bluuebunny/crossref_metadata_2025 huggingface dataset:
Russellite: a second occurrence
Authors: Hodge L. C.
Published: December 2006

Russellite Bi2O3. WO3 occurs in a small pegmatite near Poona, Western Australia. The fine-grained yellow to pale green material is an inseparable mixture of russellite, bismite, koechlinite, and bismutite. X-ray powder diffraction, physical, and chemical data agree in general with the original description of the mineral from Cornwall, England. The original analyses made on micro quantities are now supplemented by analyses on macro quantities.
The initiation of crystal growth in glasses
Authors: Rogers P. S.
Published: December 2006

The application of classical nucleation theory to the initiation of crystal growth in glasses is discussed. Its application to experimental results obtained for the rate of nucleation in three types of glass, one showing nucleation separately from crystal growth, another showing simultaneous nucleation and growth, and the third showing crystal growth after metastable liquid/liquid phase separation, is then described. Recent work on the kinetics of unmixing in glasses is outlined. The influence of so-called ‘nucleating agents’ in the glasses described appears to be exerted through changes in the anionic structure.
The kinetics of crystal growth in an aluminosilicate glass containing small amounts of transition-metal ions
Authors: Williamson J.
Published: December 2006

Quantitative data are presented for the kinetics of crystal growth in an aluminosilicate glass to which a few wt % of a transition-metal oxide had been added. The composition of the base glass was CaO 3o, MgO 2, Al2O3 15, SiO2 53 wt %, to which the oxides of iron, vanadium, chromium, or zinc were added. Glasses were prepared with a range of high to low valency ratios in cases where the transition metal oxide could exist in two or more oxidation states.

Small quantities of Fe2+, Fe3+, Zn2+, V3+, V4+, and V5+ were all found to increase the kinetics of crystal growth from the glass phase. The magnitude of the effect is compared with additions of Mg2+ to the glass. Cr2+, Cr3+, and Cr6+ were found to decrease the rates of crystal growth. Values for the apparent activation energy for crystal growth ranged from ∼50 kcal/mole to ∼150 kcal/mole. The high values were found to be of the same order of magnitude as that for the activation energy for viscous flow in the glass at the crystallization temperature.

The results are interpreted in terms of the structure of the glasses and current theories on the rate of crystallization of glasses.
Nucleation and growth of iron oxides in olivines, (Mg,Fe)2SiO4
Authors: Champness P. E.
Published: December 2006

Iron-rich olivines have been oxidized in air in the laboratory and the mechanism of their breakdown has been elucidated using X-ray diffraction and electron microscopy. Low-temperature oxidation (500–800 °C) produces well-oriented hematite- and magnetite-like precipitates together with amorphous silica. The reaction is a cellular one in which thin needles of oxide about 50–100 Å apart grow into the matrix separated by regions of amorphous silica. Nucleation of spherical colonies of the iron oxide and silica occurs on dislocations.

Although the hematite or magnetite always shows the same topotactic relationship with the matrix, the direction in which the needle-like precipitates grow is determined by the orientation of the nucleating dislocation. The small size and highly distorted nature of these precipitates accounts for the diffuseness of their X-ray reflections.

Oxidation at 1000 °C produces undistorted equiaxed grains of the oxides about 0·2 μm in size. They are surrounded by silica, which produces a disordered electron diffraction pattern. As the temperature is raised, the silica achieves more structural order and the oxide grains increase in size.
Rotational fabrics in metamorphic minerals
Authors: Powell Derek , Treagus J. E.
Published: December 2006

A model has been constructed that establishes the geometry of inclusion fabrics to be expected in near-spherical crystals that grow continuously while either the crystals rotate in a static matrix or a matrix fabric rotates about them. Garnets in certain metamorphic rocks contain inclusion fabrics whose geometry is very similar to that of the model. Such syntectonic, snowball, or rotational garnets, as they have previously been termed, can be studied in three dimensions. A method is outlined whereby the rotation axes for individual crystals viewed in thick sections can be located in certain planes of cut. The significance of the geometry of included rotational fabrics is discussed in relation to previous work and to possible mechanisms of rotation.
Exsolution during metamorphism with particular reference to feldspar solid solutions
Authors: Sturt Brian A.
Published: December 2006

In the aureole of the syn-orogenic Hasvik Gabbro a variety of exsolution phenomena developed during the contact metamorphism. These exsolution phenomena have similar textures to those produced during the age-hardening of metal alloys. In feldspars the exsolution precipitates are closely related to the pattern of excesses over and deficiencies from the formula requirements, particularly in terms of SiO2 and Al2O3. The exsolution products are dependent upon both the temperatures of ageing and the details of the mineral chemistry. Evidence supporting an exsolution theory of origin for myrmekite is provided, and it is shown that precipitation from heterotype feldspar solid solutions during metamorphism may in fact be a complex process involving a number of precipitate phases.

The nucleation of sillimanite as a precipitate phase is discussed and it is shown how sillimanite develops at a number of distinctive sites within feldspar and garnet. From the textural relations of sillimanite precipitates and their host minerals a control exerted by the thermal stress configuration over the nucleation of precipitate phases is suggested. It is argued that the thermal stresses exerted across grain boundaries and within minerals during metamorphism may have considerable relevance to the interpretation of textures in metamorphic rocks.
Experimental modification of naturally deformed galena crystals and their grain boundaries
Authors: Stanton R. L.
Published: December 2006

It is now well established that naturally—occurring sulphides—particularly galena—are frequently found in the deformed state and that at least part of this deformation can be eliminated by annealing in the laboratory.

