The following four papers are a representative subset of the talks that were given during a lively session at the 1989 Winter Meeting of the Mineralogical Society (19 and 20 December), organised by us on behalf of the Metamorphic Studies Group on the theme of ‘Geothermometry and Geobarometry’. The talks at the meeting fell broadly into three groups: those dealing with fundamental problems of obtaining the requisite compositional and thermodynamic data, case studies of the applications of thermometers and barometers, and discussions of the limitations of the approach imposed by kinetic factors.

The first group of talks included contributions by T. J. B. Holland and J. Lieberman (co-authors M. Engi and R. G. Berman) dealing with perhaps the major hurdle in the way of reliable geothermometry and geobarometry: the question of activity-composition relationships for natural minerals. Holland took the examples of the pyroxenes and the plagioclase feldspars and showed, respectively, how Landau theory could be utilised to take account of ordering, and how Darken's quadratic formalism can take account of a structural break.

In vein material from the abandoned copper mine at Dhurode, County Cork, Republic of Ireland, the sulphosalt meneghinite is partly replaced by later minerals, notably a symplectite of galena and bournonite. Mineral analyses and proportions indicate that the bulk Pb/Sb ratio in the symplectite is almost identical to that of the meneghinite. It is inferred that Pb and Sb were the relatively immobile elements whose short-range segregation controlled the scale of symplectite intergrowth, during a diffusive replacement reaction in which Cu and S were added from the vein-forming fluid. This is the second sulphosalt-bearing symplectite for which immobile elements have been identified. In both cases, the inferred replacement reaction causes a volume increase, approximately 15% in the present example.

Allanite-(La) (containing up to 7.80 wt.% V2O3 and with La/ΣREE and La/Ce atomic ratios up to 0.54 and 1.45, respectively) and allanite-(Ce) (up to 8.46 wt.% V2O3) occur in close association with vanadian muscovite, barian tomichite and vanadian titanite in the main ore zone of the Hemlo gold deposit, Ontario, Canada. Allanite-(Ce) generally occurs as a minor constituent in cross-cutting veins or along foliation planes, whereas allanite-(La) invariably occurs in direct contact with titanite. The high V concentrations in the allanites from Hemlo are readily attributable to an adequate local source of this element, and are most likely controlled mainly by a simple substitution of V for Al in octahedral coordination. Vanadian allanite-(La) and vanadian allanite-(Ce), without any systematic differences in other constituents, are clearly distinct in REE composition, in respect to both the relative concentrations of La and Ce and abundances of other REE. The formation of both allanites (Ce- and La-rich) indicates very localised remobilisation and concentration of REE during a late hydrothermal alteration. The unusual REE composition of vanadian allanite-(La) directly reflects partitioning of REE with coexisting titanite, and the formation of this unusual phase may be attributable to replacement of earlier titanite with redistribution of REE in the solid state.

Over the past several decades, thinking about chemical processes in rocks had been dominated by experimental and theoretical treatments of mineral equilibrium, which is the state from which the time variable has been excluded. But, to an extent exceeding that of any of our sister sciences, we in geology are concerned with the behaviour of things as a function of time; thus equilibrium is but one of several interesting boundary conditions. Textures, (defined as the spatial relations within and among minerals and fluids, regardless of scale or origin) provide a means to sort out and identify successive states. In fact, it is the pattern of evolution of those states that enables us to deduce the processes. We may well draw the analogy with thermodynamics and kinetics, respectively:

equilibrium textures and phase assemblages, via thermodynamic analysis → definition of conditions of equilibration,

whereas

kinetics, as displayed in disequilibrium textures → sequence of events and processes of mineralization.

The interpretation of textures is one of the most difficult yet important aspects of the study of rocks and ores, and there are few areas of scientific endeavour that are more subject to misinterpretation. Although the difficulties are many, the opportunites for new understanding are also abundant. Textural interpretations have many facets: some are well established and accepted; some that are accepted may be wrong; others are recognised to be speculative and controversial; and we trust that still other textural features remain to be described and interpreted. This paper will deal principally with low-temperature, epigenetic ore deposits, and will emphasise silica and sphalerite; but extension to other materials is not unreasonable.

Ore and gangue minerals react internally, or with their environment, at widely ranging rates, ranging from the almost inert pyrite, arsenopyrite, well-crystallised quartz, and tourmaline to the notoriously fickle copper/iron and copper/silver sulfides. Arrested or incomplete reactions may be identifed by textural criteria and, when appropriately quantified, can provide guides to the duration of geological processes.

