Crystal-chemical analyses of the serpentine polymorphs, sampled from macroscopic veins in the serpentinite rocks from Sasso della Mantesca and Sasso di S. Zanobi, are reported. The samples were examined by X-ray diffraction, thermal analysis (TG and DTG), scanning and transmission electron microscopy, IR absorption spectroscopy, microprobe, and chemical tests.

The results show: chrysotile 2Orc1 in tubular fibrils placed perpendicularly to walls of iso-oriented ‘lenticular veins’; and polygonal serpentine formed by flat layers of lizardite 2H and/or 1T, arranged as an outer shell of a central core of chrysotile 2Mc1 fibres; this type of serpentine with high crystallinity and unusual morphological development is included in ‘green veins’, which show variable Al, Fe, and Mg contents.

A genetic mechanism correlating the petrographic observations with the mineralogical results is discussed.

The stability field of analcime solid solutions in the system NaAlSi3O8-NaAlSiO4-H2O has been previously determined by Kim and Burley (1971a). These experiments are re-examined with a view to determining the variations of the room-temperature cell parameters of analcime as a function of temperature of synthesis and composition. It is shown from this evidence that most of the analcime solid solutions in these experiments are equilibrium compositions. The increase in the cell dimension of quenched analcime solid solution is found to be 5 × 10−5 Å/°C. It is suggested that birefringence in analcime is induced by low water-pressure relative to total pressure. A phase transition appears to be observed in quenched analcimes of this study and is thought to be the same as that in the Golden analcime reported by Yoder and Weir (1960).

Spotted eye-like structures, made up of densely packed biotite ‘pupils’ mantled by quartz and feld-spar, constitute about 10% by volume of a high-grade quartzofeldspathic gneiss located on the road to Purnamoota Homestead about 10 km north of Broken Hill. These spotted structures have an uneven distribution within rocks exposed at this locality. In some parts of the outcrop they tend to be closely spaced (fig. 1), whereas elsewhere they are less abundant and the spotted gneiss merges into an essentially homogeneous gneiss composed of quartz (34%. by volume), plagioclase (24%), Kfeldspar (17%), biotite (20%), and garnet, muscovite, and opaques (5%). This homogeneous gneiss is believed to have developed into a ‘matrix’ gneiss phase holding the ‘eyes’ which have a felsic mantle to mafic clot ratio of 4:1.

The whole eye-like structure appears to be a distorted prolate spheroid with its axis set close to the foliation plane of the gneiss. The internal biotite clot forms another prolate spheroid with its axis inclined at approximately 70° to that of the enveloping quartz-feldspar mantle. Although most of the eyes are isolated within the matrix, some are linked to hold three or four biotite aggregates (fig. S1) and rarely a vein-like patch which contains some ten or more partly linked spots of biotite may be found. In addition to the biotite, xenoblasts of almandine occur within some of the eye-like structures.

Whole-rock chemical analyses (including both major oxides and trace elements) show that the homogeneous gneiss and the spotted gneiss are very similar. The most notable feature of the geochemistry of the spotted rocks is the close chemical correspondence between the matrix and the whole eyes. Moreover, the compositions of corresponding minerals in these phases are very similar. For example, plagioclase is consistently about An35–40, the biotites are all very iron-rich (Fe/(Fe + Mg) ≈ 0.87) with compositions near siderophyllite, and K-feldspar has the composition Or94Ab6.

It has been suggested that the biotite spots formed by alteration of garnet probably during a retrograde metamorphic event. However, studies of the microstructure of the biotite and garnet in the spotted gneiss show that biotite of the spots does not replace garnet. Furthermore, the chemical similarity between the spots and the matrix is best explained by an isochemical rearrangement of components of the matrix phase to form the eyes. Thus, a more likely origin is best related to the displacement of matter along chemical potential gradients possibly induced by deformation in the rock system—a process described by the term metamorphic differentiation.

Veins in the Purnamoota Road gneiss are of two main contrasting types—regularly-disposed veinlets which are composed almost entirely of K-feldspar and quartz, and irregularly shaped discontinuous trondhjemitoid variants rich in biotite spots and carrying rare garnet porphyroblasts. The field relationship between these two main types of veins is difficult to discern. In some parts of the out-crop the veins occur adjacent to one another and locally appear to merge. Veins with modal (and chemical) compositions intermediate between the spotted and the K-feldspar-rich veins have also been recorded from the outcrop.

Despite a wide whole-rock chemical variation, the minerals in all the veins are similar chemically and compare closely with corresponding minerals in the host gneiss. One exception may be the plagioclase in veins rich in K-feldspar where normative calculations indicate a composition near An14.

