Manganotantalite replacement by (1) microlite and (2) ferrotantalite, and changes in composition of uranoan microlite from rare-metal pegmatites in South Africa and Namibia have been investigated with the electron microprobe. A uranmicrolite from Karibib, Namibia contained 14.35% UO2, 1.03% PbO, 56.12% Ta2O5, 13.18% Nb2O5, 0.58% Fe2O3, 6.87% CaO, 0.54% SrO, 0.59% MnO, 0.86% Na2O and 0.47% F. Analyses along traverses across a 1.3 mm uranoan microlite, Tantalite Valley, Namibia, revealed two essentially distinct compositions: a more hydrated rim area of 200 µm radius containing 7% higher Ta2O5, 10% lower CaO and 1.3% lower F than a main central area of slightly variable composition. Back-scattered electron images reveal zoning and distinctive subspheroidal structures. New data and structural features are given for radioactive uranoan microlite from Namaqualand, South Africa. These crystals contain remnants of a bismuth phase and are in various stages of replacement. In the microlites replacing manganotantalite, the microlite reflects the composition of the replaced mineral. At Rubicon Mine, Karibib, a narrow marginal zone of mangantantalite is replaced by ferrotantalite along cleavages; a zone of intermediate composition is apparent. Detailed traverses have been completed across all of these crystals.

Chemical analyses of bannisterite from the Kamo mine, Toba City, Mie Prefecture, Japan, give Ba : Ca = 69:31–55:45, yielding the ideal formula with Ba > Ca, i.e. (Ba,Ca)(K,H3O)(Mn2+ · Mg,Fe2+)21 (Si,Al)32(O,OH)92 · nH2O, where Ba > Ca, Mn2+ > Mg, Fe2+,Si ≫ Al, and O > OH. The unit cell parameters calculated after the indexing of the X-ray powder pattern are: a = 22.95, b = 16.52, c = 25.66 Å, β = 94.2°. It occurs as dark brown veinlets cutting massive caryopilite-rhodochrosite ore, which is also cut by veinlets of manganoan chlorite (Mn/(Mg + Mn + Fe) = c. 0.30–0.39) with minor barian orthoclase (Ba/(K+ Ba) = 0.05 ∼ 0.06).

Surfaces of a natural sample of arsenopyrite (FeAsS) were oxidized by a range of inorganic oxidants, and the resultant surface alteration products studied using various spectroscopic techniques. The oxidants used were air during heating to relatively low temperatures (150°C), steam, ammonium hydroxide, hydrogen peroxide, and sulphuric acid. Electrochemical oxidation in water was also undertaken. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and spectral reflectance measurements, were used to characterize the surface compositions. New data are proposed for the binding energies of core electrons in arsenopyrite based on the fitted XPS spectra: 706.9 eV for the Fe 2p3/2 level, 161.2 eV for the S 2p level, and 40.7eV for the As 3d level. Spectroscopic analyses of the surfaces following oxidation indicated a range of iron oxides and hydroxides (Fe1−xO, Fe3O4, Fe2O3, FeOOH and Fe(OH)3), arsenic oxides (As2O3 and As2O5), sulphur and iron sulphates (FeSO4, Fe2(SO4)3). The relative proportions of the different phases present in the surface layer are related to the strength of the oxidant employed and, where relevant, the Eh/pH conditions prevalent during oxidation. The conclusions regarding the nature of the oxidation of arsenopyrite are discussed in relation to arsenopyrite extraction by flotation and leaching, and the breakdown of arsenopyrite in natural systems.

Recent advances in the use of time-dependent order parameter theory to describe the kinetics of order/disorder transitions are reviewed. The time dependence of a macroscopic order parameter, Q, follows, to a good approximation:

For systems in which the order parameter has a long correlation length (large ξ) and is not conserved (small ξC), the Ginzburg-Landau equation provides a general kinetic solution:

Specific rate laws can be derived from this general solution depending on whether the crystals remain homogeneous with respect to the order parameter, Q. The advantages of the overall approach are, firstly, that it does not depend on the detailed structure of the material being examined; secondly, that the order parameter can be followed experimentally through its relationship with other properties, such as spontaneous strain, excess entropy, intensities of superlattice reflections, etc.; and, finally, that conventional Landau expansions in Q may be used to describe the thermodynamic driving forces.