In recent years so much emphasis has been placed on isotopes, fluids, chemistry, and deposit and process models that the textural features have been ignored. In part this oversight occurs because we have grown accustomed to using superposition, cross-cutting, pseudomorphism, mutual intergrowths, exsolution and so on as off-the-shelf tools, to be grasped and applied without evaluation or even description. Surely science must build on previous work without constant and exhaustive reassessment, but for mineral textures a little reassessment may yield substantial benefit.

Metamunirite, β-NaVO3, is found in cavities in sandstone in San Miguel County, Colorado, occurring as fine, fibrous, colourless needles. X-ray powder and precession photographs show the crystals to be orthorhombic, space group Pnma, with a = 14.134(7), b = 3.648(2), c = 5.357(2) Å. They are optically biaxial positive with nα = 1.780(2) (‖c), nβ = 1.800(2) (‖a), nγ ≫ 185 (‖b, fibre axis; positive elongation), 2Vz moderate. A crystal structure analysis, confirming the previously determined structure of β-NaVO3, shows the presence of (VO3)n chains made up of zig-zag VO5 square pyramids. Metamunirite is probably formed by dehydration of munirite, NaVO3.2H2O.

The ferromagnesian silicate minerals, such as garnets, pyroxenes, micas, and amphiboles, appear in a variety of geothermometers and geobarometers. Where complete chemical analyses are available and regardless of bulk composition (metamorphosed pelitic or mafic), the aforementioned minerals commonly contain ferric iron. In mineral analyses using the electron microprobe, ferric and ferrous iron are not distinguished, and all the iron is treated as FeO. In ferric Fe-bearing minerals, this treatment results in (1) low analytical sums and (2) excess cations in the mineral formulae. Assuming ideal stoichiometry (ideal formula cations and oxygens) allows direct ferric estimates in garnets and pyroxenes; amphiboles require additional assumptions concerning site occupancies, and, for micas, no acceptable constraint exists for a ferric estimate. Based on ferric iron determinations for some metamorphic ferromagnesian silicates, the proportion of ferric to total iron increases at higher XMg values. The influence of ferric estimates on T and P calculations depends on the model used and on the extent the ferric estimate alters the relative proportions of end-members. Several examples suggest that, in general, if ferric estimates (or determinations) are made, they should be made for all the relevant minerals.

Zoned hibonites from a kalsilite, leucite, spinel, corundum, perovskite gneiss from the southern Indian granulite terrain near Punalur, Kerala, have rims that are the most Ti-rich yet recorded (0.83–0.87 Ti atoms per 19 O) and are essentially free of REE elements (ΣREE < 0.01 atoms per 19 O) while the cores are the most REE-rich compositions yet recorded (ΣREE = 0.55–0.65 atoms per 19 O). Within the limits of analytical uncertainty, the compositions of the hibonite can be related to the theoretical end-member CaAl12O19 by the substitutions REE R2+ ⇌ CaAl and R2+R4+ ⇌ Al2 with the REE-rich cores containing in excess of 50% of the REE R2+ Al11O19 end member. Minor substitution of Na for Ca occurs in the rims, while non-stoichiometry in both the cores and rims is indicated by partial 12-fold site occupancy. Ion-microprobe analysis of the REE-rich hibonites reveals strong enrichment in LREE with La/Lu c. 250 000.

In a diffusion-controlled process of nucleation and growth, adjacent porphyroblasts compete with one another for nutrients. When the effects of this competition are evaluated quantitatively for garnet porphyroblasts in pelitic rocks from the Picuris Range of New Mexico (U.S.A.), significant correlations arise between crystal sizes and the volumes of the domains from which the crystals drew their nutrients. These correlations strengthen the conclusion drawn from earlier work on spatial dispositions, zoning patterns, and crystal size distributions that the kinetics of intergranular diffusion governed the crystallisation of these porphyroblasts.

Computer simulations indicate that competition for nutrients during diffusion-controlled growth may have small but detectable effects on crystal size frequency distributions. Diffusional competition therefore introduces relatively minor inaccuracies into attempts to extract quantitative information on crystallisation processes from size distributions using models for the growth of isolated porphyroblasts. In contrast, the effects of diffusional competition on patterns of compositional zoning may be substantial, especially for porphyroblasts in rocks for which chemical inhomogeneity of the precursor leads to strongly clustered spatial dispositions. In such rocks, clustering may alter the patterns of compositional zoning in ways that obscure evidence for diffusion-controlled growth.

