Mica Minerals Research Articles

Seismic velocity anisotropy in mica-rich rocks: an inclusion model

Summary We calculated seismic wave velocity anisotropy caused by the preferred orientation of mica minerals by using the differential effective medium method (DEM). Spheroidal biotite crystals with their c-axes coinciding with the symmetry axis of the spheroid are embedded in an isotropic matrix up to a volume ratio of 30 per cent. All crystals are aligned with their c-axes parallel to the symmetry axis of the effective homogeneous medium, which shows transverse isotropy. The effect of crystal shape on anisotropy was studied by changing the aspect ratio (the ratio between the minor axis and the major axis of the spheroid) from 0.01 (flat spheroid) to 1 (sphere). The S-velocity anisotropy becomes large as the crystal shape becomes flat, whereas the P-velocity anisotropy shows only small changes with changes of the crystal shape. In particular, biotite generates large S-wave anisotropy, and the anisotropy becomes stronger as the aspect ratio of the biotite crystal becomes smaller. When the volume ratio of the mica mineral is large, the P-wave phase velocity surface shows considerable deviation from the ellipse, and the SV-wave phase velocity surface forms a large bulge and crosses the SH-wave phase velocity surface (singularity) in the plane including the symmetry axis. These results show an interesting contrast when compared with the effect of crack or pore shape on seismic velocity anisotropy: crack (or pore) shape affects the P velocity more than the S velocity. We also calculated Thomsen's anisotropic parameters, e, γ and δ as functions of the crystal aspect ratio and the mica volume ratio.

Seismic velocity anisotropy in mica-rich rocks: an inclusion model

We calculated seismic wave velocity anisotropy caused by the preferred orientation of mica minerals by using the differential effective medium method (DEM). Spheroidal biotite crystals with their c‐axes coinciding with the symmetry axis of the spheroid are embedded in an isotropic matrix up to a volume ratio of 30 per cent. All crystals are aligned with their c‐axes parallel to the symmetry axis of the effective homogeneous medium, which shows transverse isotropy. The effect of crystal shape on anisotropy was studied by changing the aspect ratio (the ratio between the minor axis and the major axis of the spheroid) from 0.01 (flat spheroid) to 1 (sphere). The S‐velocity anisotropy becomes large as the crystal shape becomes flat, whereas the P‐velocity anisotropy shows only small changes with changes of the crystal shape. In particular, biotite generates large S‐wave anisotropy, and the anisotropy becomes stronger as the aspect ratio of the biotite crystal becomes smaller. When the volume ratio of the mica mineral is large, the P‐wave phase velocity surface shows considerable deviation from the ellipse, and the SV‐wave phase velocity surface forms a large bulge and crosses the SH‐wave phase velocity surface (singularity) in the plane including the symmetry axis. These results show an interesting contrast when compared with the effect of crack or pore shape on seismic velocity anisotropy: crack (or pore) shape affects the P velocity more than the S velocity. We also calculated Thomsen's anisotropic parameters, ε, γ and δ as functions of the crystal aspect ratio and the mica volume ratio.

Hydrothermal fluid evolution and structural control of the Guarim gold mineralisation, Tapajós Province, Amazonian Craton, Brazil

Fluid inclusion and structural studies were carried out at the Guarim gold deposit in the Palaeoproterozoic Tapajos province of the Amazonian craton. Guarim is a fault-hosted gold deposit cutting basement granitoids. It consists of a quartz vein, which is massive in its inner portions, grading laterally either to a massive or to cavity-bearing quartz vein associated with hydrothermal breccias. The wallrock alteration comprises chlorite, carbonate, white mica and sulphide minerals, with free gold occurring within quartz grains and spatially associated with sulphide mineral grains. Petrographic, microthermometric and Laser Raman investigations recognised CO2-rich, mixed H2O–CO2, and H2O fluid inclusions. The coexisting CO2 and H2O–CO2 inclusions were interpreted as primary immiscible fluids that formed the gold-bearing vein. The H2O inclusions were considered a product of later infiltration of fluids unrelated to the mineralising episode. The mineralising fluid has CO2 ranging typically from 5–10 mol%, contains traces of N2, has salinities of ∼5 wt% NaCl equiv., and densities varying between 0.85 and 0.95 g/cm3. The P–T estimations bracket gold deposition between 270–320 °C and 0.86–2.9 kb; ƒO2–ƒS2–pH estimates suggest a reduced, near-neutral character for the fluid. Variations in the physico-chemical properties, as demonstrated by the fluid inclusion study, resulted from a combination of fluid immiscibility and pressure fluctuation. This interpretation, combined with textural and structural evidence, suggests the emplacement of the mineralised vein in an active fault and at a rather shallow level (4–7 km). The geological and structural setting, deposit-scale textures and structures, wallrock alteration and physico-chemical fluid properties are compatible with those of epizonal to mesozonal orogenic lode gold deposits.