The present contribution is concerned principally with galena, for which it is indicated: that tectonic deformation leads to natural work hardening, which may be reduced by simple heating in vacuo; that the rate of softening is related to the annealing temperature; that softening is related to recovery at low temperatures and to both recovery and recrystallization above 250–300 °C that single crystals of galena exhibiting minor slip and kinking recrystallize yielding new, unstrained grains elongated parallel to the old slips and kinks, i.e. a foliation involving unstrained grains may form under a regime devoid of compressive stress; and that where deformed galena and deformed sphalerite coexist, the galena may be made to recrystallize completely (e.g. at 325 °C in 50 days) while leaving the deformation in the sphalerite entirely intact; apparently the coexistence of a strained component A and an unstrained component B in a deformed ore or other metamorphic rock does not necessarily indicate differences in age or deformational history—it may simply indicate that post-deformational temperatures have exceeded the recrystallization temperature of one component but not of the other.
Replacement of OH by OD in layer silicates, and identification of the vibrations of these groups in infra-red spectra
Authors: Russell J. D. , Farmer V. C. , Velde B.
Published: December 2006

Conditions necessary for exchange between lattice OH groups and D2O in expanding layer silicates are reported. From spectral changes in the 200–1200 cm−1 region resulting from this exchange, in-plane librations of OH co-ordinated to (A1A1), (A1Mg), (A1Fe3+), and (MgMgMg) have been identified in montmorillonite, beidellite, saponite, hectorite, and vermiculite. Both in-plane and out-of-plane librations have been identified for pyrophyllite and celadonite, from comparison of synthetic specimens containing OD with natural or synthetic OH forms. Comparison of hydrogen and deuterium forms of Mg- and Ni-talcs leads to the identification of translational and librational OH vibrations.
A metallographic and microprobe study of the Barranca Blanca meteorite
Authors: Axon H. J. , Faulkner D.
Published: December 2006

The distribution of kamacite, taenite, and plessite suggests that kamacite nucleated at the sulphides, which appear to have been plentifully dispersed in the parent taenite. Such a process could take place without the undercooling that is usually assumed in normal octahedrites. The plessite fields show internal boundaries that are probably relics of grain boundaries in the parent taenitesulphide aggregate. Microprobe investigations have been made of nickel distributions in and between kamacite, taenite, plessite, and schreibersite, and particular attention has been paid to the nickel distribution at the internal grain boundaries in the plessite. Evidence is presented for the diffusion of nickel along these boundaries and for their migration during the later stages of formation of the microstructure.
The metal phase of Horse Creek, Mount Egerton, and Norton County enstatitic meteorites
Authors: Wai C. M.
Published: December 2006

Results of an electron microprobe study of the metal phase and perryite inclusions in the metal of Horse Creek, Mount Egerton, and Norton County enstatitic meteorites are given. The origin of the ‘pseudo-octahedral’ structures observed in the metal phase of the Horse Creek and Mount Egerton ‘anomalous enstatitic meteorites’ is discussed.
Genesis of hydrothermal K-feldspar (adularia) in an active geothermal environment at Wairakei, New Zealand
Authors: Steiner A.
Published: December 2006

Hydrothermal almost pure potassium feldspar (adularia) forms incrustations on fissured wall rocks in an active geothermal environment. Measured max. temperature, 265 °C, and concentration of K+ and Na+ cations in the geothermal fluid require K-feldspar and Na-feldspar to be deposited under equilibrium conditions. On fissure walls only K-feldspar is precipitated, whereas andesine phenocrysts of host rocks are replaced by both K-feldspar and Na-feldspar, and groundmass of host rock is replaced by K-feldspar.

Optical data indicate that K-feldspar forming incrustations is partly monoclinic, partly triclinic. The triclinic material displays polysynthetic and ‘microcline’-like twinning, though X-ray diffractograms indicate only monoclinic structure. A primary origin for the triclinic adularia is postulated. Wairakei hydrothermal K-feldspar, max. 0·5 Myr old, is compared with Arkansas adularia 287 or 214 Myr old, also containing both monoclinic and triclinic material. The comparison suggests that, after crystallization, the structural state of adularia changes little, if at all, during geological time.
Hornblende—anthophyllite and hornblende—cummingtonite parageneses
Authors: Choudhuri A.
Published: December 2006

The associations hornblende and anthophyllite or cummingtonite are dealt with briefly in the light of experimental data and naturally occurring assemblages. On the basis of the distribution of Mg and Fe2+ in coexisting amphiboles and on the calculated and experimental slopes of a reaction in which the amphiboles hornblende and anthophyllite are formed a tentative P-T-x diagram is suggested for these parageneses.
Structural analysis of the most commonly encountered habit faces of calcite crystals from skarn deposits
Authors: Kashkai M.-A. , Aliev R. M.
Published: December 2006

This paper is a report on the various features of the crystallogenesis of calcite as shown in specimens collected from iron-ore skarn deposits. Although there are several forms, each showing a variety of habit, a detailed study shows that there is a unified sequence of successive habit substitution. A structural analysis of calcite based on P.B.C. (Periodic Bond Chain) vectors is then considered.
Plumbogummite-group minerals from Mull and Morvern
Authors: Bain D. C.
Published: December 2006