Comparison of the infrared spectra of georgeite and a phase which can be reproducibily synthesised in the laboratory shows that the mineral is an amorphous analogue of malachite, Cu2CO3(OH)2. Synthetic studies also explain the chemical conditions under which georgeite may form, as well as those which can cause it to react to either malachite or chalconatronite. Parallels may be drawn between the laboratory observations and known mineral associations of georgeite.

Chemical compositions of goldfieldite-tetrahedrite series minerals from the Iriki mine in Japan are reported for the frst time. The compositional variations of coexisting goldfieldite [atomic proportion Te > (Sb + As + Bi)], tellurian tetrahedrite [Sb > (Te + As + Bi)], and Te-free tetrahedrite from this locality are considered alongside data from the literature. These show that the substitution of (Sb, As)3+ for Te4+ in natural goldfieldite-tetrahedrite series minerals occurs by the following two mechanisms: (i) tetrahedrite, and goldfieldite, form a continuous solid-solution according to the coupled substitution of Cu+Te4+ for (Cu,Fe,Zn)2+(Sb,As)3+, and (ii) goldfieldite, and the ideal end-member, might also form a continuous solid-solution by the coupled substitution of ☐(vacancy)Te4+ for Cu+(Sb,As)3+. According to this work, the general formulae of tellurian tetrahedrite and goldfieldite are, therefore, respectively, , with x = 0 to 2 and , with y = 0 to 2.

The garnet-orthopyroxene Al barometer specifically considers the Al content of orthopyroxene in equilibrium with garnet resulting from Mg-Tschermaks substitution. It is demonstrated that P-T calibrations of this barometer derived solely from experimental data for the MAS system, such as that favoured by Finnerty and Boyd (1984, 1987) based on the data of MacGregor (1974), cannot be expected to yield meaningful pressure estimates for natural garnet lherzolite assemblages. The presence of additional CaO, FeO and Cr2O3 components in natural garnet lherzolites can be expected to influence substantially the Al partitioning between orthopyroxene, garnet and/or spinel at any particular P and T. Thus a more comprehensive barometer formulation is required, such as the one provided by Nickel and Green (1985) that is based on experimental data for the CMAS and SMACCR systems with thermodynamic modelling and addition of an Fe correction term.

It is further emphasised that for orthopyroxenes in natural garnet lherzolites the amount of Al introduced as Mg-Tschermaks substitution cannot be assessed simply as the total Al cation content since such orthopyroxenes frequently contain Al cations linked to Na substitution in M2 sites or to Cr, Ti and possibly Fe3+ in M1 sites. Revised algorithms for the calculation of specific orthopyroxene contents are presented. Application to analytical data sets for garnet lherzolite zenolith suites in the Thaba Putsoa and Mothae kimberlites generates revised upper mantle P-T arrays which refute the widely accepted advocacy by Finnerty and Boyd (1984, 1987) and Finnerty (1989) of an upper-mantle palaeogeotherm beneath northern Lesotho that is markedly inflected to a higher thermal gradient at the depths of derivation of the more chemically fertile, porphyroclastic textured, xenoliths.

In the central granulite belt of Sri Lanka, garnet-pyroxene granulites of granitic and gabbro-noritic composition are the most abundant rock types. The micro-structures and mineral chemistry data prove complete attainment of textural and large-scale chemical equilibrium during and following a phase of extreme penetrative deformation at conditions of the granulite facies (800–850 °C, 5 to 9 kbar). On a local scale, especially along the intergranular system, continued cation exchange decoupling from the early ceasing net-transfer reactions destroyed the near-peak metamorphic equilibrium. The extreme compositional variation of the coexisting ferro-magnesian phases (Fe/(Fe + Mg): gar 0.98−0.65, opx 0.92−0.40, cpx 0.88−0.28) and the near-isothermal conditions of equilibration throughout the studied area enabled examination of the effects of non-ideal mixing in garnet and pyroxenes on the equilibrium constants of reactions used in geothermobarometry, and tests on the quality of commonly applied thermometers/barometers and the validity of activity models adopted in the calibrations. The Sri Lankan data set reveals more or less pronounced compositional dependences for all of the tested gar-opx/gar-cpx Fe-Mg exchange thermometers and the opx + plg ⇌ gar + qtz barometers. Evidently the recommended solution models do not adequately describe the mixing properties of the involved ferro-magnesium mineral phases (garnet, orthopyroxene and clinopyroxene).

