas for the illite show less K and Al and more Si than muscovite, octahedral Mg, and both a tetrahhedral and an octahedral layer charge. The Kubler crystallinity index and smectite content increase systematically as particle size decreases, consistent with an increase in smectite in the interstratification in the smaller size fractions. Intermediate argillic alteration consists of smectite, illite, and kaolinite that occur primarily as a replacement of plagioclase phenocrysts. A few veins of intermediate argillic minerals are present in the quartz monzonite porphyry. Illite is interstratified with an average of 6 percent smectite (R >3 ordering), but a few samples contain a regular interstratification of illite with 40 to 50 percent smectite (R1 ordering). The smectite replacing plagioclase is montmorillonite, and smectite that occurs in one vein is beidellite. Statistical trends in intermediate argillic alteration mineralogy within the quartz monzonite porphyry show that illite and kaolinite abundances increase generally upward and toward the northeast. Smectite abundance increases generally downward and toward the southwest. In a cross section near the center of the deposit, abundances of kaolinite and illite decrease as abundance of smectite increases with increasing depth. The occurrence of interstratified illite/smectite averaging 6 to 7 percent smectite together with discreet smectite suggest formation near 200°C. A temperature difference between phyllic and intermediate argillic alteration could not be resolved from the mineralogy. Temperature estimates from fluid inclusions for phyllic alteration at other porphyry copper deposits generally range from 200° to 350°C. The activity ratio of Na + /K + for the fluid calculated from Na + /K + in illite averages 5.0 for a phyllic vein that crosscuts propylitic alteration, 1.4 for a phyllic vein that crosscuts potassic alteration, and 2.8 for intermediate argillic alteration superimposed on potassic alteration. Log f H 2 O / f HF of the hydrothermal fluid computed from the fluorine content of illite averages 6.1 for a phyllic vein that crosscuts propylitic alteration, 5.8 for a phyllic vein that crosscuts potassic alteration, and 5.7 for intermediate argillic alteration. Log f H 2 O / f HF of fluids forming late-stage biotite is about 5.0. Intermediate argillic alteration could not be correlated with sulfides in the quartz monzonite porphyry. An increase in illite upward and toward the northeast may correlate with massive sulfides in sediments recently discovered in that region.

Transmission Electron Microscopy Study of Very Low-Grade Metamorphic Rocks in Cambrian Sandstones and Shales, Ossa-Morena Zone, Southwest Spain

AbstractThis study examines the evolution of the texture, structure, and chemical composition of rocks derived from clastic materials of the Ossa-Morena Zone (Hesperian Massif, Spain). Previous studies of phyllosilicates in these rocks (by X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray analysis, and electron microprobe) indicated a temperature decrease from bottom (epizone conditions) to top (diagenetic conditions) of the rock section.At the nanometer scale, phyllosilicate packets form large angles where grains intersect with no preferred orientation. With metamorphic grade, packets are wide and defect free, compared to packets at lower grade. These packets are ∼ 15 layers under diagenetic conditions to >80 layers in the epizone. Dioctahedral K-rich micas (muscovite, phengite, and illite) have coexisting 1Md, 1M, and 2M polytypes. Long-period polytypes of 4, 5, and 6 layers are reported for the first time in dioctahedral K-rich micas. The chemical compositions of the micas are nearly identical in the anchizone and the diagenetic zone, comprising an illitic (0.8 atoms per formula unit, a.f.u., of K) and a phengitic component (0.15 a.f.u, of Mg and 0.13 a.f.u, of Fe). Fe may correspond to a ferrimuscovitic substitution. Epizone samples have a high phengitic content (Mg = 0.24 a.f.u.) and almost no illite component. One diagenetic sample has coexisting berthierine, trioctahedral chlorite, sudoite, and corrensite. Berthierine and chlorite are identical in composition. Because of the clastic nature of the system, the composition of corrensite is not typical of other corrensites, with higher Al content, Fe/Mg ratio at ∼1, and K as the exchangeable cation.Textural differences between the diagenetic zone and the anchizone are the progressive increase in the size of dioctahedral K-rich mica grains, which involves an increasing illite crystallinity based on the Kübler index. The chemical compositions of these micas are illite (diagenesis and anchizone) and phengite in the epizone. There are no intermediate phases, suggesting a compositional gap between illite and phengite. The coexistence of different polytypes of dioctahedral K-rich micas and the absence of chemical homogeneity indicate disequilibrium in the Cambrian pelitic rocks studied.

