The nepheline syenites of the Pilansberg alkaline complex (South Africa) have undergone extensive subsolidus equilibration and alteration with a deuteric Cl- and Na-rich fluid phase. Complex assemblages of secondary minerals result from the replacement of primary alummosilicates, rinkite, eudialyte and fluorapatite. The composition of apatite group minerals formed during these alteration processes reflects the Sr- and rare earth element (REE) content, Na/Cl ratio and pH of the deuteric fluids. Apatite-group minerals are observed to have formed in the following sequence: orthomagmatic fluorapatite; strontian britholite-(Ce); strontian fluorapatite; Sr-apatite; REE-rich Sr-apatite; Sr-Na-REE- rich minerals approaching the stoichiometry of belovite-(Ce) and deloneite-(Ce); britholite-(Ce). Increasing alkalinity of the deuteric fluids is reflected by increasing amounts of Sr replacing Ca in apatite and culminates in the formation of Sr apatite containing 62.1 wt.% SrO (∼4.17 a.p.f.u. Sr). Pilansberg apatite-group minerals form a near-complete solid solution between fluorapatite and a fluorine analogue of Sr apatite with limited solution towards belovite-(Ce), Si-rich belovite-(Ce) and strontian britholite-(Ce).

Chalcedony and agates from a variety of world-wide hosts have been examined using cathodoluminescence (CL). Gaussian fitting of the experimental data shows that there are two dominant spectral emissions at ∼400 and ∼660 nm. A third subordinate peak is also found at ∼470, ∼560 or ∼620 nm. An age-related link is shown between the respective decreasing and increasing relative intensities of the 660 and 620 nm emissions. It is proposed that this change is due to a condensation reaction between neighbouring Si–OH groups eliminating water and forming a strained Si-O-Si bond.

Agates from a variety of hosts and regions produced no clear demonstrable CL distinctions. However, a set of Western Australian agates was examined from host rocks that had been subjected to burial metamorphism. Cathodoluminescence produced different spectral emissions in the petrographic fibrous and granular regions of these agates. One agate shows a partial transformation of the petrographic fibrosity into granularity. This conversion is characterized by emission bands at 570 nm and 460 nm. Similar emission-band changes were produced by heating Brazilian agates for 35 days at 300°C. The identification of these changes in agate could serve as an indicator of palaeoheating within the parent rock.

Chemical modification of chromian spinel at low-T alteration was examined in detail for a podiform chromitite from a Tethyan ophiolitic mélange belt at Rayat, northeastern Iraq. The chromitite is highly brecciated and the matrix has been completely altered, producing chlorite and carbonate (dolomite and calcite). High-Cr, low-Fe3+ spinel has formed along the margins and cracks of chromian spinel grains throughout the alteration, associated with unaltered primary spinel and magnetite without ferritchromite. In associated harzburgites, only ferritchromite is found instead of the high-Cr, low-Fe3+ spinel. The high-Cr, low-Fe3+ secondary spinel apparently has chemical properties of mantle origin, plotted at the extension of ordinary mantle spinels on compositional spaces. The character is due to subtraction of Al as chlorite with the addition of an amount of magnetite component from the silicate matrix, which is small in volume relative to peridotite and composed of highly magnesian olivine (up to Fo97). We should treat high-Cr chromian spinels with caution in highly altered mantle-derived rocks, especially chromitite and other rocks with highly magnesian olivine, as well as in detrital particles for provenance study.

Deformed jadeite grains in the UHP jadeite-quartzites from the Dabie Mountains, eastern China show evidence (by transmission electron microscopy, TEM) of plastic deformation: free dislocations, dislocation loops, dislocation networks, dislocation arrays and regularly spaced subgrain boundaries by dislocation walls. Electron energy-loss spectroscopy and TEM studies reveal the existence of water-rich bubbles within dislocations and imply a relationship with water-defect dislocation by water weakening in the jadeites. These observations suggest that the water-rich bubbles are a major weakening effect facilitating plastic deformation of jadeite by dislocation creep.

Zirconium and Hf are nearly identical geochemically, and therefore most of the crust maintains near-chondritic Zr/Hf ratios of ∼35–40. By contrast, many high-silica rhyolites and granites have anomalously low Zr/Hf (15–30). As zircon is the primary reservoir for both Zr and Hf and preferentially incorporates Zr, crystallization of zircon controls Zr/Hf, imprinting low Zr/Hf on coexisting melt. Thus, low Zr/Hf is a unique fingerprint of effective magmatic fractionation in the crust. Age and compositional zonation in zircons themselves provide a record of the thermal and compositional histories of magmatic systems. High Hf (low Zr/Hf) in zircon zones demonstrates growth from fractionated melt, and Ti provides an estimate of temperature of crystallization (TTiZ) (Watson and Harrison, 2005). Whole-rock Zr/Hf and zircon zonation in the Spirit Mountain batholith, Nevada, document repeated fractionation and thermal fluctuations. Ratios of Zr/Hf are ∼307–40 for cumulates and 18–30 for high-SiO2 granites. In zircons, Hf (and U) are inversely correlated with Ti, and concentrations indicate large fluctuations in melt composition and TTiZ (>100°C) for individual zircons. Such variations are consistent with field relations and ion-probe zircon geochronology that indicate a >1 million year history of repeated replenishment, fractionation, and extraction of melt from crystal mush to form the low Zr/Hf high-SiO2 zone.