To material scientists the term texture means the crystallographic orientation of grains in a polycrystal. In contrast, geologists use the term more generally to refer to the spatial arrangement or association of mineral grains in a rock. In this contribution we are concerned with the materials science definition. There are several established techniques available for the determination of crystallographic textures in rocks. It has also been realised that the scanning electron microscope (SEM) is applicable to the study of crystallographic textures via the electron channelling (EC) effect. This provides an image of mineral/rock microstructure (via orientation contrast), as well as a means of accurately indexing their crystal orientations (via electron channelling patterns, ECP). Both types of EC image result from the relationship between incident electron beam and crystal structure, and the subsequent modulation of the backscattered electron (BSE) emission signal according to Bragg's Law. It is a simple matter to switch between the two imaging modes. A related effect, electron backscattering, provides only the diffraction patterns, but has superior spatial resolution and pattern angles.

Due to crystal symmetry restrictions, there is only a limited range of ECP configurations possible for any mineral. Individual patterns can therefore be identified by comparison with the complete ‘ECP-map’. The location of an individual pattern within the map area is determined by spherical angles, the exact definition of which depends on the type of fabric diagram (e.g. inverse pole figure, pole figure or orientation distribution function). Originally, these angles were measured manually. A computer program (CHANNEL) has been developed which uses a digitisation approach to pattern recognition, derives the required fabric diagrams and also constructs ECP-maps from standard crystal data (i.e. unit cell parameters etc.).

The combination of SEM/EC and CHANNEL dramatically facilitates the study of crystal textures in minerals and rocks, making statistical crystallographic analysis from individual orientations a practicality. The following example applications are considered: (1) crystal structure representation of the Al2SiO5 polymorph system; (2) local crystal texture relationships (epitaxial nucleation) between andalusite and sillimanite grains; (3) bulk rock crystal textures of quartzites; and (4) physical properties (e.g. elastic constants and seismic velocities) determined from bulk rock texture.

Silver-bearing zinc-iron tetrahedrite-tennantite and freibergit fahlores approximating the simplified formula (Ag,Cu)10(Fe,Zn)2(As,Sb)4S13 have been equilibrated with excess electrum (AuxAg1−x) and chalcopyrite + pyrite + iron-bearing sphalerite (CuFeS2 + FeS2 + Fe0.05Zn0.95S) in evacuated silica tubes at 300 °C, in reversed silver-copper exchange experiments with less than 0.1 mg NH4Cl added as a transport medium. A thermodynamic formulation and parameters describing As-Ag incompatibility at 400 °C (Ebel and Sack, 1989), which incorporate large temperature dependencies of standard-state properties and composition-ordering systematics, are shown to apply equally well to these 300 °C results. A generalised graphical model for this mineral assemblage is presented, describing fahlore composition as a function of temperature and the compositions of coexisting electrum and (Fe,Zn)S, which define the Ag(Cu)−1 and Fe(Zn)−1 exchange properties controlling fahlore compositions.

Deep azure-blue beryl is reported from veins in Upper Devonian conglomerate from southeast Ireland. Microprobe analysis, Mössbauer spectroscopy and loss on ignition suggest an unusual chemistry with an exceptionally high degree of substitution of Fe and Mg for Al, and extreme amounts of Na and water held within channel sites in the structure. The calculated structural formula is:

Refractive indices (1.603 and 1.595) and the cell edge, a, (9.292 Å) are also unusually high. Colourless beryl from the same locality, some of which forms cores to blue beryl prisms, has less octahedral substitution and a lower Fe/Mg ratio. The origin of the beryl is tentatively linked to contemporaneous volcanic activity.

Metamorphic petrology, and in particular quantitative thermobarometry, offer the possibility of identifying faults in metamorphic terrains by the metamorphic and/or thermobarometric breaks that occur across them. Furthermore, the sense of the thermobarometric break (e.g. warmer, deeper rocks on top of colder, shallower ones) and its magnitude could be useful tools for determining the sense and magnitude of the fault. The sensitivity of thermobarometry to tectonic variables such as fault throw and uplift rate, has been tested by a series of one-dimensional numerical re-equilibration models for both thrust and normal faults. In each of these models, the post-tectonic re-equilibration of a model garnet-biotite geothermometer is simulated by coupling one-dimensional numerical thermal and garnet diffusion models. After cooling and uplift, a thermobarometric temperature was calculated by using a near-rim garnet composition (5–10 µm from the rim) calculated by the re-equilibration model and biotite from the model matrix. The models varied the thermal structure of the model orogen prior to faulting, the depth of the fault, the structural throw of the fault, and the uplift rate following faulting.