An excellent fossil wood cell texture with primary uranium minerals at a sandstone-hosted roll-type uranium deposit, NW China

A fossil wood cell texture with pitchblende and coffinite found at a sandstone-hosted roll-type uranium deposit, Xinjiang, NW China, is first reported here for the country. In the mineralized sandstone, detrital grains consisting of quartz, feldspar, rock fragments, carbonaceous trashes, mica and accessory minerals were deposited in early Turassic time and were cemented by clays and minor authigenic calcite and quartz. Pitchblende and minor coffinite are principal ore minerals at the deposit, and selectively replaced carbonized fossil wood remnants or filled fossil wood cells. An excellent fossil wood cell texture with primary uranium minerals formed. Replacement of organic debris by primary uranium minerals may be due to a local reducing environment resulting from the production of CH 4, H 2S or H 2SO 3 in its decomposition, and a biochemical reaction indicated by the common presence of framboidal pyrite.

Synthesis and Properties of Poly(acrylic acid)/Mica Superabsorbent Nanocomposite

A novel poly(acrylic acid)/mica superabsorbent composite was synthesized by graft polymerization reaction between partially neutralized acrylic acid and ultrafine mica mineral powder. The water absorbency of the composite is higher than 1 100 g H2O/g. The influence of the neutralization degree of acrylic acid, as well as of the amounts of mica and crosslinker on the absorbing properties are discussed in this paper.

Tubular and spheroidal halloysite in pyroclastic deposits in the area of the Roccamonfina volcano (Southern Italy)

Halloysite-bearing weathered pyroclastic deposits of the Roccamonfina volcanic area (Southern Italy), consisting generally of light grey clay masses richer in slightly weathered pumice grains, mottled patches of whitish clayey material and yellowish-brown veins along vertical root channels, were characterised by chemical analysis, X-ray diffraction (XRD), infrared spectroscopy (FT-IR), optical (OM), scanning (SEM) and transmission (TEM) microscopy, electron diffraction (ED) and energy dispersive X-ray analysis (EDXRA). Both tubular and spheroidal forms of halloysite were observed, different parts of the deposits being characterised by a particular morphology. No consistent differences in terms of the chemical and mineralogical composition of the deposit materials were found. However, the chemical composition of the different particle-size clay fractions indicated that less silica, with concomitant relative enrichment in aluminium, and loss of bases characterised the <0.5 μm compared with the 0.5–2.0 μm fraction reflecting their differences in the content of primary minerals. The amounts of Fe, Al and Si extracted by dithionite–citrate–bicarbonate (DCB) and oxalate reagents suggested the presence of short-range-order aluminosilicates but this could not be proved by other physical techniques. X-ray analysis revealed the presence of hydrated (1 nm) and dehydrated (0.7 nm) halloysite, mica, sanidine and plagioclase minerals. TEM/EDXRA analysis of the <0.5 μm clay fraction revealed an amorphous Si-rich phase in the sample containing spheroidal halloysite. The differential X-ray diffraction analysis (DXRD) from untreated and DCB treated yellowish-brown material along with the electron diffraction (ED) of the <0.5 μm clay fraction showed the presence of goethite and ferrihydrite. The various weathered pyroclastic materials showed a major difference in terms of their physical characteristics, particularly in terms of their compactness and porosity/permeability as revealed by microscopic and sub microscopic observations. It is suggested that the different morphological forms of halloysite may result from conditions of growth in the different microenvironments related to variability in physical conditions.