Interpretation of the trace element data counts against the veins having formed by partial melting and the variable K-feldspar contents place both main vein types well away from ternary minima in the system Q-Ab-Or. Calculations based on the compositions of co-existing biotite and garnet in the host quartzofeldspathic gneiss and in associated prograde pelitic schists indicate an equilibrium temperature of 650° ± 50 °C at pressures between 3 and 4 kilobars. This fact, together with trace element data and the calcic nature of the plagioclase in the homogeneous gneiss and the matrix phase, places some doubt on the suggestion that partial melting has played a signficant role in the genesis of the veins.

Although mechanisms of vein formation such as igneous injection or metasomatism are considered as other alternatives for vein formation, we suggest that metamorphic segregation (as proposed for the origin of the spots in the gneiss) will also account for the development of the veins.

The chemical stabilities of mendipite, Pb3O2Cl2, diaboleïte, Pb2CuCl2(OH)4, chloroxiphite, Pb3CuCl2O2(OH)2, and cumengéite, Pb19Cu24Cl42 (OH)44, have been determined in aqueous solution at 298.2 K. Values of standard Gibbs free energy of formation, ΔGf°, for the four minerals are −740, −1160, −1129, and −15163±20 kJ mol−1 respectively. These values have been used to construct the stability diagram shown in fig. I which illustrates their relationships to each other and to the minerals cotunnite, PbCl2, paralaurionite, PbOHCl, and litharge, PbO. This diagram shows that mendipite occupies a large stability field and should readily form from cold, aqueous, mineralizing solutions containing variable amounts of lead and chloride ions, and over a broad pH range. The formation of paralaurionite and of cotunnite requires a considerable increase in chloride ion concentration, although paralaurionite can crystallize under much less extreme conditions than cotunnite. The encroachment of the copper minerals on to the stability fields of those mineral phases containing lead(II) only is significant even at very low relative activities of cupric ion. Chloroxiphite has a large stability field, and at given concentrations of cupric ion, diaboleïte is stable at relatively high aCl−. Cumengéite will only form at high concentrations of chloride ion.

Electron-microscope observations have been used to follow the low-temperature dehydration of boehmite, and show that a coherent skeleton of γ-Al2O3 with a porous texture is produced. For material fired in air the development of a number of pore systems is described. Of these the most important, as far as the dehydration process is concerned, is a lamellar pore system parallel to (001)γ and having a regular spacing of the order of 35–40. Å. A much faster dehydration process is induced by exposure to the electron beam and results in the formation of a more randomly oriented pore structure. The observed microstructure and the calculated internal surface area are compared with the results of previous adsorption studies.

The substitution of 2A13+ for Si4+ +M2+ (M = Mg or Ni) has been studied by infra-red spectroscopy in 7 Å trioctahedral minerals over the composition range from Al = 0.00 to Al = 2.00. Significant differences are observed in the infra-red spectra and these make possible an estimation of the amount of Al in tetrahedral coordination. The most striking effect of the substitution is observed in the stretching vibrations of the outer hydroxyl groups and that of the silicate anion.

The aluminous Mg-serpentines can be readily distinguished from their analogues, the aluminous Niserpentines, by the absence of the Mg-OH absorption band between 570 and 615 cm−1 in the latter.

The ideal end-members reddingite, Mn32+ H2 O)3(PO4)2, phosphoferrite, (H 2O)3(PO4)2, and kryzhanovskite, (OH)3(PO4)2 form a complex triple series. Similarities in crystal axes and pronounced differences in site preferences have led to erroneous indexing of the powder data and subsequent errors in cell refinements. Writing the general formula M(1)M(2)2[(H2O),(OH)]3(PO4)2, the following end-member names apply:

Type landesite,Ca0.4Mg1.2(OH)3.2(H2O)8.8(PO4)8.0, has a = 9.458(3) Å, b = 10.185(2) Å, c = 8.543(2) Å, space group Pbna. R = 5.2% for 1821 independent reflexions (Mo-Kα radiation). The distance averages are M(1)-O = 2.098 Å, M(2)-O = 2.205 Å, P-O = 1.539 Å. Cotype kryzhanovskite,Ca0.5Mg0.4(OH)7.3(H4O)4.7(PO4)8.0, has a = 9.450(2) Å, b = 10.013(2), c = 8.179(2). R = 7.2% for 1,703 independent reflexions (Mo-Kα radiation). The distance averages are M(1)-O = 2.017 Å, M(2)-O = 2.115, P-O = 1.542.