All models produced zoned garnet porphyroblasts that recorded P-T conditions that were different from those at the time of fault motion. Most of the models produced thermobarometric breaks that were in the same sense as the temperature break at the time of fault motion. Normal faults produced a normal thermobarometric gradient with higher temperatures recorded below the fault than above it. Many, but not all of the thrust models produced a thermobarometric inversion near the fault, with higher temperatures locally recorded above the fault than below it. However, the magnitude of thermobarometric break correlated poorly with the fault throw. Most models developed thermobarometric breaks that were much smaller than the breaks that existed at the time of fault motion. The size of the thermobarometric break was commonly of the same magnitude as would be generated from microprobe analytical error. The models suggest that for metamorphic rocks whose thermal peak does not exceed a narrowly defined closure temperature, thermobarometry faithfully recorded the P-T conditions of the metamorphic peak. The closure temperature increases slightly with increasing uplift rate, but overall is rather insensitive to the uplift rate or other tectonic variables. For rocks whose thermal peaks is above the closure temperature, however, the model thermobarometer recorded a temperature that was very close to the closure temperature.

Orbicules in diorite from the Grenville Front zone of eastern Labrador are defined by shell structures alternately enriched and depleted in biotite, epidote and magnetite. Hornblende occurs locally in orbicule cores and the matrix, but not in the shells. The shells enclose plagioclase-rich (An40–45), leucodioritic cores containing biotite, epidote, magnetite and/or hornblende-bearing mafic clots. The matrix of the orbicules is mineralogically-similar to the orbicule cores, but is mesocratic, and contains relatively sodic plagioclase and accessory quartz and K-feldspar. In places, hornblende contains quartz oikocrysts, implying the resorption of early-formed clinopyroxene, and is rimmed by biotite and epidote. The latter phases also occur as inclusions in quartz-free hornblende interpreted to have crystallized directly from the magma. Epidote has a pistacite content of 21 to 26 and occurs as (1) tiny, idiomorphic crystals (‘epidote I’) enclosed by plagioclase or hornblende, and (2) relatively large (to 1 mm) grains with vermicular textures (‘epidote II’), particularly where in mutual contact with biotite (or hornblende) and plagioclase. These microstructures suggest that epidote is a magmatic phase which formed by direct crystallization from the magma, and by reaction of previously-formed minerals with the magma.

The following approximate paragenetic sequence has been inferred for orbicule cores and the matrix: clinopyroxene (clinopyroxene resorbed [→ poikilitic hornblende]), epidote I, Ca-Na plagioclase, biotite, hornblende (biotite and/or hornblende ± plagioclase resorbed [→ epidote II]), quartz + K-feldspar. Biotite compositions became progressively more Fe-rich during crystallization (XMg ⋍ 0.6 → 0.4), and the first-formed plagioclase (inclusions in quartz-free hornblende in orbicule cores) is more calcic (An51) than the last (matrix grains: An35).

The appearance of epidote early in the crystallization history of the diorite testifies to elevated PH2O and PTotal (PT). The most aluminous hornblende indicates maximum PT of 5 to 6 kbar. Orbicule shell structures are interpreted to have crystallized from supercooled boundary layers enclosing water-saturated globules within the dioritic magma. Although sufficient to suppress the formation of hornblende in the shell structures, the extent of magma supercooling did not permit the development of comb layering in the orbicules. Supercooling is attributed to an influx into the magma of water from an unidentified source.

Ion imaging by SIMS (ion microprobe) techniques provides a novel approach to the identification of mechanisms of fluid-mineral interaction during hydrothermal experiments. In scanning ion imaging, the primary ion beam is focussed to a fine spot which is rastered across the polished sample surface, and the secondary ion signal from the mass spectrometer is synchronised with the primary beam. The resistive anode encoder provides a direct ion image of a sample surface illuminated by a defocussed primary beam. The latter system is susceptible to charging artifacts during the imaging of insulating geological materials, and has a lower dynamic range than the scanning ion imaging system.

Application of both systems to the study of fluid–mineral interaction during hydrothermal experiments are reviewed in which fluid or solid phases have been labelled with 18O-enriched isotopic tracers. These include studies of micropermeability and microporosity in feldspars, solid-state diffusion versus solution-reprecipitation, textural equilibration mechanisms, and oxygen isotope exchange in silicate–carbonate systems.

Wonchi volcano (Ethiopia) is the second locality for wilkinsonite, the new mineral related to aenigmatite via the coupled substitution 2Fe3+ = TiFe2+. Wilkinsonite occurs as relics surrounded by rims of riebeckite and biotite in a syenitic ejectum which also contains microphenocrysts of aenigmatite. The two minerals do not represent an equilibrium pair. They both formed in the magma chamber under plutonic to hypabyssal conditions, but during two following stages of crystallisation separated by a major drop in ƒO2 conditions.