Hydrochemical Monitoring and Heavy Metal Contaminations at the Narim Mine Creek in the Sulcheon District, Republic of Korea

The Narim gold mine is located approximately 200 km southeast of Seoul within the Sulcheon mineralised district in the Yeongnam massif, Korea. In this study, environmental geochemical analyses were undertaken for soil, sediment and water samples collected in April, September and November in 1998 from the Narim mine creek. The mine area consists mainly of granitic gneiss; however, mineral constituents of soil and sediment near the mine were mainly composed of quartz, feldspar, mica, amphibole, some pyrite and clay minerals. Also were found some pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, malachite, goethite, various hydroxide and unidentified secondary minerals. Generally, high concentrations of heavy metals in the soil and sediment are correlated with a high proportion of secondary minerals. Hydrochemical compositions of water samples are characterised by relative significant enrichment of Na+ + K+ and alkali metals in the ground water, whereas the surface and mine waters are relatively enriched in Ca2+ + Mg2+ and heavy metals. Anion contents of the ground waters are typically enriched in HCO3−, NO3− and Cl−, whereas the surface and mine waters are highly enriched in HCO3− and SO42−. The pH and EC values of the surface water from the non-mine creek are relatively lower compared with those of the surface water around the mine and waste dump. The range of δD and δ18O values (d parameters) of the water samples are shown in distinct two groups for the April waters of 10.1–13.1, and for the November waters of 5.8–7.9, respectively. This range variation indicates that two group water were composed of distinct waters because of seasonal difference. Geochemical modelling showed that mostly heavy toxic metals may exist largely in the form of free metal (M2+) and metal-sulphate (MSO42−), and SO42− concentration influenced the speciation of heavy metals in the mine water. These metals in the ground water could be formed of CO3− and OH− complex ions. Using a computer program, saturation indices of albite, calcite, dolomite in mostly surface water show undersaturated and progressively evolved toward the saturation state, however, ground and mine waters are nearly saturated. The gibbsite, kaolinite and smectite are supersaturated in the surface and ground water, respectively. Calculated water-mineral reaction and stabilities suggest that the weathering of silicate minerals may be stable kaolinite. The clay minerals of K-illite and Na-smectite will be transformed to more stable kaolinite owing to the continuous reaction.

Dissolution and structural alteration of phlogopite mediated by proton attack and bacterial oxidation of ferrous iron

Microbiological weathering of a research-grade mica mineral, phlogopite, was studied using ferrous sulfate media that were inoculated with an acidophilic iron-oxidizing bacterium, Thiobacillus ferrooxidans. Weathering due to dissolution was monitored by analysis of Si, Al, Fe, K, Na, Mg, and Ca in the leach solutions and in chemical controls at pH 1.0, 1.5 and 2.0. Structural alterations of phlogopite were analyzed by X-ray diffraction. At pH 2, the oxidation of Fe(II) by T. ferrooxidans was accompanied by the formation of jarosite within 7 days of incubation at 22°C. The precipitation of jarosite was coupled with partial alteration of phlogopite to vermiculite and an interstratified (mixed-layer) phlogopite/vermiculite. Similar results were obtained with chemical controls containing 120 mM ferric sulfate. The data suggested that K incorporated into jarosite was released from interlayer positions in phlogopite; thus, jarosite constituted a sink for K. The formation of jarosite and expansible layer silicate phases was pH-dependent. At pH<1.5, jarosite was not formed and phlogopite weathering was due to chemical dissolution without detectable structural alteration.

