Showing papers in "Mineralogical Magazine in 1985"

Phase equilibria and mineral parageneses of metabasites in low-grade metamorphism

Abstract: The system Na20-CaO MgO A1203 SiO2-H20 is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe203 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quanti- tative effects on P-T displacement and phase composi- tions are discussed. The XFe 3 + isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T XFe~+ relations were established for continuous and discontinuous reactions relating such facies transi- tions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric A1-Ca-Fe 3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-A1 silicates in the presence of quartz, aibite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean- floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpelly- ite and epidote with increasing metamorphic grade are oversimplified.

The conversion of smectite to illite during diagenesis: evidence from some illitic clays from bentonites and sandstones

Abstract: Diagenetic illitic clays from seven North American bentonites of Ordovician, Devonian, and Cretaceous ages and from three subsurface North Sea sandstones of Permian and Jurassic ages have been examined by X-ray diffraction (XRD) and transmission and scanning electron microscopy (TEM and SEM). XRD indicates that the clays from the bentonites are randomly and regularly interstratified illite/smectites (I/S) with 30-90 % illite layers, whereas the clays from the Jurassic and Permian sandstones are regularly interstratified I/S, with 80 90 % illite layers, and illite respectively. TEM of shadowed materials shows that randomly interstratified I/S consists primarily of mixtures of elementary smectite and 'illite' particles (10 and 20A thick respectively) and that regularly interstratified I/S and illite consist mainly of 'illite' particles 20 50/~ thick and > 50 A thick respec- tively. Regularly interstratified I/S from bentonites and sandstones are similar with regard to XRD character and particle thickness distribution. These observations can be rationalized if the interstratified XRD character arises from an interparticle diffraction effect, where the smectite interlayers perceived by XRD, result from adsorption of exchangeable cations and water or organic molecules at the interfaces of particles generally < 50A thick. A neoformation mechanism is proposed by which smectite is converted to illite with increasing depth of burial in sedimentary rocks, based on dissolution of smectite particles and the precipitation/growth of 'illite' particles occurring within a population of thin phyllosilicate crystals.

Granites and hydrothermal ore deposits: a geochemical framework

Abstract: The geochemical evolution of tin tungsten deposits and their associated sulphides can be discussed in terms of four sequential processes: acquisition of the ore-forming elements (OFEs) by the granitic magma, emplacement of these elements in minerals and residual melt of the crystallizing granite, release of the OFEs to the circulating hydrothermal fluids and transport to the depositional sites, and finally, deposition of ore minerals through interaction of these fluids with the wall rock. Based on their crystallographic behaviour, it is useful to distinguish three principal classes of OFEs, here identified as BOC, LHC, and ALC elements. BOC (bivalent octa- hedral cation) elements are similar to ferrous iron and here are represented mainly by Zn, Mn, and perhaps Cu. Li also belongs to this class, although it is monovalent. LHC (large highly charged cations) elements encompass As, Nb, Mo, Sn, Sb, Ta, and W and they are similar to ferric iron or titanium in their crystallographic role. ALC (alkali-like cations) are capable of occupying alkali positions and are represented mainly by Pb, Ag, and Hg. LHCs are rejected from the polymerized silicate liquid network and become enriched in the roof of the acid magma chamber, where more non-bridging oxygens are available. Transport to the roof may be enhanced by the formation of hydrous complexes, as is the pronounced enrichment of Na and Li. BOCs, along with C1, F, and B, fractionate strongly into the vapour phase during vesiculation. HC1 in the ore fluid is crucial for the alteration process and can be produced during boiling by a hydrolysis reaction of the NaCI dissolved or immiscibly present in the silicate magma. Considerable laboratory information is available con- cerning release mechanisms of the OFEs to hydrothermal fluids. We can distinguish congruent and incongruent dissolution, both in response to acid buildup, as well as congruent and incongruent exchange not involving HC1. Melt-fluid fractionation is also thought to be important, though the physical mechanisms are not well understood. Any of these release mechanisms may be coupled with reduction or oxidation reactions. LHC, BOC, and ALC elements respond differently to each of these mechanisms, and these differences may in part be responsible for the observed separation of ore minerals in space and time. It is suggested that LHC elements are released preferentially during acid, non-oxidizing conditions typical of early