For a simple second-order transition in crystals which remain homogeneous in Q the rate law is:

If the free energy of activation varies with the state of order of the crystal, this becomes:

Simplifying assumptions can be introduced into the mathematics, or the integrals can be solved numerically. For crystals which remain homogeneous, the simplest solution valid only over small deviations from equilibrium is:

For crystals which develop heterogeneities in Q, the rate laws change significantly and we find as an extreme case:

where the A coefficient may be temperature dependent.

Experimental data available for a limited number of minerals (omphacite, anorthite, albite, cordierite and nepheline) are used to demonstrate the practical implications of the overall approach. As anticipated from the theory, modulated structures commonly develop during kinetic experiments, the observed rate laws depend on whether the critical point of the ordering is located at the centre or boundary of the Brillouin zone, and the rate laws for ordering and disordering can be quite different. The importance of different length scales, not only in the different techniques for characterizing states of order (IR, NMR, calorimetry, X-ray diffraction, etc.) but also for interpreting observed mechanisms and rate laws, is also outlined.

Use of the order parameter in Landau expansions and in Ginzburg-Landau rate laws provides, in principle, a means of predicting the equilibrium and non-equilibrium evolution of minerals in nature.

Phase relations in the binary system anorthite-K-feldspar at 10 kbar were studied with a solid-media piston cylinder apparatus. The eutectic character of this system is confirmed, with the eutectic point located at An30Kf70 (± 3 mol%), and 1215 ± 15°C, 10 kbar. The mutual solubilities between anorthite and K-feldspar at the solidus are greatly increased at 10kbar compared with those at 1 atm. The maximum solubility of K-feldspar in anorthite is c. 5% at 1 atm, c. 18% at 10 kbar; and that of anorthite in K-feldspar is c. 3% at 1 atm, c. 7% at 10 kbar. The increased solubilities are attributed to the increase in the eutectic temperature at high isostatic pressures. The results imply increased ternary feldspar solid solutions at high pressures and support the existence of homogeneous feldspars with extensive ternary composition under high-P, T and anhydrous conditions.

The infrared (IR) spectra of almandine-grossular and almandine-pyrope garnet solid solutions have been measured using the powder method. Frequency shifts of a band related to internal vibrations associated with the 8-co-ordinate dodecahedral site are nonlinear in almandine-grossular garnets and mimic the form of its molar volume of mixing curve. Almandine-pyrope solid solutions have nearly ideal molar volumes of mixing and the frequency shift of this same 8-co-ordinate site-related band is linear. The IR data support the empirically based crystal chemical model of Equivalent Site (ES) behaviour (Newton and Wood, 1980). The IR spectra give no indication of long-range ordering between Ca and Fe2+ in garnet, but thermodynamic calculations involving Ca-poor garnets might be affected by small volume or short-range ordering anomalies.

In a study of the liquidus surface in the silica-undersaturated part of the system Q-An-Ne-Ks at P(H2O) = 5 kbar, several two-feldspar plus liquid assemblages were obtained. The compositions of the feldspars were determined by electron microprobe analysis. The products of short runs (one or two days) consisted of fine intergrowths of two feldspars which may have crystallized during the heating period, or in the early stages of the run. Runs of four days were of sufficient length to coarsen these intergrowths such that individual euhedral crystals could be observed. However, analyses of the feldspar crystals from the longer runs showed a greater amount of ternary solid solution than expected. This may be attributed to the persistence of a small quantity of the second feldspar intergrown in the crystals. The slope of the tie-line joining one of the inferred pairs has been shown to be reproducible for gel and crystalline starting materials, hence the slopes of the tie-lines are believed to represent their equilibrium slopes.

