Subsolidus phase relations in the plagioclase system are investigated using an Ising-type model. The model treats ordering of Al,Si and Na,Ca cations in three dimensions using a version of the cluster variation method. The interactions within the T (Al,Si) and M (Na,Ca) sublattices and the interactions between the T and M sublattices are constrained by the available experimental information on orderdisorder transitions in the system. The paper suggests a thermodynamic explanation for the existence of two distinct structural series (high and low) and gives constraints on the thermodynamics of low plagioclases.

Lattice defects in lawsonite have been studied by transmission electron microscopy. It is proposed that twinning and easy glide systems are 1/2[110] {110}; the easy glide planes are coincident with twin planes. This mineral displays high sensitivity to the electron beam, even at low temperatures. In situ precipitates appear as a consequence of beam irradiation. The precipitation takes places first on dislocations, then on twin boundaries and then in the matrix, causing ‘coffee-bean’ contrast features typical of precipitates. The studies were performed at low temperature (∼110 K) in order to investigate the low temperature displacive transitions from space group Cmcm to Pmcn and P21cn and elucidate their microscopic character. No characteristic microstructural texture, such as antiphase domains associated with the transition, were observed, however. This is probably due to the high mobility of protons under the electron beam. The development of regularly spaced dislocations along twin planes is hypothesized as the only evidence that a phase transition takes place at a nanoscale.

Structural refinements of lawsonite have been obtained at pressures up to 16.5 GPa using angle-dispersive powder diffraction with synchrotron radiation on a natural sample contained in a diamond anvil cell. Lawsonite compresses smoothly and relatively isotropically up to 10 GPa. Its bulk modulus is 126.1(6) GPa (for K’ = 4), consistent with previous results. A trend of decreasing Si–O–Si angle indicates that compression is accommodated partly through the narrowing of the cavities containing Ca and H2O in the [001]ortho direction. At 10–11 GPa there is a phase transition from Cmcm to P21/m symmetry. The occurrence of a mixed-phase region, spanning >1 GPa, indicates that the transition is first order in character. The phase transition occurs through a shearing of (010)ortho sheets containing AlO6 octahedral chains in the [100]ortho direction, which causes an increase in βmono. Across the transition, the number of oxygens coordinated to Ca increases from 8 to 9, causing an increase in the average Ca–O bond length. The compressibility of P21/m lawsonite could not be determined due to solidification of the methanol/ethanol pressure-transmitting medium. On the basis of an experiment in which the P21/m lawsonite structure was heated to 200°C at 12.0 GPa, we predict a shallow positive P-T slope for the phase transition, and therefore no stability field for P21/m lawsonite in the Earth.

Laumontite is a common zeolite mineral indicative of low-grade metamorphism of lavas and volcaniclastic sediments. Stoichiometric laumontite (CaAl2Si4O12·4.5H2O) dehydrates in air at 298.15 K, 1 bar to leonhardite (CaAl2Si4O12·3.5H2O) via loss of water from the W1 crystallographic site. Consideration of reported X-ray diffraction and calorimetric data indicates that the standard molal volume and entropy for dehydration of laumontite to leonhardite + liquid water are ∼13 cc/mol and ∼8 cal/molK, respectively, at 298.15 K, 1 bar. Equilibrium between laumontite and leonhardite occurs at 70 to 80% relative humidity at 298.15 K, 1 bar, corresponding to a standard molal Gibbs energy and enthalpy of reaction of ∼170±40 and ∼2630±100 cal/mol, respectively. Calculated univariant equilibrium for this reaction is nearly linear from 46±3°C at 1 bar to 235±3°C at 5000 bar. Comparison of geological observations with these results suggests that laumontite forms as ‘leonhardite’ during metamorphism and diagenesis. Consideration of ‘leonhardite’, instead of laumontite, in low-grade metamorphic phase relations facilitates prediction of the relative stabilities of zeolites in natural and geologic systems, where calculated entropies of reaction incorrectly predict that assemblages bearing fully hydrated laumontite are stable at lower temperatures than the zeolites heulandite and stilbite.

Some zircons from the Hercynian Soultz granite, NE France, exhibit complex structures, with double cores and oscillatory-banded overgrowths. These glomeroporphyritic crystals are uncommon for zircons, in which synneusis has not previously been described. The cores of the zircons are considered to correspond to very quickly formed {100} or {110} prisms. These could slowly sediment in the magma chamber and cluster together while sinking. The overgrowth rims correspond to a slower growth rate.

U/Pb ion-probe dating of the cores and the overgrowth rims provided the same age of 327±6 Ma. Chemical electron microprobe analyses indicated that cores and rims have the same composition for major and trace elements.

The sinking of zircons accounts for the abnormally high percentages of zircon in the lower parts of the granite. The formation of clusters in the early stages of crystallization needs stable P/T conditions, a hot, liquid silicate melt with a high fluid content, and no other minerals crystallizing (liquidus phase).