Sandstones from Mull and Morvern, most of which are from the Greensand formation, have been shown by X-ray diffraction to contain minerals of the plumbogummite group in very small amounts in the < 1·4µm fraction. The minerals were concentrated by HF digestion of the clay minerals. X-ray spectrographic traces show concentrations of Sr, La, Ce, Yt, and Ba, and a semiquantitative spectrochemical analysis also shows a concentration of Ca and Pb and the presence of numerous rare-earth elements. Individual minerals, which have a complex composition and can not be assigned to any one named species, are disseminated throughout the rocks as particles with an estimated size of between 0·1 and 0·25µm.
Hallimond Lecture 1970 Electron-Optical study of phase transformations
Authors: McConnell J. D. C.
Published: December 2006

This paper is primarily concerned with the interpretation of phenomena associated with phase transformations in single crystals, as observed by current experiments using electron-optical techniques. An analysis of permissible metastable behaviour in such single crystals, in terms of lattice theory, is provided. The dominant role of symmetry in defining the constraints on permissible be-haviour of this kind is explained. The resulting theory indicates that, at low temperatures, permissible thermal fluctuations in local chemical composition, or order, become strongly degenerate and lead to the development of non-Bragg diffraction effects. The theory and practical study of the phenomena by electron microscopy are also described.

The theory of metastable behaviour in a single crystal subject to symmetry constraints is then tested by re-examining the experimental data available for two systems that have been studied recently where such behaviour might be suspected. The first of these concerns the low-temperature behaviour of adularia, and the second deals with incipient exsolution phenomena in the alkali feldspar from a pantellerite. The data in each case appear to be compatible with the general theory proposed. Finally the theory is used to analyse the low-temperature behaviour of the intermediate plagioclase feldspars. In this case the theory indicates that both antiphase phenomena and schiller effects are associated with metastable behaviour in a system subject to strong symmetry constraints at low temperature. The implications of the analysis in relation to peristerite exsolution phenomena are discussed.
A microprobe study of chromites from the Andızlık-Zımparalık area, south-west Turkey
Authors: Engin T. , Aucott J. W.
Published: December 2006

Compositional changes in chromites are shown to be related to the degree of alteration of the grains to ‘ferritchromit’. The chromium content of late stage pyroxenites and tremolites may be related to the development of the ‘ferritchromit’. Analysed grains from the chrome ores are shown to have a different chemistry to the accessory chromites in the peridotites.
Host control of recrystallized quartz grains
Authors: Ransom D. M.
Published: December 2006

The results of a petrofabric study on recrystallization of quartz in nature are presented and compared with recent experimental work. The orientations of newly recrystallized quartz grains are shown to be dependent on the orientation of host grains from or in which they grew. The results are compatible with the experiments of Hobbs (1968). The orientation of new grains appears to be unrelated to stress.
The Molteno meteorite
Authors: Frost Michael J.
Published: December 2006

The Molteno meteorite was seen to fall in South Africa in 1953. It is a howardite, with pyroxene varying from En76Fs22Wo2 to En38Fs58Wo4 and then to En33Fs26Wo41. Also present are twinned plagioclase (An 84·4%) and small quantities of olivine (Fa 12%), troilite, nickel-iron (Ni 4%), ilmenite, and an aluminian chromite with 0·71% V2O3. A bulk analysis and analyses of the metal, troilite, and combined silicates and oxides are given.
A metallographic study of the angra dos Reis (iron) meteorite
Authors: Axon H. J. , Waine C. V.
Published: December 2006

The Angra dos Reis (iron) has been studied metallographically and an attempt has been made to discuss the circumstances under which the following elements of structure formed: clear etching and frosty etching kamacite, decorated Neumann lines, giant rhabdites, plate rbabdites, rhabdite clusters, microrhabdites, cohenite, and remelted troilite. The remelted troilite is taken to indicate a shock event. However, since there are no metallographically visible indications of shock in the kamacite and since the back reflection X-ray diffraction pattern shows only very faint Debye-Scherrer arcs superimposed on a pattern of sharp spots, it is concluded that the shock event took place at a temperature that allowed shock effects to anneal out of the kamacite almost completely. A submicroscopic precipitate in the metallic matrix is observable with the electron microscope and may represent the final precipitation of phosphide from shocked kamacite.
Rhyodacite glass in Réunion basalt
Authors: Upton B. G. J. , Wadsworth W. J.
Published: December 2006

The local development of strongly oxidizing conditions during crystallization of a recent basalt lava lake on Piton de la Fournaise, Réunion, led to the production of approximately 15 % residual rhyodacite liquid. This was segregated into the vesicle cavities as the lava solidified, and congealed as a glass containing microlites of feldspar, clinopyroxene, and hematite. The rhyodacite is compared chemically with the residual glasses recorded from other basaltic and doleritic occurrences, and with the Mauna Kuwale rhyodacite of Oahu.
Phenocrystic olivines from the eastern Azores
Authors: Boone Gary M. , Fernandez Louis A.
Published: December 2006

Phenocrystic and xenocrystic olivines and one olivine nodule sample, all from volcanics in the eastern Azores, range in composition from Fo87 to Fo69. Most are phenocrysts from the Nordeste alkali basaltic complex, eastern São Miguel Island, in which olivine-bearing rocks range from ankaramite to latite. CaO, MnO, and NiO trends are related to Mg-Fe contents, both for the concentrates as a whole, and also within analysed zoned crystals. It is shown that anomalous plots of Δ2ϑ(220LiF—062olivine) are related to pronounced Mg-Fe zonal gradients and, in samples from some flows, to mixtures of different generations of olivine. Regressions of Δ2ϑ vs. Fo (mol %) and FeO (wt %) for the least-zoned olivines are in close accord with those for Hawaiian olivines reported by Murata and others (1965).