Computed powder pattern intensities from the structure data admitted extensive revisions of earlier published Miller indices and revised powder data are presented.

Oxygen isotope fractionation between rutile and water has been studied from 300 °C to 700 °C, PH2O = 1 kb, using aqueous oxidation of titanium metal as the equilibration reaction. The mechanism of rutile formation (which is critical to the assessment of isotopic equilibrium) is an ‘armouring’ reaction in which rutile grows around grains of titanium metal by solution-precipitation processes. Mean fractionation factors expressed as 103 In αTiO2-H2O obtained in the present study are:

−6.20±0.23‰ at 304±5 °C

−6.64±0.27‰ at 405±6 °C

−6.11±0.16%. at 508±6 °C

−4.45±0.28%. at 608±6 °C

−3.38±0.15%. at 698±6 °C.

These data agree with those obtained at temperatures above 500 °C by Addy and Garlick (1974) but do not accord with theoretical predictions by Bottinga and Javoy (1973). A minimum in the calibration curve 103 ln α versus 106T−2 occurs between 300 °C and 500 °C but from 500 °C to 700 °C 18O fractionation between rutile and water may be expressed by the equation:

103 ln α = −(4.72±0.40)106T−2+(1.62±0.53).

Oxygen isotope analyses of rutile and quartz from metamorphic eclogites and schists from the Tauern Window, Austria, yield isotopic temperatures at about 550 °C in agreement with results obtained on similar rocks from the Sesia Zone (Western Alps, Italy) and elsewhere by other workers. Petrologic studies indicate that the latest metamorphism of the Tauern eclogites reached about 450 °C Thus the measured partitions of 18O between rutile and quartz indicating temperatures around 550 °C have been inherited from an earlier metamorphic event.

Awaruite is not known from Awarua River or Awarua Bay, Westland, New Zealand, but was first described from alluvial sands of the Gorge River and soon thereafter, in situ in serpentinite from the same valley. The name is a misnomer.

Transmission electron microscopy of several chondritic meteorites reveals exsolution products of two kinds in olivine. The coarser variety comprises particles that have nucleated heterogeneously on subgrain boundaries. Electron microprobe data confirm that these are chromite. The second variety, consisting of smaller precipitates distributed randomly, is interpreted as a homogeneously nucleated analogue of the first. It is potentially important as an indicator of cooling history. The aluminium-poor compositions of heterogeneously exsolved chromites suggest that some Cr may have entered tetrahedral sites in olivine during initial rapid crystallization from the melt.

The morphologies of hydrothermally grown reedmergnerite crystals change systematically with time and rate of change of B, Si order. Three stages in this process are recognized: (1) Growth of well-faceted crystals. (2) Development of etching leading to the formation of irregular grains. (3) Modification of irregular grains to form rounded or ellipsoidal crystals. A solutionreprecipitation process is responsible for the morphological changes and the increase in degree of order of B and Si with time at constant temperature and pressure.

Small-scale assimilation of limestone during the intrusion of an olivine basalt feeder dyke into an Eocene argillaceous, siliceous biomicrite in the Tokatoka area has resulted in the incorporation of large amounts of calcium into the parent magma. Initial effects of assimilation have caused partial resorption of the early-formed igneous mineralogy and the precipitation of calcic, iron-rich clinopyroxenes (ferrosahlite to hedenbergite), wollastonite, schorlomite, and pyrrhotine. Pyroxene compositions show a trend of strong enrichment in Catschermak and ferrosilite components. Derivative hydrothermal solutions, rich in Ca, Si, Al and alkalis have precipitated and altered anhydrous phases to tobermorite, thomsonite, prehnite, pectolite, cebollite, hydrogrossular, gismondine, analcime, Sr- and Ba-bearing zeolites, and calcite.

Modelling of the basalt-limestone assimilation process by least-squares mixing methods has shown that the observed chemical variation can largely be accounted for by the dilution of the basalt with up to 30 wt. % decarbonated limestone. Desilication of the liquid, a result of this dilution effect, has been accommodated in the chemistry of the early-formed mineralogy rather than by the crystallization of minerals characteristic of an undersaturated rock type.

Contemporaneous with the intrusion of the basalt was the high-temperature contact metamorphism of the limestone. This produced assemblages of rankinite, kilchoanite, larnite, spurrite, grossular, and tobermorite. Subsequent injection of the basalt and hybrid phases into fractures has resulted in the alteration of the primary metamorphic assemblage to wollastonite, scawtite, foshagite, hydrogrossular, calcite, and vaterite.