Three polymorphs of tridymite (MC, PO-10 and MX) have been examined by transmission electron microscopy (TEM), in crystals which also contain lamellae of cristobalite. MX is the least common, and is highly unstable in the electron beam. It occurs as small regions (< 1 µm2) in PO-10 tridymite and at MC/PO-10 boundaries; these regions are interpreted as being strained. A consistent association between PO-10 and the cristobalite lamellae is attributed to the fact that PO-10 can more closely match the low-cristobalite structure than MC can, at boundaries parallel to the plane in which the crystal structures contain layers of SiO4 tetrahedra. An analogous interpretation explains the observation that twin boundaries (associated with 60° or 120° rotation of the layer) are commonly parallel to the layer plane in PO-10 but at high angles to it in MC. Abundant lamellar features, parallel to the same plane in PO-10, are tentatively interpreted as representing twinning by reflection and 180° rotation, which may also account for c* streaking in diffraction patterns.

Yellow blades on joint surfaces in granite from Merrivale quarry, Dartmoor, Devon, are shown to be of a slightly aresenatian variety of phurcalite by X-ray powder diffraction, infrared spectroscopy, electron microprobe and thermal analyses and optical methods. The results from these techniques are tabulated, including assignments for infrared absorption bands. Comparison of X-ray powder diffraction, infrared spectroscopic and optical data with those obtained from samples of ‘nisaite’, an incompletely described mineral from Nisa, Portugal, indicates that the latter is also phurcalite.

The analytical characteristics of a W/Si multilayer dispersion element (MLDE) with a nominal 2d spacing of 60 Å are assessed and compared with those of a conventional thallium acid phthalate (TAP) crystal for the determination of fluorine in silicate and phosphate minerals by electron microprobe. Measured count rates for fluorine were found to be up to fourteen times higher using the MLDE compared with TAP giving improved detection limits of 0.02 to 0.08 wt. % F (six sigma, 100s count time) compared with equivalent data of 0.06 to 0.12 wt. % F by TAP. At equivalent spectrometer angles, peak widths by MLDE were broader by a factor of 2 to 3. However, an important advantage of MLDE was found to be the absence of multiple order diffractions higher than the second order. In consequence, the troublesome third order P-Kα line interference on the F-Kα line, normally encountered in the analysis of apatites, is absent in determinations made using the MLDE. Conversely small interferences on the F-Kα line were detected from the first-order Lα line of Fe and second-order Lα line of Mg. X-ray data and spectra of selected minerals are presented to demonstrate these interferences. Detection limits for elements that can also be determined using the MLDE were 0.07 to 0.15 wt. % oxygen and 0.34 wt. % carbon.

High-alumina subsilicic calcic amphiboles, including sadanagaite and subsilicic ferroan pargasite, are found in rock samples from the contact aureole in the Nōgō-Hakusan area, central Japan. They occur in the reaction zones between dark fragments and the surrounding crystalline limestone of the pyroxene hornfels facies zone. The dark fragments which underwent K-metasomatism are originally basaltic rocks. The sadanagaite and subsilicic ferroan pargasite have high Al2O3 (16–19 wt. %) and K2O (3.6–4.3 wt. %) contents. The Si value ranges from 5.38 to 5.64 and the total Al ranges from 3.10 to 3.43 when cation ratios are calculated on the basis of O = 23. The calculated unit cell parameters of sadanagaite are a 10.00 (1), b 18.06 (2), c 5.355 (4) Å, β105.52(7)°, V 932(1) Å3. The A-sites of the amphiboles is occupied almost entirely by K and Na; the amphiboles are saturated with the edenite component. The amphiboles show a larger extent of tschermakite-type substitution [(Mg,Fe)Si⇌AlAl] than does ordinary pargasite. Sadanagaite is probably stable at the temperature above the upper amphibolite facies.