MnO percentages increase linearly with FeO; this and the Hawaiian trend are indistinguishable. Limited data for the most magnesian olivines indicate that NiO increases with FeO up to ∼ 15 wt % of FeO; the trend then decreases with further iron enrichment. This trend is apparent from other olivine data, and its relationship to the onset of pyroxene crystallization is discussed. CaO in olivine phenocrysts is inversely related to normative anorthite percentages in host rocks; the trend appears to be controlled largely by co-precipitation and fractionation of calcic plagioclase.
The herzenbergite-teallite series
Authors: Chang Luke L. Y. , Brice William R.
Published: December 2006

Equilibrium relations of lead-tin sulphides in the system SnS-PbSnS2 were studied using a sealed silica-tube technique. A complete series of solid solutions forms in the system, and lattice parameters were found to vary lineally with composition. The melting temperature of the solid solution decreases with increasing amount of PbS from herzenbergite at 880 °C to teallite at 725 °C.

Evidences were found to indicate the existence of this series of solid solution in ore deposits.
Chemical composition and physical properties of lithium-iron micas from the Krušné hory (Erzgebirge), Czechoslovakia and Germany. Part B: Cell parameters and optical data
Authors: Rieder M. , Píchová A. , Fassová M. , Fediuková E. , Černý P.
Published: December 2006

Unit-cell dimensions and refractive indices of lithium-iron micas decrease with decreasing iron and increasing lithium. Indices β and γ as well as parameters a and b can be used to estimate the composition of lithium-iron micas but basal spacing and 2Vα are poor indicators of composition.

The chemical composition of natural lithium-iron micas from the Krušné hory and the Erzgebirge along the Czechoslovak—German border was discussed in Part A of this study (Rieder et al., 1970). It was concluded that the composition and crystallography of these micas fit best the series siderophyllite-polylithionite. The compositions were expressed by the ratio A′ = LR/(LR+‘Fe’). In this expression, LR is the subscript value of Li or octahedral R3+ (whichever is the smaller) in the crystallochemical formula, ‘Fe’ is the sum of the values in the formula of Fe2+ and Mn2+. A′ therefore defines the position of a particular mica on the siderophyllite-polylithionite join. This paper deals with the correlation between composition, cell dimensions, and refractive indices.
Further investigations into the nature of the materials chlorophaeite and palagonite
Authors: Stokes K. R.
Published: December 2006

Specimens of chlorophaeite and palagonite obtained from eight countries were examined using a variety of techniques. Investigations showed the materials to be not simple substances but multiphase composites. The confusion that existed in the literature necessitated a revision of the nomenclature and construction of criteria for identifying the two materials. Their nomenclature has been expanded: chlorophaeite to include the following phases; phase I, green and isotropic; phase II, golden yellow and isotropic; phase III, darker than phase II and anisotropic; a fourth phase, named associated material (A), microcrystalline, pale green and sometimes pleochroic. Similarly for palagonite, a golden yellow glassy phase, essentially basaltic glass (no new name needed); phase X, a hydrated phase slightly darker in colour than the glassy phase, also isotropic; phase Y, dark brown to orange, microcrystalline, anisotropic, found adjacent to phase X; associated material (M) green, anisotropic, often chlorite or chloritic. Both of the above descriptions were based on neotypes, established during the work. X-ray diffraction data for the two materials are given together with microprobe analyses of some of the phases of the two materials.
Synthetic ‘deweylite’
Authors: Speakman K. , Majumdar A. J.
Published: December 2006

Materials similar to the natural hydrated magnesium silicate mineral deweylite have been synthesized hydrothermally under low-temperature-low-pressure conditions from magnesia-silica gels. Optical, X-ray, infra-red, and DTA examinations have shown that both the natural and synthetic materials are mixtures of badly crystallized talc and badly crystallized serpentine. The mineral is thought to be a coarse mixture of these two components while the synthetic products are intimately interlayered. These materials are believed to be closely related to the hydrated magnesium silicates detected in high-magnesia cement hydration products.
Elba manganese ores and their origin, South-eastern Desert, U.A.R.
Authors: Basta E. Z. , Saleeb W. S.
Published: December 2006

The manganese ores of Elba, South-eastern Desert, occur as numerous veins located within a narrow belt trending N.W.-S.E. Three ore-types were distinguished: hard crystalline ore consisting mainly of pyrolusite or ramsdellite or both, banded colloform ore consisting mainly of psilomelane, and in places cryptomelane, and soft nodular ore consisting of todorokite with minor amounts of psilomelane (or cryptomelane), nsutite, and pyrolusite. Black calcite and baryte occur in some of the veins and increase with depth.