Processes of mineral alteration involving the mobilization and deposition of more than 30 chemical elements during bauxite formation and epigenesis have been studied on specimens from the Devonian Schugorsk bauxite deposit, Timan, Russia. Chemical analyses of the minerals were obtained by electron microprobe and element distribution in the minerals was studied by element mapping. Interpretation of these data also utilized high-resolution BSE and SE images.

The main rock-forming minerals of the Vendian parent rock are calcite, dolomite, feldspar, aegirine, riebeckite, mica, chlorite and quartz; accessory minerals are pyrite, galena, apatite, ilmenite, monazite, xenotime, zircon, columbite, pyrochlore, chromite, bastnaesite and some others. Typically, the grainsize of the accessory minerals in both parent rock and bauxite is from 1 to 40 µm. However, even within these rather small grains, the processes of crystal growth and alteration during weathering can be determined from the zonal distribution of the elements. The most widespread processes observed are: (1) Decomposition of Ti-bearing minerals such as ilmenite, aegirine and riebeckite with the formation of ‘leucoxene’, which is the main concentrator of Nb, Cr, V and W. Crystal growth can be traced from the zonal distribution of Nb (up to 16 wt.%). Vein-like ‘leucoxene’ is also observed in association with organics. (2) Weathering of columbite and pyrochlore: the source of Nb in ‘leucoxene’ is now strongly weathered columbite, while the alteration of pyrochlore is expressed in the growth of plumbopyrochlore rims around Ca-rich cores. (3) Dissolution of sulphide minerals and apatite and the formation of crandallite group minerals: ‘crandallite’ crystals of up to 40 µm size show a very clear zonation. From the core to the rim of a crystal, the following sequence of elements is observed: Ca → Ba → Ce → Pb → Sr → Nd. Sulphur also shows a zoned but more complicated distribution, while the distribution of Fe is rather variable. A possible source of REE is bastnaesite from the parent rock. More than twelve crandallite type cells can be identified in a single ‘crandallite’ grain. (4) Alteration of stoichiometric zircon and xenotime with the formation of metamict solid solution of zircon and xenotime: altered zircon rims also bear large amounts of Sc (up to 3.5 wt.%), Fe, Ca and Al in the form of as yet unidentified inclusions of 1–2 µm. Monazite seems to be the least altered mineral of the profile.

In the parent rock, an unknown mineral of the composition (wt.%): ThO2 – 54.8; FeO – 14.6; Y2O5 – 2.3; CaO – 2.0; REE – 1.8; SiO2 12.2; P2O5 – 2.8; total – 94.2 (average from ten analyses) was determined. In bauxite, another mineral was found, which has the composition (wt.%): ThO2 – 24.9; FeO – 20.5; Y2O5 – 6.7; CaO 2.0; – ZrO – 17.6; SiO2 – 8.8; P2O5 – 5.4; total – 89.3 (F was not analysed; average from nine analyses). Presumably, the second mineral is the result of weathering of the first one. Although the Th content is very high, the mineral is almost free of Pb. However, intergrowths of galena and pyrite are observed around the partially decomposed crystals of the mineral. Another generation of galena is enriched in chalcophile elements such as Cu, Cd, Bi etc., and is related to epigenetic alteration of the profile, as are secondary apatite and muscovite.

Tausoare Cave is renowned in Romania for its gypsum and mirabilite speleothems. Of interest are the white crystalline speleothems formed on the floor of the ‘Sala de Mese’ (Dining Room) that were previously described as consisting of mirabilite. The samples we collected reveal crystals of two different habits. One shows the characteristic mirabilite fibrous crystals (cotton-like speleothem) while the other formed bladed and short prismatic crystals which comprise the delicate ‘cave flowers’. The mineral association was characterized by means of X-ray analysis, scanning electron microscopy and electron microprobe. Beside thenardite (dehydration product of mirabilite) we also identified three sulphate minerals: leonite [K2Mg(SO4)2·4H2O], syngenite [K2Ca(SO4)2·H2O] and konyaite [Na2Mg(SO4)2·5H2O]. Of these, leonite and konyaite have never been reported in a cave environment. This paper describes the mineralogy of this particular sulphate deposit and offers some viewpoints on the crystallogenesis.