Disposal of high-level nuclear wastes: a geological perspective

Abstract: Most countries intend to dispose of their high-level radioactive wastes by converting them into a solidified wasteform which is to be buried within the earth. SYNROC is a titanate ceramic wasteform which has been designed for this purpose on the basis of geochemical principles. It comprises essentially rutile TiO2, 'hollandite' Ba(A1,Ti)Ti6016 , zirconolite CaZrTi2OT, and perovskite CaTiO a. The latter three phases have the capacity to accept the great majority of radioactive elements occur- ring in high-level wastes into their crystal lattice sites. These minerals (or their close relatives) also occur in nature, where they have demonstrated their capacity to survive for many millions of years in a wide range of geological environments. The properties of SYNROC and the crystal chemistry of its constituent minerals are reviewed in some detail and current formulations of SYNROC are summarized. A notable property of SYNROC it its extremely high resistance to leaching by groundwaters, particularly above 100 ~ In addition, it can be shown that the capacity of SYNROC minerals to immobilize high-level waste elements is not markedly impaired by high levels of radiation damage. Current investigations are focused on developing a satisfactory production technology for SYNROC and progress to- wards this objective is described. The high leach- resistance of SYNROC at elevated temperatures increases the range of geological environments in which the waste may be finally interred; in particular, SYNROC is well adapted for disposal in deep drill-holes, both in con- tinental and marine environments. The fact that SYNROC is comprised of minerals which have demon- strated long-term geological stability is significant in establishing public confidence in the ability of the nuclear industry to immobilize high-level wastes for the very long periods required.

Compositional variation within some sedimentary chlorites and some comments on their origin

Abstract: A B S T R A C T. A range of authigenic sedimentary chlorites from sandstones has been studied by analytical transmission electron microscopy. Selected area (single crystal) electron diffraction patterns are of the Ib (fl = 90 ~ polytype confirming the earlier observations of Hayes (1970). TEM analyses show all samples to be relatively rich in both A1 and Fe. In the general formula (Mg,Fe,A1), [Sis_xAlxO20](OH)~6, x varies between 1.5 and 2.6; Fe/(Fe + Mg) between 0.47 and 0.83 and n between 10.80 and 11.54. Octahedral AI is close to 3 in this formulation and Fe 2§ predominates over Fe 3+. Swelling chlorites have significantly different compositions which are consistent with smectite/chlorite interstratifications. The Ib (fl = 90 ~ ) polytype appears to be stable under conditions of moderate to deep burial. It replaces berthierine and swelling chlorites formed at lower temperatures. As commonly seen in grain coatings, however, it precipitates from porewater; solutes probably being contributed from several mineral decomposition reactions.

A Survey of White Mica Crystallinity and Polytypes in Pelitic Rocks of Snowdonia and Llŷn, North Wales

Abstract: Pelitic rocks in North Wales, ranging in age from late Precambrian to Silurian were sampled. XRD analysis was used to determine the mineralogy and white mica crystallinity of separated 0.43Δ°2θ; the 1Md polytype dominates < 2 µm fractions, occurring as K-and Na-micas. Stage II metapelites show variable cleavage development and crystallinities in the range 0.26-0.43Δ°2θ; 1M d and 2M 1 polytypes occur and K- and Na-micas are commonly regularly interstratified. Stage III metapelites are strongly cleaved with crystallinities < 0.26Δ°2θ; the 2M 1 polytype is dominant, occurring as K-mica and paragonite. Pelites bearing pyrophyllite, rectorite, and corrensite are found close to plug-like intrusions and were contact altered prior to regional metamorphism. 1M mica is common in deeply buried but relatively undeformed volcaniclastic rocks of the Arfon Group. Contours of equal crystallinity (isocrysts) are plotted with metabasite zones on a metamorphic map. This shows that stage I metapelites are equivalent to the subpumpellyite zone (≡?laumontite zone). Stage II metapelites are equivalent to the prehnite-pumpellyite facies and stage III metapelites to the clinozoisite and biotite zones of the greenschist facies.