Based on the field and mineralogical evidence the origin of the ore is discussed. It is suggested that the ore is a very low-temperature epithermal fissure deposit of black calcite type that occurs near the surface (oxidation zone) in brecciated zones along faults.
Crystal structure of coalingite
Authors: Pastor-Rodriguez J. , Taylor H. F. W.
Published: December 2006

The crystal structure of coalingite (Mg10Fe2(OH)24(CO3)·2H2O) has been determined using single-crystal X-ray methods. The mineral is trigonal, with space group Rm, aH = 3·12, cH = 37·4 Å, Z = ½, and (0001) cleavage. The structure is of a layer type, and is based on a structural element about 12·5 Å thick in the c-direction and consisting of two brucite-like layers and one disordered layer containing carbonate ions and water molecules and resembling those in sjögrenite and pyroaurite. The unit cell comprises three of these structural elements stacked together in the c-direction. The Mg2+ and Fe3+ ions are randomly distributed among all the octahedral sites of the brucite-like layers. The structure closely resembles those of sjögrenite and pyroaurite, but has two brucite-like layers between each CO32−−H2O layer where these have one. There is a tendency to random interstratification, and the crystals appear to contain intergrown regions of brucite and of sjögrenite or pyroaurite. Coalingite-K probably has a similar structure, but with three brucite-like layers between each -H2O layer; its idealized formula is probably Mg16Fe2(OH)36(CO3).2H2O.
Distribution of potassium feldspar polymorphs in intrusive sequences
Authors: Parsons Ian , Boyd Rognvald
Published: December 2006

K-feldspar variation is described for four salic plutonic igneous complexes that show normative mineral variation in keeping with the course of fractionation predicted from experimental studies. These are a sodic syenite (Loch Ailsh), potassic syenites (Loch Loyal), a peralkaline syenite and soda granite complex (Puklen), and a calc-alkaline pluton (Foyers). Additional data are given for examples from the literature. Microcline becomes more abundant relative to orthoclase as members of these intrusive sequences approach thermal minima on the liquidus in the system Ab-Or-An-Qz. This relation exists irrespective of the compositional range of the rock suites, the bulk composition of the alkali feldspars, grain size, or field relations. Early orthoclase-bearing rocks retain orthoclase when enclosed as xenoliths in microcline-bearing rocks, or when cut by minor bodies of microcline-bearing rocks. The K-feldspar variation may reflect increasing water content or the peralkaline or peraluminous character of the fractionating magmas. These factors affect the feldspar structure at the time of initial crystal growth and dictate whether microcline will develop on cooling.
The stability and natural occurrence of margarite
Authors: Velde Bruce
Published: December 2006

The upper stability of margarite has been determined to be 450 °C at 1 kb and 530 °C at 2 kb total pressure (PH2O = Ptotal). Other experiments delimit in a general way the lower stability of this mineral and its paragenesis in more SiO2 rich compositions. The presence of the 060 reflection at 1·475 Å can be used to distinguish between margarite and paragonite or muscovite.
Twinning and intergrowth of olivine crystals in chondritic meteorites
Authors: Dodd R. T. , Calef Charles
Published: December 2006

The angular relationships among sets of olivine plates in barred chondrules were investigated on the universal stage to test a suggestion that such chondrules represent exsolution and inversion from a pre-existing jadeitic pyroxene. In one of sixty-seven cases studied, adjacent plates represent olivine twinning on (100); the others vary randomly and correspond to neither olivine nor pyroxene twinning. On this basis and from other evidence, we conclude that the mineralogy and texture of barred chondrules are primary and due to rapid crystallization of ultrabasic magma in a low-pressure environment.
Conditions for the formation of bayerite and gibbsite
Authors: Mchardy W. J. , Thomson A. P.
Published: December 2006

Aluminium hydroxide gels have been prepared by the hydrolysis of amalgamated aluminium in water and by precipitation from aluminium salt solutions with an anion exchange resin in the hydroxyl form. The products crystallizing from such gels have been examined by electron microscopy and by X-ray and electron diffraction. Bayerite crystallizes as cone or pyramid-shaped particles and gibbsite as hexagonal plates or prisms. Two types of gel are postulated. The first type, pseudoboehmite, predominates in the absence of acids, is uncharged and rapidly crystallizes to bayerite; the second type, pregibbsite gel, occurs in carboxylic acid solutions, is positively charged and, in the absence of inorganic anions, crystallizes slowly to gibbsite.
The relationship between optical properties and occurrence of some black tourmalines from Northern Nigeria
Authors: McCurry Patricia
Published: December 2006

Seventy-nine black tourmalines collected over a 12 000 km2 area of Precambrian to lower Palaeozoic rocks in northern Nigeria show a close correlation between maximum R.I., absorption colour, and occurrence in non-feldspathic micaceous schists and schorl rocks on the one hand, and in gneisses, migmatites, and feldspathic pegmatites on the other. Pleochroism is known to depend upon the total iron content and the Fe2+Fe3+ interaction. Colour variations may thus provide useful indicators of the geochemical environment during crystallization.

Limited published data, supplemented by additional measurements on four tourmalines from outside the original area, suggest that similar correlations would be obtained for the rest of Nigeria.