The coupled thermal oxidation and deprotonation in air of iron-rich biotite (FeO + Fe2O3 = 34%) has been investigated by EXAFS and XANES spectroscopy at the Fe-K edge and by XANES spectroscopy at the Ti-K edge. Samples annealed for 5 h at temperatures between 250° to 600°C have been studied. Distortions mainly of the Fe-Fe correlation within the octahedral layers are reflected in increasing Debye-Waller factors of the Fe-Fe correlation peak proportional to the annealing temperature. Unchanged Fe-O nearest-neighbour and Fe-Fe next-nearest-neighbour coordination numbers show that these distortions, nonetheless, do not change the structural topology of the octahedral layers. A model is introduced to demonstrate that increasing distortions are compatible with the expected heterogenous deprotonation mechanism in biotite. Titanium occurs in octahedral coordination. It was found to be unaffected by the coupled oxidation/deprotonation process. Both the coordination number and the valence state stay constant during the annealing process, in spite of dramatic changes of the Fe2+/Fe3+ ratio. Thermally activated hopping conduction involving Ti according to Fe2+Ti4+ → Fe3+Ti3+ is, therefore, not a significant process during thermal deprotonation and oxidation in biotite.

High-resolution X-ray emission spectra (XES) are presented for minerals with a variety of structures. The participation of the Si 3p orbitals in bonding is influenced by the local structure around the silicon atom. In orthosilicates the distortion of the SiO44--tetrahedron influences both peak-width and the intensity of the high-energy shoulder of the Si-Kβ spectrum. In minerals containing Si-O-Si bonds there is mixing of the Si 3s and 3p orbitals giving rise to a peak on the low-energy side of the main Si-Kβ peak. When combined with X-ray photoelectron spectra (XPS), a complete molecular orbital picture of bonding can be established.

Sodic-calcic and alkali amphiboles from benmoreitic members of the Igaliko Dyke Swarm contain up to 4.13 wt. % ZrO2. It is proposed that Zr enters the amphiboles by a coupled substitution of

(where C = octahedral site and T = tetrahedral site) to produce the richest Zr-bearing amphiboles so far identified, with compositions ranging up to

These amphiboles crystallize at a late stage from magmas which were Zr-rich, highly peralkaline and hydrous, with an fo2 close to the synthetic QMF buffer. The incorporation of Zr in to the amphibole is a consequence of the failure of other Zr-bearing phases (such as zircon, baddeleyite, eudialyte) to crystallize.

The equilibrium solubility of mimetite has been determined in aqueous solution at 298.2K. For the reaction Pb5(ASO4)3Cl(s,mimetite)+6H+(aq)⇌5Pb2+(aq)+3H2AsO4−(aq)+Cl−(aq) at this temperature log KH+, extrapolated to zero ionic strength, is equal to −27.9(4). This value is equal, within experimental error, to that corresponding to pyromorphite, Pb5(PO4)3Cl, derived from the literature, and redetermined here under analogous conditions. Distribution coefficients in terms of both HXO42− and H2XO4−(aq) ions (X = P,As) have also been determined for solid phases of the pyromorphite-mimetite solid solution series containing from 5 to 95 mol. % mimetite. Although the two end-members are isostructural without being strictly isomorphous, the solid solution series behaves ideally over the whole compositional range; that is, the composition of the solid phase reflects the ratio of arsenate to phosphate species in aqueous solution at pH values corresponding to naturally-occurring aqueous solutions generally associated with the oxidized zones of base metal orebodies. Some relationships between mimetite and other secondary lead(II) and copper(II) arsenate minerals have been explored.

Some picritic lavas from near the base of the volcanic succession in the Lebombo region of the Karoo Province contain phenocrysts of olivine and skeletal microphenocrysts of ilmenite, the latter indicating rapid crystallization from the magma. These ilmenite grains contain up to 6% MgO. The near-liquidus crystallization and the high MgO content of the ilmenite are highly anomalous for ilmenite in tholeiitic rocks. These features reflect the atypical nature (high MgO and TiO2) of the parental magma and/or highly reduced conditions of crystallization.

Deerite has been found in layers of Fe-rich metasediments, metamorphosed under conditions of blueschist to eclogite facies (T = ca. 480°C). Geochemical comparisons with other deerites described in the literature show that the Ile de Groix deerites most closely resemble the Mn-poor, alpine type, and Sifnos deerites. The P-T conditions of the Ile de Groix metamorphism do not conform with the stability field of deerite. In the assemblages examined here deerite seems to be a relic from earlier stages of prograde metamorphism.