The rates of dissolution of olivine, plagioclase, and quartz in a basalt melt

Abstract: The dissolution rates of spheres of two magnesian olivines, two plagiodases, and quartz in tholeiitic basalt have been determined at three super- liquidus temperatures and one-atmosphere pressure. There are considerable differences in the rates among the minerals, e.g. at 1210 ~ 12 ~ above the liquidus temperature of the basalt, labradorite dissolves at 86 #m/h. and the magnesian olivines at 9 and 14/~m/h. The rates are not time dependent and this, coupled with the existence of concentration gradients in the composition of quenched melt adjacent to partially dissolved crystals, indicates that the dissolution rates are dictated by a combination of diffusion and convection of components to and from the crystal-liquid interface. Values for the activation enthalpy of dissolution are small for quartz and plagioclase (40- 50 kcal mol 1) but large for olivine 73-118 kcal mol-~). Dissolution of plagiodase in rock melts seems to be a much more rapid process than crystal growth, whereas olivines apparently dissolve and grow at similar rates. Crystal dissolution is sufficiently slow that ascending, crystal-bearing magma may become superheated and yet fail to dissolve the crystal fraction before quenching; this may be the reason that olivine phenocrysts are often rounded. K E Y W O R D S : olivine, plagioclase, quartz, tholeiite, basalt, dissolution.

Low-grade metamorphism and accretion tectonics: Southern Uplands terrain, Scotland

Abstract: Previous studies of low-grade meta- morphism in the Southern Uplands accretionary terrain indicated prehnite-pumpellyite facies/anchizone condi- tions developed throughout the area, except for local preservation of trench-slope sediments and an accreted seamount at zeolite facies/advanced diagenetic grade. New graptolite reflectance data are presented that show a general northward increase in temperature in the Southern Uplands. The results from two cross-strike traverses in the southern and central belts in contem- poraneous sequences, using illite crystallinity, illite lateral spacing (bo) , and graptolite reflectance, indicate the development of systematic accretion-related low-grade metamorphism. Well-developed and constant anchizone conditions occur throughout the NE (Langholm) traverse, associated with common, F1 accretion-related folding and a regionally penetrative S1 cleavage. In the SW (Kirkcudbright) traverse, however, the youngest, last accreted packets are preserved at a transitional diagenetic stage and lack a penetrative S1 cleavage. Illite crystal- linity, graptolite reflectance, and bo increase systematic- ally northward through earlier accreted packets, reaching values of the NE traverse only at the northern end. The concomitant increase of b o with illite crystallinity suggests the relatively high P-low T trajectory characteristic of subduction zones. Integration of metamorphic and struc- tural data relates increasing intensity of aceretion-related F1 folding, developmertt of S1 fabric, and onset of later fold phases to grade of metamorphism and structural level within the accretionary pile. K E Y W 0 R D S : metamorphism, low-grade, iUite, graptolite, Southern Uplands, Scotland.

Multi-Component Origin of Caledonian Lamprophyres of Northern England

Abstract: Mafic lamprophyre dykes in northern England were emplaced soon after the final closure of the Iapetus Ocean at the end of the Caledonian orogeny. High Mg/(Mg+Fe) ratios and Cr and Ni contents are consistent with equilibrium with mantle peridotite. Incompatible trace element abundances suggest that the mantle sources were metasomatized prior to the melting events. Three components are recognized in the lamprophyre chemistry: (i) a depleted mantle source, taken to be that overlying the subducting lithosphere; (ii) a H2O-rich subduction zone component, related to the dehydration of the subducting oceanic crust; and (iii) a CO2-rich phase thought to result from the degassing of the mantle after ocean closure. This multi-component origin may be applicable to other lamprophyres of calc-alkaline affinity.