Contents, modes of occurrence and origin of chlorine and bromine in coal

Contents, modes of occurrence and origin of Cl and Br in individual coal samples from 34 deposits worldwide (Bulgaria, Australia, the United States, Japan, Canada, South Africa, China, and Ukraine) were studied. Some relationships of Cl and Br contents with chemical and mineral composition, rank, age, ash yield and geographic location of coals are described. Despite of similar chemical and geochemical properties of Cl and Br some distinct differences in the association, behaviour and occurrence of both elements in coal were found and discussed. Chlorine and Br in coal may occur, in decreasing order of significance, as organic compounds, as impurity components in the crystalline and amorphous inorganic constituents, in the fluid constituents and as discrete minerals. Both elements could have an organic occurrence and association with water-soluble (mainly ionic) and water-insoluble (ionic and covalent) organic combinations. Chlorine was identified as an impurity in minerals and inorganic phases such as clay minerals, mica, feldspars, polyhalite, gypsum, siderite, volcanic glass, phosphates and other carbonates and sulphates. A characteristic Br association with illite and to a lesser extent with mica, kaolinite and Fe-bearing minerals was also found. Water molecules, hydroxyl groups and exchangeable cations in various minerals play a leading role for the inorganic occurrence and distribution of both elements. Discrete Cl minerals such as sylvite, halite, chlorapatite and probably carnallite were also identified. Significant amounts of Cl ions may associate with solutions in the inherent moisture of the coal mesoporosity. Limited proportions of both elements may also occur in gas–liquid inclusions of different solid phases. The favourable conditions for Cl and Br enrichments in coal are also discussed.

Ammonium-bearing mica and mica/smectite of several pottery stone and pyrophyllite deposits in Japan: their mineralogical properties and utilization

Many pottery stone and pyrophyllite deposits have been developed in southwestern Japan. These deposits are hydrothermal in origin and have produced dioctahedral mica (sericite) minerals usually used for porcelain, tiles, sanitary ware, etc. Sericite specimens from two pottery stone deposits (Izumiyama and Tobe) and three pyrophyllite deposits (Yagi, Mitsuishi and Horo mines) are mineralogically characterized in terms of utilization. These sericite clays are chemically characterized by extensive substitution of NH 4 for K corresponding to the muscovite–tobelite series, resulting in the basal spacing being enlarged from 10.00 Å (muscovite) to 10.36 Å (tobelite), which is diagnostic of the interlayer composition. Infrared absorbance of the NH 4 band (1430 cm −1) standardized to the OH band (3620 cm −1) is also useful for determination of the NH 4 content. DTA of a NH 4-dominant specimen from the Tobe deposit reveals the liberation of NH 4 around 490°C and double steps dehydroxylation in the 600–800°C range, as well as crystallization of mullite and cristobalite at 1000°C. These thermal properties are considered to be effective in sintering and refractoriness of the sericite materials when fired. Most of the sericite specimens are interstratified mica/smectite with <20% smectite, which may be responsible for plasticity when treated in the wet refinement process. Of the sericite clays from the five deposits, the Tobe, Yagi and Mitsuishi specimens are characterized by a varied interlayer NH 4/K ratio, interstratification and lower Fe and Ti content, which favor refractoriness, plasticity and whiteness, respectively: these are excellent qualities for their use as raw materials for porcelain.

Regolith geochemical exploration using acid insoluble residues as a sample medium for gold and base metal deposits in the Cobar region, N.S.W., Australia

Acid insoluble residues of regolith samples after treatment with aqua regia (3HCl : 1HNO 3) and subsequently with H 2SO 4 have been tested as a sampling medium for determining geochemical signatures around gold and base metal mineralisation at the McKinnons and Wagga Tank deposits and at the Lower Tank prospect, in the Cobar region, N.S.W., Australia. The residues, composed of quartz, chalcedony and minor mica, retain primary geochemical signatures related to mineralisation/alteration events. Gold and base metal deposits in the Cobar region occur within clastic sediments and volcaniclastics of the Early Devonian Cobar Basin and its equivalents (e.g. Mt Hope Trough). Common alteration styles are silicification and chloritisation with some carbonitisation and sericitisation. Within the regolith, clay mineral alteration is significant. At both the McKinnons and Wagga Tank deposits the weathering profile is about 80–100 m thick. The regolith at McKinnons consists of red-brown soil containing ferruginous pisoliths underlain by saprolite consisting of quartz, clay minerals (mainly illite) and muscovite, pyrite, goethite and haematite. In the Wagga Tank area, the weathering profile consists of soil overlying a residual ferruginous layer underlain by a kaolinite-rich zone which passes into goethitic saprolite containing alunite–jarosite family minerals. Distributions of elements in the acid insoluble residues of rock and regolith (including surface soil) exhibit similar geochemical signatures at the McKinnons and Wagga Tank deposits. There are two main groups of elements of interest. The first group (Cu, Zn, Pb, Ni, As, Mo, Ag, Sb, Ba, Bi and W) are ore-related and are enriched in and around the mineralisation. The second group (K, Fe, Ca, Ti, Mn, Cl, Ga, Rb, Sr, Zr, Y, V, Al, Na, Mg, Tl and F) are generally depleted in acid insoluble residues of the rock and regolith near and around the mineralisation. The depletion of some elements such as K, Al, Ca, Na, Rb and Sr is considered to reflect element loss from the hydrothermal system as a result of feldspar and mica destruction followed by silicification. These signatures are considered to be mainly the result of primary geochemical dispersion and despite the weathering effect are retained within the regolith. The previously defined regolith-hosted base metal anomalies at the Lower Tank prospect are also reflected by anomalous Cu, Zn, Pb, Ni, Mo and As in the acid insoluble residues of both saprolite and drainage sediments. However, K, Fe, Ca, Na, Rb, Sr and Ga are not depleted, which does not support the possibility of wall rock alteration and associated buried mineralisation.