Crystallisation Kinetics and Phase Relations of Wollastonite by Real Time Synchrotron Powder Diffraction

The crystn. kinetics of 1T-wollastonite in the CaO-SiO2 system starting from a stoichiometric glass was investigated in situ by x-ray synchrotron powder diffraction at 800-900�. At higher temp., 1T-wollastonite converts into the 2M polytype. The collected data were analyzed by the Avrami models and kinetic parameters seem to indicate a nucleation deceleratory controlled mechanism with 1D growth. The preliminary results of this study are part of a general project aimed to investigate polymorphism and polytypism of wollastonite and the related phases, esp. the role of the kinetic parameters in the formation of the polytypes in the pyroxenoid series. [on SciFinder(R)]

Oxygen Isotope Compositions of Mixed-Layer Serpentine-Chlorite and Illite-Smectite in the Tuscaloosa Formation (U.S. Gulf Coast): Implications for Pore Fluids and Mineralogic Reactions

AbstractOxygen isotopic compositions were determined for coexisting mixed-layer serpentine-chlorite (Sp-Ch) and illite-smectite (I-S) from 5 Tuscaloosa Formation sandstone cores sampled between 1937 and 5470 m burial depth. High gradient magnetic separation (HGMS) was used to concentrate Sp-Ch and I-S from the <0.5 μm fraction of each core sample into fractions with a range in the Sp-Ch: I-S ratio, and end-member δ18O compositions were determined by extrapolation. The Sp-Ch δ18O values range from + 10.4 to 13.7% and increase with burial between 3509 and 5470 m. The only exception is Sp-Ch from 1937 m, which has an anomalously high δ18O value of +12.6‰ The I-S δ18O values range from +16.1 to 17.3% and do not change significantly between 3509 and 5470 m burial depth.Pore water δ18O compositions calculated from Sp-Ch and I-S values and measured borehole temperatures range from −2.6 to +10.3‰ The isotopically light values indicate that Sp-Ch formed at shallow burial depths in the presence of brackish to marine water and/or meteoric water. The depth-related increase in δ18O of Sp-Ch is attributed to oxygen exchange between mineral and pore water during diagenetic mineral reactions. Increasing δ18O values, in conjunction with XRD and SEM data, indicate that transformation of serpentine layers to chlorite layers and Ibb polytype layers to Iaa polytype layers occurred on a layer-by-layer basis when individual layers dissolved and recrystallized within the confines of coherent crystals. Possible explanations for the variation in I-S δ18O values include depth-related differences in pore water δ18O values present at the time of I-S crystallization, contamination by detrital 2M, mica and 1M polytype rotations that facilitated oxygen exchange.

Refinement of the crystal structure of zinnwaldite 2M1

Crystal structure of zinnwaldite 2M(1) was refined in the Cc space group with R equal to 5.8% and R(w) 7.5%. Its octahedral sheet is hetero-octahedral, with octahedra around M1 and M3 having the same mean cation-anion bond length, whereas the third (around M2) has the mean bond significantly shorter. In terms of electrons, the cation ordering between M1 and M3 is less pronounced (16.2 e- vs 17.4 e-) than that between M1 (M3) and M2 (14.3 e-). Li and Fe2+ prefer M1 (M3) to M2, Al does the opposite. Thus, like zinnwaldite 1M (Guggenheim & Bailey, 1977), the 2M(1) polytype is hetero-octahedral. This is the first two-layer hetero-octahedral mica polytype known to date. The degree of tetrahedral cation ordering is very low in both zinnwaldites. Also, distortion characteristics of individual octahedra and tetrahedra in these zinnwaldites are very close to each other. A comparison of the details of structures of 1M and 2M(1) biotite and 1M and 2M(1) zinnwaldite gave no indication that a geometrical or other advantage is associated with either 2M(1) or 1M stacking in zinnwaldite.