Refinement of the crystal structure of botallackite

Abstract: The crystal structure of botallackite, Cu2(OH)3Cl, monoclinic, a = 5.717(1), b = 6.126(1), c = 5.636(1)A, β = 93.07(1)°, V = 197.06(5)A3, space group P21/m, Z = 1, has been refined to an R index of 3.8% for 350 observed (4σ) reflections, measured with Mo-Kα X-radiation on an automatic four-circle diffractometer. The structure consists of sheets of edge-sharing Cuϕ 6 (ϕ = unspecified anion) octahedra of composition Cu2(OH)3Cl; these sheets are // {100}, and are joined by hydrogen bonding between oxygen and chlorine anions, thus accounting for the perfect {100} cleavage and the platy habit of botallackite.

Rare Earth and Other Trace Element Mobility Accompanying Albitization in a Proterozoic Granite, W. Bergslagen, Sweden

Abstract: Zones of albitization 20 m wide are developed in the peraluminous, undeformed Proterozoic Bastfallshrjden granite, W. Bergslagen, central Sweden. During albitization Na, Si, Mg, Ni, Zn, and Ga are added, while Fe 2+, Fe 3+, Mn, K, Sc, Rb, Cs, Ba, Pb, U, and F are lost, together with the rare earth elements (REE) in decreasing amounts with increasing atomic number. Ti, A1, P, and Y were immobile. Trace element data for chlorites separated from hydrothermally altered country rocks and from a quartz-chlorite vein in the albitized granite show similar REE patterns indicating a common origin: the most altered granite has a similar REE pattern, probably resulting from interaction with the same hydro- thermal fluid which produced the chlorites, in which seawater is thought to have been an important con- stituent.

Low-Temperature Recrystallization of Franciscan Greywackes from Pacheco Pass, California

Abstract: Low-temperature metamorphism of the Franciscan complex at Pacheco Pass occurs at P-T conditions near the so-called jadeite isograd. Along Highway 152 in the Diablo Range, from west to east, four distinct prograde metamorphic zones are defined: (I) albite-quartz zone, (II) albite-rich, albite-quartz clino- pyroxene assemblage, (Ili) clinopyroxene-rich, albite- quartz clinopyroxene assemblage, and (IV) clino- pyroxene zone. Metamorphic pyroxenes are restricted to zones II, III, and IV in metagreywackes, but metabasites contain augite-rich pyroxenes even in zone I. With increasing grade, the compositions of pyroxene in meta- greywackes change in Xjd from nearly 100 in zone II to about 60 in zone III. The diopside component has little effect on the Xjd of pyroxene. The apparent change of Xjd reflects differences in pressure and temperature rather than in bulk rock composition. In metagreywackes, at the onset of zone II, albite breaks down to form Jdl00 according to the reaction Ab = Jd+Qz. With in- creasing grade, this reaction leads to less jadeitic pyr- oxene. From zones II to III, a continuous reaction is delineated: Qz+2 pyroxenes (JdsoAClsAUgs)=pyr- oxene (Jd60mc30 Auglo ) + Ab. The metamorphic temperatures are estimated to be about 170 ~ in zones I and II, and about 230 ~ in zones II and III. The temperature variation recorded in a single rock is probably less than 30~ The pressure estimate depends on the choice of experimental data for the jadeite albite-quartz curve; it ranges from 4.5 to 8 kbar. A gently folded thermal structure of the Cretaceous sub- duction zone metamorphism is postulated for this area.

The distribution of the rare earth elements within the Carnmenellis Pluton, Cornwall

Abstract: The Carnmenellis pluton is a post-orogenic granite of Hercynian age, comprised largely of porphyritic biotite granites which possess LREE enriched patterns with slight negative Eu anomalies. Electron microprobe and ICP spectrometry data are presented for monazite, which occurs as an accessory mineral in all granite types, and it is demonstrated that this mineral is the principal host for LREE in the biotite granites. HREE are strongly partitioned into the accessory minerals xenotime, apatite, and zircon; only Eu substitutes significantly into the essential minerals. The behaviour of the REE during granite differentiation is controlled by the behaviour of the radioactive accessory minerals, which limits the usefulness of these elements in the petrogenetic modelling of granitic rocks.

