In the Fen complex (Telemark, S.E. Norway), carbonatites of different compositions have penetrated feldspathic fenites (alkali feldspar(s) + aegirine augite ± alkali amphibole) or older carbonatites, inducing different types of contact metasomatic alterations in their wall-rocks. (1) Pyroxene søvite has induced alkali metasomatism (i.e. fenitization s.s.), with alkali feldspars remaining stable and aegirine-augite transformed to nearly pure aegirine. (2) Søvite and dolomite carbonatite with phlogopite and/or alkali or alkali-calcic amphibole have caused replacement of feldspathic fenite by phlogopite, i.e. magnesium metasomatism. (3) Granular (dyke facies) ferrocarbonatite has increased the ferromagnesian components in calcite in wall-rock søvite. (4) Heterogeneous (pyroclastic) ferrocarbonatite induced pseudomorphic replacement of phlogopite by chlorite (leaching of alkalis). The different contact metasomatic processes reflect contrasts in compositional character among carbonatite magmas in the Fen complex, which may be evaluated in terms of differences in alkali and magnesium carbonate activities. The different types of carbonatite magma represent the products of local evolutionary trends, and are genetically related to spatially associated silicate rocks, rather than to a single ‘primitive’ carbonatite parent magma.

The mechanism by which secondary porosity in sandstone reservoirs is produced is poorly understood. Previous hypotheses to account for the increased dissolution of framework silicates have invoked the introduction of acidic and/or organic-rich fluids capable of complexing metals. In order to evaluate the relative effects of these factors, the rates of K-feldspar dissolution have been measured at 70°C and 95°C, at pH values of 1, 4, and 9, with and without oxalic acid, at a total pressure of 50 MPa, using direct sampling autoclaves. The presence of oxalic acid increased the dissolution rate of Kfeldspar at pH 4 and 9, but decreased the rate at pH 1 at both temperatures. The maximum increase in dissolution rate was observed at pH 4, 95°C by a factor ∼4. In oxalic-free experiments, the rate of dissolution increased an order of magnitude with each unit decrease in pH in the pH regime 1–4. In the presence of oxalic acid, this dependency of dissolution upon pH decreased to approximately an order of magnitude increase with each decrease in pH by 2pH units. The dissolution process was observed to be stoichiometric under all conditions, indicating that the mechanism by which rates are increased was not due to preferential complexation of aluminium, but rather by an increase in the overall solubility of feldspar in the neutral pH region. This regime of increased rate of reaction of feldspar coincides with that expected to be naturally buffered by the introduction of organic acids into a sandstone reservoir system.

A combination of ion-microprobe (for Li) and electron-microprobe (for other major elements including F) methods has been used to analyse Li-rich micas from the S.W. England batholith (mainly the St Austell granite) and the Massif Central, France. Rocks showing various degrees of hydrothermal alteration were studied in order to separate the original compositional trends from alteration trends. The original compositional trend is essentially one of increasing Li with increasing degree of evolution. The main atomic substitution in the original micas is 3Li substituting for A1 and 2 vacancies in octahedral sites; substitution of Li for R2+ (Fe, Mn, Mg) in octahedral co-ordination is generally subordinate. Alteration trends involve a loss of Li, Fe, F, Rb and Cs, and a gain in A1. The effects of volatile elements on phase relations of granites are reviewed and it is concluded that the original Li-micas were primary, i.e. crystallized from the melt. It is suggested that the late-magmatic stage passed transitionally into the hydrothermal stage leading inevitably to subsolidus recrystallization (autometasomatism) of the primary minerals, so introducing further textural and mineralogical complexities to the rocks.

Compositional data are presented for Nb-Ba-K-V titanates from micaceous kimberlite (Star mine, S. Africa). These data significantly extend the previously known range of solid solutions in naturally occurring members of the hollandite group. Two distinct suites of crystals occur. One is a suite of primary groundmass prismatic crystals that are Ba-K-V-rich and represent solid-solutions from the priderite series towards mannardite. The second suite consists of anhedral xenocrysts that are Ba-free. In this suite relatively Nb-rich varieties (>4.0% Nb2O5) represent solid-solution towards a niobian-bearing potassian analogue of mannardite, whereas relative Nb-poor (<3.0% Nb2O5) varieties are Nb-bearing vanadian priderites. These hollandite-group minerals have compositions that differ significantly from priderites found in lamproites.

Holtite, (Si2.25Sb0.75)B[Al6(Al0.43Ta0.27□0.30)O15(O,OH)2.25], is orthorhombic a = 4.6914(5), b = 11.896(2), c = 20.383(4)Å, space group Pnma. Full-matrix least-squares refinement yielded a final R = 0.030, for 1664 reflections with I ⩾ 3σ(I). Its crystal structure is closely related to that of dumortierite, but with the difference that extensive substitution of Si by Sb occurs in holtite. SbO3 triangular pyramids replace the SiO4 tetrahedra arranged around the inside edge of the hexagonally shaped tunnels which are the feature of the dumortierite and holtite structures. Associated with these replacements is the replacement of Al by Ta in the octahedral site centred in these tunnels, and the development of vacancies in this metal site and its attached oxygens.

Mössbauer spectroscopy has been used to identify the iron-containing products that are formed during the thermal decomposition of ankerite in a CO2 atmosphere. The decomposition takes place in three stages and evidence is produced to show that the first stage involves decomposition of ankerite to yield a periclase-wustite solid solution, (Mg,Fe)O, along with calcite and CO2, the periclase-wustite then reacting with CO2 to produce magnesioferrite (MgFe2O4) and CO. In the second stage the magnesioferrite and calcite react to produce periclase and dicalcium ferrite. The third stage does not involve reaction of Fe-containing phases and corresponds to the decomposition of calcite to CaO.

Vochtenite, a new mineral from the Basset Mine, southeast of Camborne in Cornwall, England, is a ferrous-ferric magnesium-bearing hydroxy uranyl phosphate mineral. It is monoclinic with a = 12.606, b = 19.990, c = 9.990 Å, β = 102.31° Z = 3; the ideal formula is:

The strongest lines in the X-ray powder diffraction pattern are: 9.998(100)(020), 4.999(30)(040), 4.892(45)(002), 3.475(70)(311), 3.333(50)(060), 3.087(40)(232), and . Vochtenite is brown in colour with a bronzy lustre and is non-fluorescent. Mohs hardness is 2.5 and the density (calc.) = 3.663 g/cm3. There is a prominent (010) cleavage. Vochtenite is optical biaxial negative, 2V (calc.) = 89(3)° its dispersion is indiscernible. Refractive indices are α = 1.575(2), β = 1.589(2), γ = 1.603(2), and the pleochroism is very weak. Orientation X(α)∥b is perpendicular to (010) and Z(γ)∧c is small. The mineral occurs as subparallel (0.5–1.0 mm) crystal aggregates with a pseudo-quadratic outline. The mineral is named vochtenite after Prof. Dr. Ing. R. F. C. Vochten of the State University of Antwerpen, Belgium.

Gypsum crystals were grown at 25, 35, and 45 °C from solutions containing different ionic impurities (Mg2+, Na+, Cd2+). Curved gypsum crystals formed from the solutions enriched in magnesium or cadmium, whereas, in the presence of sodium, gypsum always grew undeformed. The mechanism of the curvature is described and an interpretation based on crystallographic considerations is given. Curvature is the result of impurity incorporation followed by crack formation and mechanical twinning which releases the internal stresses.

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.