Selective processes of boron metasomatism may have been important in the association of different coloured tourmalines with feldspathic and non-feldspathic rocks, and the apparent lack of biotite in most tourmaline-rich rocks. The source of the boron probably lay within the upper Proterozoic to lower Palaeozoic metasedimentary belts with which the tourmaline is most strongly associated.
On aluminous and edenitic hornblendes
Authors: Leake Bernard E.
Published: December 2006

Based on nearly 1500 published amphibole analyses the maximum possible Alvi in hornblendes is shown to increase with increase of Aliv. New analyses of hornblendes from amphibole-corundum rocks, with and without anorthite, are given and after critical examination of the available data it is concluded that the maximum verified Alvi-rich calciferous amphibole that approaches the closest to hypothetical tschermakite comes from a kyanite-bearing aluminous high-pressure-crystallized schist from Lukmanier, Switzerland. Pure natural edenite or ferroedenite is unknown, but a new analysis of the nearest known natural edenite, from Mysore, India, agrees with the postulated view that extraordinarily low temperatures are needed for edenite-ferroedenite crystallization, much below that possible in magmas and only rarely achieved in metamorphic rocks containing amphiboles. The limit of the approach of igneous hornblendes to edenite-ferroedenite and tremolite-ferroactinolite is outlined.

At least 1100 °C is required for complete expulsion of water from some amphiboles.
The uranium-bearing mineral bolivarite: new data and a second occurrence
Authors: Van Wambeke L.
Published: December 2006

A second occurrence of the mineral bolivarite has been found in the Kobokobo pegmatite, Kivu, Republic of Congo (Kinshasa). This amorphous mineral was previously considered to be evansite. Its physical properties are: weak birefringence; biaxial to uniaxial positive; n = 1·50–1·51; specific gravity 1·97–2·05; hardness 3 to 3½.

Three chemical analyses have been performed on the Congolese material. The general formula of the mineral is Al2(PO4)(OH)3.4–5 H2O. Both known bolivarites from Spain and Congo contain abnormal amounts of U. The bright green UV fluorescence is due to the presence of U. The DTA curve of the Congolese bolivarite is given. Its genesis is discussed. This uranium-bearing mineral may be an indicator of uranium mineralization in Spain.
Chemical changes accompanying spherulitic crystallization in rhyolitic lavas, Central Volcanic Region, New Zealand
Authors: Ewart A.
Published: December 2006

Spherulitic devitrification is a post-eruptive process affecting many rhyolitic lavas of the Taupo region. The spherulites consist of cryptocrystalline intergrowths ofα-cristobalite and alkali feldspar (calcic anorthoclase), with minute granules of magnetite, hematite, and secondary (?)goethite. The effects of chemical fractionation occurring during progressive spherulite growth has been studied from a suite of samples from the Aratiatia rhyolite. The most significant effect is the progressive enrichment of both bulk spherulite compositions and the coexisting residual glass in potash with increasing spherulite development. This effect is due both to the very low potash in the earliest formed spherulites and to the consistently higher Na/K ratios of the spherulites relative to the total rock compositions. These differences progressively decrease with increasing spherulite crystallization. The bulk rock compositions, however, evidently remain essentially constant. The degree of potash enrichment in the residual glasses during advanced stages of devitrification is greater than expected by reference to the ternary feldspar and quartz-feldspar systems. This post-eruptive alkali fractionation during spherulite formation is superimposed on the pre-eruptive phenocryst-liquid fractionation.
The origin of the granitic sheets and veins in the Loch Coire migmatites, Scotland
Authors: Brown P. E.
Published: December 2006

Most of the granitic sheets and veins in the Loch Coire migmatite complex do not have chemical compositions in accordance with a formation by local anatexis. Regional metasomatism by fluids from outside the exposed confines of the complex was an important factor in the development of the sheets and veins, although there was probably some overlap with magmatic conditions.
Calc-aluminous insets in olivine of the Sharps chondrite
Authors: Dodd R. T.
Published: December 2006

A 250 µm olivine crystal in Sharps contains 1 to 5 µm insets identified by microprobe analysis as ferroan monticellite, spinel, and fassaite with average compositions: olivine (host), (Fe0·44Mg1·51Ca0·05) (Si0·99Al0·01)O4; ferroan monticellite, (Ca0·89Fe0·41Mg0·66) (Si1·00Al0·02)O4; spinel, (Fe0·41Mg0·59)(Fe0·063+Cr0·23Al1·69Si0·01Ti0·01)O4; and fassaite, Ca0.96(Fe0·01Mg0·59)(Al0·18Ti0·07Fe0·163+)(Si1·53Al0·47)O6. Textural and experimental data suggest early crystallization of spinel from a calcic olivine melt, exsolution of ferroan monticellite from the host olivine, and reaction of spinel and ferroan monticellite to form fassaite. The severe depletion of alkalies and silica in this crystal suggests it is a residue from vapour fractionation.
The cause of coloration in Derbyshire Blue John banded fluorite and other blue banded fluorites
Authors: Mackenzie K. J. D. , Green J. M.
Published: December 2006

The cause of coloration of Blue John fluorite from Castleton, Derbyshire, and blue banded fluorites from Ashover, Derbyshire, and Weardale, Co. Durham, has been investigated by a number of techniques, including mass spectrometry, optical spectroscopy, and paramagnetic resonance measurements on natural and irradiated samples. In all respects Blue John is indistinguishable from the other blue banded fluorites. Although traces of hydrocarbons were found in all the natural fluorites including Blue John, they are shown not to be the cause of the colouration. The optical spectra and bleaching behaviour are consistent with colouration by colloidal calcium rather than F-centres. The causes of colour banding are discussed.
The join CaMgSi2O6-Ca2MgSi2O7-CaTiAl2O6 in the system CaO-MgO-Al2O3-TiO2-SiO2 and its bearing on titanpyroxenes
Authors: Onuma Kosuke , Yagi Kenzo
Published: December 2006