The Tugtutôq Older Giant Dyke Complex: Mineralogy and Geochemistry of an Alkali Gabbro-Augite-Syenite-Foyaite Association in the Gardar Province of South Greenland

Abstract: The Older Giant Dyke Complex is a differentiated alkaline intrusion of Proterozoic age (1154±16 Ma) and is the earliest of the late Gardar intrusions in the Tugtutoq-Ilimaussaq region. The dyke is approximately 20 km long by 0·5 km broad and comprises (i) marginal ‘border group’ rocks of alkali olivine gabbro, grading inwards to ferro-syenogabbro and (ii) an axial ‘central group’ of salic rocks ranging from augite syenite in the WSW to sodalite foyaite in the ENE. Chilled margins contain plagioclase (An53), olivine (Fo53), magnetite, ilmenite, and apatite as liquidus phases and later-crystallized augite (Di69Hd27Ac4) and biotite (Annite32). The coexisting Fe-Ti oxides indicate f O2 and T values just below the synthetic QFM buffer curve. In the border group, plagioclase cores zone into anorthoclase and soda-sanidine rims, olivines reach Fo16, pyroxenes Di32Hd59Ac9, and biotites Annite86. Interstitial pargasitic amphibole appears close to the innermost margins. In the central group, feldspars are all perthitic alkali feldspars and nepheline becomes a major, early crystallizing phase. Olivines range from Fo10-Fo4, in the augite-syenites where they coexist with ferro-salites Di50Hd47Ac3, but olivine is absent from foyaitic assemblages in which the pyroxenes range through aegirine-augite to pure aegirine. Interstitial amphiboles range from ferro-pargasite or hastingsite to katophorite and thence towards arfvedsonite, but are absent from the most differentiated rocks, whereas biotite occurs throughout the entire group in the range Annite71-Annite100. The parental magma, represented by the chilled margins, was a relatively anhydrous alkali olivine-basalt with an initial 87Sr/86Sr ratio of 0.70326. Its high Ti, P, Ba, and F contents are inferred to be features inherited from a primary magma, derived from the mantle as a small partial melt fraction which involved significant amounts of fluor-apatite and phlogopite. While all lithologies are considered as differentiates from this parental magma, there is both a well-defined field junction and a compositional hiatus between the border group and the central group rocks. Mineralogical considerations and REE patterns suggest that the later, more salic (?benmoreitic) magma from which the central group crystallized, related to the parental magma by ol-fsp-ap-FeTi oxide fractionation. Congelation in both border group and central group occurred by side-wall crystallization, but the salic magma became compositionally stratified, with upward concentration of alkalis and volatiles to produce a phonolitic upper facies which is preserved at the ENE end of the intrusion owing to subsequent axial tilting.

The accuracy of X-ray diffraction methods for determining mineral mixtures

Abstract: An X-ray diffraction method has been successfully applied to the quantitative determination of mineral mixtures. The absorptive properties of the samples and the number of components determine the analytical procedure to be used. The methods described (external standard plus empirical determination of the mass attenuation coefficient) provides an accurate and rapid alternative to the direct measurement of the mass attenuation, Compton diffusion or internal standard methods. The relative accuracy obtained is of the order of 10% at the 0.1% level, independent of the sample composition.

Structural and phase equilibria studies in the system Pt-Fe-Cu and the occurrence of tulameenite (Pt2FeCu)