A statistical study on the modal analysis data of eight minerals of the Meldon aplite is made with a view to determining their distribution patterns in the rock. Various parameters and statistics have been calculated and the results are tabulated.

Topaz and elbaite are considered as censored samples; ‘normalization’ in the case of these two minerals is obtained by double probit transforms. Findings of the statistical study are correlated with the modes of formation of the minerals.

Grandidierite has been identified in two aluminous metasedimentary xenoliths from quartz-latitic volcanic rocks from Mt. Amiata and Mt. Cimino, central Italy. Physical and electron microprobe data for the grandidierites and petrological data for the grandidierite-bearing xenoliths are presented. The grandidierites formed by a reaction involving pre-existing aluminium rich minerals, possibly at temperatures of at least 800 °C and at low pressures. The grandidierite from Mt. Amiata replaces sillimanite. Several common characteristics can be demonstrated for magmatic and metamorphic grandidierite-bearing rocks. It is suggested that metamorphic rocks in which grandidierite occurs have often undergone partial melting.

Spectral reflectance measurements on three uniaxial ore minerals, tellurium, chalcopyrite, and stibioluzonite, which are opaque at least in the visible part of the spectrum have revealed that the reflectance curve of the ordinary ray varies with crystallographic orientation of the polished section. The three minerals possess symmetries capable of exhibiting optical activity in transmitted light. A possible explanation, therefore, of the anomalous behaviour is that the optical constants, i.e. the refractive index and the absorption coefficient, and thus also the reflectance, of the ordinary ray may differ for sections cut normal to c where optical activity probably has its maximum effect and for sections cut parallel to c where there is probably little or no complication due to optical activity. There would therefore appear to be a need to extend the theory of reflection from absorbing media to include reflection from optically active absorbing minerals.

Anomalous iron meteorites are those which do not have Ni, Ga, and Ge contents appropriate to one of the twelve chemical groups; they account for 14% of all irons. The chemistry of irons in the twelve groups can be largely understood in terms of primary fractionation in the nebula, which established the bulk composition of the groups, and secondary fractionation in the parent bodies (probably fractional crystallization), which produced the chemical trends within groups. Logarithmic element-Ga graphs containing data for groups and anomalous irons reveal that anomalous irons experienced the same primary and secondary fractionations as affected the groups.

The uniformity of chemical trends within groups allows possible genetic relationships between anomalous irons and groups and among anomalous irons to be tested. It is concluded that the sixty-nine anomalous irons are samples from fifty-odd additional groups, which had similar histories to the twelve groups. Less than five of the anomalous irons could be compositional end- members or reprocessed irons from the groups.

Because ‘anomalous’ means abnormal, some other term for the irons which do not belong to the twelve groups would be a useful reminder that these irons formed in a similar way to irons in the major groups. They could be called members of minor groups or grouplets.

The Verkhne Dnieprovsk material (BM 51183) in the collection of the British Museum (Nat. Hist.) is chemically group IIE and therefore distinct from Augustinovka. Structurally it is a finest octahedrite, with a kamacite band-width (0.05 mm) and bulk Ni content (Scott and Wasson, 1976) appropriate to a cooling rate of 830 K Myr−1. Subsequent pre-terrestrial shock has produced localized deformation and shock-heating effects, which include mm-size globules of metal-phosphide melt product with a dendritic texture indicating resolidification at c.10 000 K sec−1. It is suggested that the phosphides were already fissured at the time of shockmelting. The structure of Verkhne Dnieprovsk, although lacking silicates, is analogous to that of Kodaikanal.

Free energies of formation of divalent metal ion zippeïtes, M2(UO2)6(SO4)3(OH)10 ·nH2O, M = Mg,Co,Ni,Zn have been determined from solution studies and metal speciation calculations, in water. It is found that in the compounds, the number of molecules of water of crystallization is equal to 8. This is at variance with a previous report (Frondel et al., 1976), but it has been found that some at least of the water content of zippeite is either nonessential or very loosely bound in the structure. Based on the octahydrate formulation, values are −3506, −12695, −12683 and −12870±4 kJ mol−1 for the Mg,Co,Ni and Zn end-members, respectively. Almost all of the differences in the values are accounted for by those values for the metal ions alone with the exception of Znzippeïte where a discrepancy of some 22 kJ mol−1 is found. Even this value is small however, and the chemical studies indicate that extensive mutual solid solution between all end members is to be expected. These findings agree perfectly with observations on the composition of naturally occurring zippeite minerals of this group.