The phase equilibrium diagrams of CaMgSi2O6-CaTiAl2O6 with 10, 20, and 30 wt % Ca2MgSi2O7 in the join CaMgSi2O6-Ca2MgSi2O7-CaTiAl2O6 were determined. Six three-phase assemblages, six four-phase assemblages, and two five-phase assemblages (isobaric trivariant, divariant, and univariant) were confirmed. The univariant assemblages are diopsidess+forsterite+åkermanitess + perovskite + liquid and diopsidess+åkermanitess+perovskite+spinel + liquid. Because of the complex solid solutions of diopside and åkermanite the crystallization ceases before invariant assemblages are reached and the final phase assemblage in the diopside-rich region is diopsidess+åkermanitess+perovskite, in the diopside-poor region diopsidess+åkermanitess+perovskite + spinel. Two invariant points, diopsidess+forsterite+åkermanitess+perovskite+spinel+liquid and diopsidess+åkermanitess+anorthite+perovskite+spinel+liquid in the silica-poor region of the system CaO-MgO-Al2O3-TiO2-SiO2 are estimated. The diopside solid solution in this join is discussed with its bearing on the natural titanpyroxenes from alkalic rocks including melilite.
The synthesis of birnessite, cryptomelane, and some other oxides and hydroxides of manganese
Authors: McKenzie R. M.
Published: December 2006

Detailed methods are described for the laboratory preparation of the manganese minerals manganosite, hausmannite, manganite, partridgeite, birnessite, cryptomelane, pyrolusite, and todorokite. New data are presented on the conversion of birnessite to cryptomelane, and on the exchange properties of potassium in these two minerals. It was found that the upper limit of potassium in cryptomelane is about 7 % K, while the lower limit lies between 0·25 % and 2·2 % K.
The composition of anomalous plagioclase glass and coexisting plagioclase from Mistastin Lake, Labrador, Canada
Authors: Currie K. L.
Published: December 2006

Anorthosite country rocks near the fissure vents of an andesitic resurgent caldera contain coexisting crystalline plagioclase, pseudomorphous plagioclase glass, and anomalously dense, massive plagioclase glass. Glass pseudomorphous after labradorite (An 53) has the composition of oligoclase (An 17). Large masses of dense plagioclase glass have compositions near An 53, but relict crystalline plagioclase within them has compositions near An 80. Devitrification products of this glass have potassium-rich compositions. These compositions are compatible with partial thermal melting in a high-temperature, moderate-pressure pulse. Such a pulse might be asscciated with a confined chemical explosion.
Charge-transfer in ferromagnesian silicates: The polarized electronic spectra of trioctahedral micas
Authors: Robbins D. W. , Strens R. G. J.
Published: December 2006

Polarized spectra (2000 to 25000 Å) have been obtained for fifteen analysed trioctahedral micas covering a wide range of compositions, and including four phlogopites, nine biotites, and two lepidomelanes. Three main contributions to the absorption have been noted: charge transfer from oxygen to ferrous iron throughout the visible and near ultraviolet, charge transfer from ferrous to ferric ions at the red end of the visible spectrum and internal d-d transitions of ferrous ions in the near infrared.

Substitutions in the brucite layer (especially Ti) cause the O → Fe2+ band to broaden, and thus to encroach on the visible region for vibration directions in the plane of the cleavage flake. The transmission window between these bands is further blocked by the Fe2+ → Fe3+ charge-transfer band, which is polarized in the plane of the flake. The overall effect is to produce very strong absorption throughout the visible region for (β, γ) vibration directions, and relatively weak absorption for α, i.e. the observed very strong pleochroism of biotite.

Many features of the spectra of chlorite, amphiboles, and tourmaline, which contain octahedral ions in brucite-like sheets, strips, and fragments respectively, may be interpreted by analogy with biotite.

Two types of solid solution effect are described: substitutional broadening, which is responsible for the extension of the O → Fe2+ band into the visible region in biotites and aluminous ortho-pyroxenes, and substitutional intensification, which permits in solid solutions transitions that are forbidden by the conventional selection rules.
Myrmekite and muscovite developed by retrograde metamorphism at Broken Hill, New South Wales
Authors: Phillips Evan R. , Ransom D. M. , Vernon R. H.
Published: December 2006

Retrograde metamorphism of gneisses and pegmatites leads in part to the destruction of feldspar and its replacement by late-stage lobate myrmekite and muscovite. Reactions promoted by retrogression suggest a range in volume of quartz production that may supplement that developed by exsolution and lead to deviations from the strict proportionality relationship suggested by previous workers. There is no need, however, to propose that quartz in myrmekite originates by constriction of pre-existing quartz within exsolved albite.
Natural garnet-pyroxene transformation in Cenozoic alkali basalt from Nigeria?
Authors: Wright J. B.
Published: December 2006