Abstract: The crystal structure and compositional limits of the ternary compound Pt2FeCu (tulameenite), formed either by quenching from above the critical temperature of 1178 ~ or by slow cooling, have been investigated using X-ray diffraction, transmission electron microscopy, differential thermal analysis and electron probe microanalysis. The crystal structure of Pt2FeCu, established using electron density maps constructed from the measured and calculated intensities of X-ray diffraction patterns of powdered specimens, has the (000) and (2121-~0) lattice sites occupied by Pt atoms and the (2t~21) and (0~2 l) sites occupied by either Cu or Fe atoms in a random manner. The resulting face-centred tetragonal structure undergoes a disordering transformation at the critical temperature to a postulated non-quenchable face-centred cubic struc- ture. Stresses on quenching, arising from the ordering reaction, are relieved by twinning along {101} planes or by recrystallization along with deformation twinning; always involving grain boundary fracturing. Phase relations in the system Pt-Fe Cu have been investigated through the construction of isothermal sec- tions at 1000 and 600 ~ At 1000 ~ there is an extensive single phase region of solid solution around Pt2FeCu and extending to the binary composition PtFe. At 600 ~ the composition Pt2FeCu lies just outside this now reduced area of solid solution in a two-phase field. Comparison of the experimental results with data for tulameenite suggests that some observed compositions may be recta- stably preserved. The occurrence of fine veinlets of silicate or other gangue minerals in tulameenite is suggested to result from grain boundary fracturing on cooling below the critical temperature of 1178 ~ and to be evidence of a magmatic origin.

Electron-optical studies of phyllosilicate intergrowths in sedimentary and metamorphic rocks

Abstract: The results of a microstructural study by backscattered scanning electron microscopy and a microchemical study using X-ray microprobe analysis of phyllosilicate intergrowths from sandstones, shales, metagreywackes, and low-grade schists are presented. The microstructural study revealed that the intergrowths thicken and become more coherent with metamorphic grade; the intergrowths change from incoherent to coherent in the anchizone. The increasing coherency is mirrored by an increase in the crystallinity indices of the illites/phengites. Chemical analysis of the individual intergrowth phases was difficult in the sediments and no systematic compositional variations were recorded. However, clear compositional trends with increasing metamorphic grade emerged in the phengites from the metagreywackes and schists, but in the chlorites only slight compositional changes were recorded.

Apatite composition and the fugacities of HF and HC1 in metamorphic fluids

Abstract: A B S T R A C T Microprobe analyses of the halogen contents of apatites from two samples of amphibolite-facies schist from Connemara, Ireland, have been used to calculate the fugacity ratios fac0/fn2o and fnr/fa2o using the experimental data of Korzhinskiy The results imply fugacities for both acids in the range 003 to 01 bars, but whereas for the lower grade rock fur >fncl, the migmatitic sample gives fHF~--fncl An independent estimate of frlv/fnzo from the biotite composition in one sample is in acceptable agreement with the result obtained from apatite KEVWORDS: apatite, halogens, metamorphic fluid, schist, Connemara, Ireland MOST studies of metamorphic fluid composition are concerned only with the principal gas species present, but aqueous fluids in particular can also carry significant quantities of dissolved material, predominantly in the form of chloride complexes at moderate to high grades but with significant dissociation of the dominant species (normally NaCI) in the greenschist facies or at lower temperatures (Poty et al, 1974; Eugster, 1977; Eugster and Gunter, 1981) It is possible to calculate the content of the major species in aqueous solution in equilibrium with a range of common mineral species from thermodynamic data compilations or directly from the original experiments, provided that a reasonable estimate can be made of the total chloride content of the fluid phase, eg from fluid inclusion studies, and provided that chloride is indeed the dominant anion It is of particular interest to know the content of acid species in the fluid phase under different conditions, because migration of fluids of different acidities has the potential for metal transport and ore formation This paper describes a means of estimating HF and HC1 fugacities in metamorphic fluids One way to measure the halogen-acid contents of metamorphic fluids is to determine the halogen concentrations in coexisting hydrous minerals in which OH can be replaced by F or CI because the extent of such substitution depends directly on the