Rengeite, Sr4ZrTi4Si4O22, is a new member of the perrierite–chevkinite group found in the jades from the Itoigawa-Ohmi district, central Japan. It is monoclinic, P21/a, a = 13.97(1), b = 5.675(7), c = 11.98(1)Å, β = 114.26(8)°, V = 866 Å3 and Z = 2. The six strongest lines in the X-ray powder diffraction pattern are 3.12 (s) (40-3), 3.05 (vvs) (31-3), 2.99 (vs) (311), 2.84 (s) (020), 2.74 (s) (004), 2.20 (s) (31-5). Electron microprobe analysis gave SiO2 22.58, TiO2 29.88, ZrO2 9.49, Nb2O5 0.24, Ta2O5 0.07, Al2O3 0.20, FeO 0.10, CaO 0.43, SrO 34.32, BaO 0.13, La2O3 0.00, Ce2O3 0.38, Pr2O3 0.10, Nd2O3 0.29, Sm2O3 0.04, total 98.25 wt.%, corresponding to (Sr3.62Ca0.08Ce0.03Nd0.02Ba0.01Pr0.01)∑3.76 (Zr0.84Ti0.09Al0.04Fe0.02Nb0.02)∑1.01Ti4.00Si4.11O22 on the basis of O = 22. The unitcell parameters and chemical composition imply that rengeite is the Sr and Zr-analogue of perrierite or high-β analogue of strontiochevkinite. It is transparent, dark brown with adamantine lustre. Its streak is pale greenish brown, and no cleavage was observed. The hardness is VHN100 606–698 kg mm−2 (Mohs 5–5.5). The calculated density is 4.12 g cm−3. It is strongly pleochroic from pale green to pale greenish brown where the REE contents are <1 wt.% and pale violet to greenish brown where the REE contents are between 3 and 10 wt.%. It occurs as anhedral grains in close association with titanite, zircon and tausonite in a pebble of blue titanian omphacite-jadeite rock from the seashore of Oyashirazu, Ohmi Town, in a boulder of lavender-coloured Ti-bearing jadeitite from the bed of the Kotaki-gawa river, Itoigawa City, and in a boulder of green jade from the bed of the Hime-kawa river, Itoigawa City, Niigata Prefecture, central Japan. Rengeite is considered to have crystallized by interaction between pre-existing minerals (rutile, anatase, titanite and zircon) and Sr-rich metamorphic fluid during later stage activity of high-P/T metamorphism. The name is for Mt. Renge near the locality and the Renge metamorphic belt where jadeitite deposits are found.

In order to assess the role played by sheet silicates in controlling base metal distribution, the mineralogy of the <2 µm fraction of waste material, surface soils, and stream sediments was investigated in the surroundings of a pyrite-chalcopyrite mine area in northern Italy. The results indicate that smectite is very abundant in the <2 µm fraction of the tailings, and it exerts an effective control on the concentrations of Zn, Ni (and Cu). Normal soils and sediments are characterized by interstratified illite-smectite and by lower base metal concentrations. Away from the mine area, smectite decreases in the stream sediments, and chlorite becomes more important in controlling base metal distribution.

Ni, Cr, Cu and Co increase with chlorite and talc, but are depleted in the <2 µm fraction compared to coarser fractions in the stream sediments, because the sand–silt fraction of sediments concentrates ophiolitic fragments, variably enriched in ores. A mechanical dispersion of chlorite is probably the controlling factor. Zinc displays a systematic enrichment in the clay fraction of waste material and in the stream sediments but the main mineral carrier is not identified.

In hypabyssal and crater-facies kimberlites of the Lac de Gras kimberlite field, perovskite occurs as reaction-induced rims on earlier-crystallized Ti-bearing minerals (magnesian ilmenite and priderite), inclusions in atoll spinels and discrete crystals in a serpentine-calcite mesostasis. The mineral is associated with spinels, apatite, monticellite, phlogopite, baryte, Fe-Ni sulphides, ilmenite, diopside and zircon. Uncommon accessory phases found in an assemblage with perovskite include titanite, monazite- (Ce), witherite, strontium-apatite, khibinskite, djerfisherite, wollastonite, pectolite, suolunite, hydroxyapophyllite and bultfonteinite. Three types of perovskite can be distinguished on the basis of composition: (I) REE-Nb-Al-poor perovskite with relatively high Sr and K contents (up to 2.2 and 0.6 wt.% oxides, respectively) occurring as mantles on priderite and inclusions in atoll spinels; (II) perovskite with elevated Al, Fe, Nb and LREE (up to 1.4, 8.3, 9.1 and 17.0 wt.% oxides, respectively) found as discrete crystals and rims on macrocrystic ilmenite; (III) perovskite significantly enriched in Na, Sr, Nb and LREE (up to 3.3, 3.4, 13.0 and 22.6 wt.% oxides, respectively) found as rims on perovskite I and II. The overwhelming majority of perovskite is represented by discrete crystals of type II. In some occurrences, this type of perovskite also has high Th contents (up to 5.5 wt.% ThO2) and Zr contents (up to 3.7 wt.% ZrO2). Textural evidence indicates that perovskite shows an overall evolutionary trend from the most primitive type I towards type III showing the highest Na, Nb and LREE contents. Perovskite of type I probably crystallized under relatively high pressures prior to the precipitation of MUM spinels. Perovskite II crystallized after magnesiochromite, pleonaste and MUM (magnesian ulvöspinel-magnetite) spinels, under increasing fO2. The most compositionally evolved type III formed during near-solidus re-equilibration of the earlier-crystallized perovskite. The compositional variation of the Lac de Gras perovskite can be adequately characterized in terms of five major end-members: CaTiO3 (perovskite), CeFeO3, NaNbO3 (lueshite), Na0.5LREE0.5TiO3 (loparite), and CaFe0.5Nb0.5O3 (latrappite).