Aggregates of clinopyroxene in some doleritic lavas have external outlines strongly reminiscent of garnet crystals. They cannot be satisfactorily identified either as Upper Mantle fragments or as cognate synneusis aggregates, and it is suggested that they may have developed by inversion from original garnets, which crystallized at very high pressures. Neither morphology nor composition can provide conclusive proof of their original nature, but there is circumstantial evidence from the presence of an unusual inclusion suite in basalts nearby. The commonest phases are zircon and corundum, minerals more usually found in kimberlites than in basalts. Corundum in kimberlite inclusions is almost invariably accompanied by calcium-rich garnet, which adds some significance to the suggestion that these clinopyroxene aggregates are pseudomorphing original garnet.
Thermal decomposition of amosite, crocidolite, and biotite
Authors: Rouxhet P. G. , Gillard J. L. , Fripiat J. J.
Published: December 2006

The oxidation of amosite, crocidolite, and biotite has been determined at temperatures up to nearly 900 °C under both a vacuum (10−2 mm Hg) and oxygen (10 and 600 mm Hg). Infrared spectra gave the loss of constitutional hydroxyl under these conditions. The loss of tensile strength of the amphiboles with increasing temperature seems to be due to thermal decomposition. For the three minerals oxidation takes place progressively over a broad temperature range. Under vacuum there is a certain temperature above which the ferric iron previously formed is reduced; this temperature corresponds to the completion of the loss of hydroxyl. The crocidolite anhydride in the literature is most probably an oxycrocidolite formed by dehydrogenation, the truly dehydroxylated zones being amorphous.
The thermal expansion behaviour of the framework silicates
Authors: Taylor D.
Published: December 2006

This paper gives a revision and expansion of an earlier interpretation of the thermal expansion behaviour of the framework silicates. The partially-collapsed and ideal fully-expanded structures of quartz, cristobalite, and sodalite are characterized by the geometric relationship between the angle of rotation of their tetrahedra from the ideal fully-expanded state, their cell parameters, and the length of the tetrahedron edge. Their thermal expansion behaviour is interpreted as due mainly to the effect of the rotation of the tetrahedra towards the fully-expanded state modified by anisotropic thermal motion of the framework oxygens and distortion of the tetrahedra from a regular form. With the leucite and sodalite groups the significance of the interframework cations is discussed.
Study of metamict Ceylon zircons
Authors: Vance E. R. , Anderson B. W.
Published: December 2006

X-ray, density, refractive index, and optical absorption measurements have been made on metamict Ceylon zircons. The appearance of a previously reported anomalous optical absorption spectrum in low Ceylon zircons has been associated with small particles of cubic or tetragonal zirconium dioxide in a radiation-damaged zircon lattice, in both heated and unheated stones. No evidence of a previously reported second phase was detected in intermediate Ceylon zircons.
Titrimetric analysis for silicon
Authors: Kalocsai G. I. Z. , Hockley J. J.
Published: December 2006

Rapid, accurate silica determinations have been made titrimetrically following decomposition by HClO4-HF in a ‘Teflon’ vessel, the volatilized silicon tetrafluoride being precipitated as K2SiF6 in a HF-KF-glycerol-alcohol mixture. After filtration and washing with alcohol the precipitate is dissolved in boiling water, then titrated with NaOH. This method has many advantages over conventional gravimetric or spectrophotometric procedures.
Structure and twinning in
spencerite
Authors: Fanfani L. , Nunzi A. , Zanazzi P. F.
Published: December 2006

A refinement of the crystal structure of
spencerite,
Zn4(PO4)2(OH)2·3H2O,
was performed on a twin crystal from Salmo,
British Columbia. Three-dimensional intensity data
were collected by a Weissenberg apparatus. The
R index was 0·098 for
335 independent observed reflections. The crystal
structure previously determined is essentially
confirmed. Co-ordination around Zn is both
octahedral and tetrahedral. The atomic arrangement
consists of complex sheets of co-ordination
octahedra and tetrahedra around Zn and P atoms
connected by layers of water molecules. The
frequent occurrence of twinning in spencerite is
explained from structural considerations.
The crystal structure of stilpnomelane.
Part II. The full cell
Authors: Eggleton R. A.
Published: December 2006

The structural formula of stilpnomelane
averaged from 37 representative literature
analyses and based on the determined structure is
(Ca,Na,K)4(Ti0·1Al2·3Fe35·5Mn0·8Mg9·3)[Si63Al9](O,OH)216·nH2O.
Ferrostilpnomelanes have up to 12
R3+
cations in the octahedral sheet,
ferristilpnomelanes commonly have between 24 and
30
R3+
cations, rarely up to 37. The two groups can be
distinguished by accurate X-ray powder
diffraction, ferrostilpnomelanes having
d001
less and a greater than
have ferristilpnomelanes. The unit cell is
triclinic, a =
b∼21·8 Å,
γ = 120°,
d001∼12·2
Å,
P.
Hexagonal groups or ‘islands’ of 24 silicon-oxygen
tetrahedra co-ordinate to the octahedral sheet,
the centres of the ‘islands’ are displaced by
5a/12,
8b/12 on either side of
the octahedral sheet as a result of warping of
this sheet to permit articulation with the smaller
tetrahedral sheet. The 24-tetrahedra ‘islands’ are
linked laterally by 6-member rings of inverted
tetrahedra, which in turn link at their apices to
like rings connecting the next sheet of ‘islands’.
Thus a single tetrahedral layer is 4 tetrahedra
thick and has symmetry
6/m. The 6-member rings
of tetrahedra act as hinges permitting up to 0·5 Å
extension or contraction of the tetrahedral sheet
to accommodate variations in the octahedral sheet
dimensions with varying
R2+/R3+
ratio.