Hydroxyl-bastnaesite-(Nd), a new mineral from Montenegro, Yugoslavia

Abstract: Hydroxyl-bastnaesite-(Nd), the Nd- and OH-dominant new member of the bastnaesite group, has been found in the red karstic bauxites near Niksic, Montenegro. It occurs as whitish, irregular aggregates of crystals usually 100–200 µm in diameter. Hexagonal with the cell dimensions a = 7.191 and c = 9.921 A, with Z = 6. The strongest lines of the X-ray powder diffraction pattern (d, I, hkl) are: 4.95 80(002); 3.596 79(110); 2.911 100(112); 2.077 29(300); 2.042 51(114); 1.914 30(302). The mineral is uniaxial positive, ω 1.715, ɛ 1.81; Dcalc 4.89 g/cm3. Electron probe analysis, using garnet, apatite and synthetic glass standards, combined with thermogravimetric and evolved gas analysis, gave Y2O3 0.2, La2O3 27.1, Ce2O3 0.3, Pr2O3 8.5, Nd2O3 31.5, Sm2O3 4.4, Eu2O3 1.3, Gd2O3 1.4, CaO 0.3, F 3.3, H2O 2.26, CO2 20.63, sum 101.19, less O = F 1.39, total 99.80%. This analysis calculates to (Nd0.41La0.36Pr0.11Sm0.06Gd0.02 Eu0.02Ca0.01)Σ0.99(CO3)1.03[(OH)0.55F0.38]Σ0.93, or ideally, (Nd,La)CO3(OH,F), with Nd > La and OH > F.

Rock-matrix diffusion and its implications for radionuclide migration

Abstract: Diffusion into the rock matrix is potentially an important retardation mechanism for nuclides leached from an underground radioactive waste repository in a fractured hard rock. A technique for measuring the intrinsic diffusion coefficient and rock capacity factor is briefly described. Simple solutions to migration model equations are used, together with diffusion results and typical hydrogeological parameters, to estimate the impact of matrix diffusion on radionuclide migration. It is shown that retardation factors in excess of 100 and reductions in the peak concentration by 3–4 orders of magnitude are possible for non-sorbed ions, which would otherwise be carried by the flow and not retarded at all.

Cements in radioactive waste disposal: some mineralogical considerations

Abstract: Cementitious matrices are being assessed for immobilization of radioactive wastes. This paper dis- cusses some mineralogical aspects of cement chemistry and the uses of siliceous minerals as selective sorbants to enhance immobilization potential. Studies of sorption and leaching of caesium from pulverized fuel ash (PFA), blast furnace slag, tobermorite, xonotlite, and clinoptilolite are reported. The role of incorporation of these additives in cement and the effect on the nature of the composite matrix on caesium behaviour has been investigated. Specific mechanisms of the interaction of additives with highly alkaline cement environment are described. While slags, PFA, and clinoptilolite undergo reaction at different rates, tobermorite and xonotlite appear to be stable in cement. SPENT fuel elements may undergo a lengthy and complex reprocessing cycle before disposal of the resulting waste streams can be accomplished; moreover, storage, transportation, and reprocess- ing operations give rise to other waste streams. The bulk of these are non-heat generating. The esti- mated United Kingdom arisings from all sources have been estimated to the year 2000 and classified according to volume and radioactivity (Duncan and Brown, 1982). It is apparent that cement and concrete will find much application in waste disposal, mainly of non-heat generating wastes. We exclude from con- sideration its structural applications, e.g. in silos, tanks, bunkers, tunnel linings, and concentrate instead on non-structural applications, e.g. as a primary matrix for immobilization of wastes or in secondary barriers such as grouts or sealants. Cements are tolerant of water, resistant to the action of certain salts, have a long history of use and lend themselves to remote handling. Moreover, they are cheap. However, they may constitute a chemically reactive, microporous matrix and the interactions between cements and waste form, or cements and their environment, or both, have many features

A transition from diagenesis to greenschist facies within a major Variscan fold/thrust complex in south-west England

Abstract: The north coast of Cornwall, from Bude to Newquay, provides a continuous section through a major Variscan fold/thrust complex. Illite crystallinity studies have revealed a transition from diagenesis in the north to greenschist facies metamorphism in the south in the Upper Palaeozoic succession. More detailed studies of mineral assemblages in both metabasites and pelitic rocks support the regional pattern of metamorphism indicated by illite crystallinity, and show that locally in the Tintagel district, the grade of metamorphism may have reached middle to upper greenschist facies. An attempt to corre- late the above data with temperatures (108-985~ derived from O-isotope geothermometers is made. Inter- pretation of the metamorphic data presented helps to emphasize the tectonic importance of the major structures seen in the fold/thrust complex.