Deerite, a typical mineral of Fe-rich metacherts metamorphosed under blueschist conditions, is not rare, but known occurrences have up to now been restricted mainly to the Tethyan collisional zone and the Western Cordillera of North America. We describe a first occurrence in the high-pressure nappes of the Escambray Massif, Cuba, in the assemblage deerite + Mg-Al-poor riebeckite + magnetite + quartz ± garnet ± phengite ± aegirine. This assemblage typically forms during exhumation and accompanies late, stress-free annealing of the quartz matrix. Mg-Al-poor riebeckite overgrows older, large, oriented crystals of glaucophane, ferroglaucophane and Mg-Al-rich riebeckite (‘crossite’) during deerite formation. Early-formed hematite was largely replaced by magnetite. Deerite is very close to ideal composition, attaining >99% Si12O40(OH)5, allowing direct application of the experimentally determined P-T-fO2 stability field (Lattard and Le Breton, 1994). In combination with oxygen-isotope thermometry on magnetite-quartz, the crystallization conditions of the deerite-bearing assemblage can be constrained to ∼470°C, >15 kbar, and an oxygen fugacity restricted closely to the quartz-fayalite-magnetite buffer (fO2 ≈ 10−23 bar). Thus, the late-stage P-T path ofthe metacherts mirrors a steep P-T gradient of 10°C/km or less, requiring subduction ofthis part ofthe Antillean Island Arc to be still active during exhumation of the Escambray nappes.

The chemistry and crystal structure of a unique Zn-rich kupletskite: (K1.55Na0 .21Rb0.09Sr0.01)Σ1.86(Na0.82Ca0.18)Σ1.00(Mn4.72Zn1.66Na0.41Mg0.12)Σ7.00 (Ti1.85Nb0.11Hf0.03)Σ1.99(Si7.99Al0.12)Σ8.11O26 (OH)4(F0.77OH0.23)Σ1.00, from analkalin e pegmatite at Mont Saint-Hilaire, Quebec, Canada has been determined. Zn-rich kupletskite is triclinic, , a = 5.3765(4), b = 11.8893(11), c = 11.6997(10), α = 113.070(3), β = 94.775(2), γ = 103.089(3), R1 = 0.0570 for 3757 observed reflections with Fo > 4σ(Fo). From the single-crystal X-ray diffraction refinement, it is clear that Zn2+ shows a preference for the smaller, trans M(4) site (69%), yet is distributed amongst all three octahedral sites coordinated by 4 O2− and 2 OH− [M(2) 58% and M(3) 60%]. Of note is the lack of Zn in M(1), the larger and least-distorted of the four crystallographic sites, with an asymmetric anionic arrangement of 5 O2− and 1 OH−. The preference of Zn for octahedral sites coordinated by mixed ligands (O and OH) is characteristic of its behaviour in alkaline systems, in contrast to granitic systems where Zn tends to favour [4]-coordinated, OH− and H2O-free sites with only one ligand species (O, S, Cl, B, I). In alkaline systems, [4]Zn is only present in early sphalerite or in late-stage zeolite-like minerals. The bulk of Zn in alkaline systems is present as discrete [6]Zn phases such as members of the astrophyllite, labuntsovite, milarite and nordite groups, a result of the formation of network-forming complexes inthe low-temperature, low-fS2, high-alkalinity and highly oxidizing systems.

High-resolution imaging of electron-transparent samples using a scanning electron microscope, here termed low voltage (LV) STEM, is a new and valuable technique for studying Earth and planetary materials. The most effective method of LV-STEM imaging uses a pair of electron detectors positioned side-by-side beneath the thin sample. The detector directly underlying the sample forms bright-field images dominated by mass-thickness contrast. Activation of the detector offset from the sample yields dark-field images with a greater component of atomic number contrast. LV-STEM images with significant diffraction contrast can also be obtained, but require careful positioning of the sample relative to the electron detectors. In this study LV-STEM was used successfully to image sub-μm sized kaolinite crystals and tens of nm-sized etch pits on the gold-coated surfaces of weathered feldspar grains. Dark-field LV-STEM was also especially effective for characterizing very fine-scale intergrowths of Mg- and Fe-rich phyllosilicates within uniformly thin samples of the Murchison meteorite prepared using the focused ion beam (FIB) technique. LV-STEM is a quick and easy method for characterizing the morphology and internal structure of mineral and rock samples and may prove to be especially useful in geomicrobiology research.

Hematite is a mineral the chemical composition of which rarely differs significantly from stoichiometric Fe2O3. As such, little attention has been paid to the mineral chemistry of hematite in Precambrian iron formations, where hematite forms monomineralic high-grade orebodies. Electron microprobe analysis of hematite from two iron-ore deposits, Cauê (Itabira district) and Gongo Soco, in the Palaeoproterozoic Itabira Iron Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil, has revealed distinct variations in chemical composition with respect to Ti and Cr. Hematite containing Ti and/or Cr is of very local occurrence in the itabirite unit and shows a spatial relationship to hematitic, palladiferous gold-bearing veins (known as ‘jacutinga’), occurring either within the veins (adjacent to, or included in, palladiferous gold grains) or in their vicinity. Where present, titaniferous hematite (to ∼1.3 wt.% TiO2) is lepidoblastic and defines a pervasive tectonic foliation (S1). In contrast, Ti-free, chromiferous hematite (to ∼6.4 wt.% Cr2O3) characteristically occurs as inclusions in palladiferous gold within S1-truncating ‘jacutinga’. Replacement of granoblastic, Ti-free, chromiferous martite with relicts of magnetite by lepidoblastic, Cr-depleted, titaniferous hematite proves that Cr and Ti were mobile during metamorphism. Chromium was ultimately fractionated into the hematite found in auriferous aggregates within cross-cutting ‘jacutinga’. A positive correlation between Cr and Pt in bulk-rock samples from the Itabira district suggests that Cr is a potential prospective guide for Au-Pd-Pt-bearing hematitic veins (‘jacutinga’).