Significance of Illite Crystallinity and bo Values of K-white Mica in Low-Grade Metamorphic Rocks, North Hill End Synclinorium, New South Wales, Australia

Abstract: A study of low-grade metamorphism in late Silurian to early Carboniferous rocks in the North Hill End Synclinorium and adjacent anticlinoria has been made by the determination of illite crystallinity and b o values of K-white mica in eighty slates and phyllites. Illite crystallinity values vary from 0.40 Δ°2θ on the Molong Anticlinorium to 0.12 Δ°2θ within the axis of the synclinorium, suggesting anchizonal to epizonal metamorphic conditions. This is in agreement with previous observations on Ca-Al-hydrosilicate assemblages which indicated a change from prehnite-pumpellyite facies in the anticlinoria adjacent to the synclinorium to middle greenschist facies in the axis. Local variations in crystallinity are attributed to variation in ak + in fluids migrating along cleavage zones. The mean b o value obtained from the pelites is 9.017 A (σn = 0.008; n = 80) which is in close agreement with that obtained from part of the adjacent Capertee Anticlinorium ( = 9.019 A; σn = 0.007; n = 52). However, ‘t’ tests indicate that two b o populations are present in the synclinorium ( = 9.019 and 9.022 A), with the lower values concentrated in the southern portion of this structure. The two populations are considered to be the result of slightly different metamorphic conditions prevailing during the deformation of the rocks in the synclinorium. A higher geothermal gradient affecting rocks giving the lower b o values is attributed to the presence of granitoids at shallower depths than elsewhere in the synclinorium.

Zirconolite and Baddeleyite from Metacarbonates of the Oetztal-Stubai Complex (Northern Tyrol, Austria)

Abstract: Zirconolite and baddeleyite from polymetamorphosed metacarbonates of the Oetztal-Stubai complex (northern Tyrol, Austria) are described in terms of their paragenetic relationship and chemical compositions. For the first time an analysis of a chemically zoned niobian zirconolite (high Nb and low Ti in core, low Nb and high Ti at rim) is presented. Genetic possibilities for the origin of the metacarbonates are discussed in the light of zirconolite- and baddeleyite-formation.

Cordierite in the peraluminous granites of the Meguma Zone, Nova Scotia, Canada

Abstract: The peraluminous granitoid intrusions of southern Nova Scotia contain several mineralogical ex- pressions of the excess alumina, including variable amounts of cordierite, in different textural types of granitoids, ranging from fine-grained aplites through coarser grained mouzogranites and granodiorites, to very coarse-grained pegmatites. A detailed study of the spatial, textural and chemical characteristics of these cordierites suggests that the majority are of relict metamorphic origin, but that primary magmatic cordierites, as well as cordierites which grew in equilibrium with a water-rich fluid phase, also occur.

Microscopic Determination of the Nonopaque Minerals

Abstract: Optical methods of determining minerals with the petrographic microscope have long been used and have been carried to a high state of development in studies of the minerals in thin sections of rocks and ores, yet out of about 1,000 mineral species comparatively few can be identified readily in thin sections. A mineral whose optical properties are known can be accurately and quickly identified, however, by the immersion method that is, by immersing its powder in liquid media whose indices of refraction are known and determining its optical constants. In this bulletin the author gives a set of tables for the systematic determination of minerals from their optical constants, describes briefly some methods for the rapid determination of optical constants, gives the results of measurements of the optical constants of more than 500 species for which data was not previously available, and presents statistics on the optical properties of minerals. The first tables prepared for general use in determinations of minerals by the immersion method were those of Van der Kolk, 1 published in 1900. Somewhat similar tables, prepared by A. F. Rogers, 2 were published in 1906, and an optical mineralogy containing tables and description of all minerals for which optical data were then available, prepared by N. H. and A. N. Winchell, 3 was published in 1909. However, the method has not received the attention that it deserves and has not come into general use, largely because the optical constants of over half the known minerals had not been determined. NEW DATA.