Ultracataclasis, sintering, and frictional melting in pseudotachylytes from East Greenland

Large volumes of pseudotachylyte (an intrusive, fault-related rock interpreted to form by a combination of cataclasis and melting) occur in Tertiary normal faults and accommodation zones along ∼400 km of the East Greenland volcanic rifted margin. Analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures, mineralogies, and chemistries in the aphanitic pseudotachylyte matrix. Three distinct types of pseudotachylyte (referred to as angular, rounded and glassy) are identified based on these characteristics. Angular pseudotachylyte (found primarily in dike-like reservoir zones) is characterized by angular grains visible on all scales, with micron-scale fragments of mica and amphibole. Its matrix is enriched in Fe 2O 3, MgO, and TiO 2 relative to the host rock, with minor increases in CaO, K 2O, and small decreases in Na 2O. Rounded pseudotachylyte is found in reservoir zones, injection veins (pseudotachylyte-filled extension fractures), and fault veins (small faults with pseudotachylyte along their surfaces). It is characterized by smooth-surfaced, compacted grains on microscopic scales, and encloses rounded, interpenetrative lithic clasts on outcrop scale. Its matrix is enriched in Fe 2O 3, MgO, TiO 2, and Al 2O 3 relative to the host rock, with minor depletion in Na 2O and K 2O. Glassy pseudotachylyte is found primarily along fault surfaces. Its matrix is characterized by isotropic, conchoidally fractured material containing microscopic, strain-free amphibole phenocrysts, and is enriched in TiO 2, Al 2O 3, K 2O, Fe 2O 3, MgO, CaO, and Na 2O relative to the host rock. These observations suggest that angular pseudotachylyte was produced by cataclasis, with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock. Cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting, producing rounded and glassy pseudotachylyte, respectively. Compositional and textural observations constrain the temperatures reached during frictional heating (700–900°C) which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip. Our results suggest that the East Greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels. Consistent relative ages and widespread occurrence of pseudotachylyte-bearing faults in East Greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin.

Petrographic-microchemical studies and origin of the Agbaja Phanerozoic Ironstone Formation, Nupe Basin, Nigeria: a product of a ferruginized ooidal kaolin precursor not identical to the Minette-type

The Campanian-Maastrichtian Agbaja Ironstone Formation of the Nupe basin, Nigeria, forms a major part of the about 2 billion tons of iron ore reserves of the Middle Niger Embayment. The ironstone deposits were previously reported to be similar to the Minette-type ironstones because of their depositional patterns, composition and inferred origin. Four rock-types are recognized within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, mud-ironstone and bog iron ore. In the ironstones, kaolinite of both the groundmass and the ooids/pisoids is of lateritic origin, whereas the associated quartz, mica and heavy minerals are of detrital origin. Ooids and pisoids were formed by mechanical accretion of platy kaolinite crystals by rolling on the sea floor in a near-shore environment, and were subsequently transported and deposited together with a fine-grained kaolinitic groundmass. Pyrite (mainly framboidal) and siderite (both exclusively occurring as pseudomorphs of goethite and/or hematite) are diagenetic whereas goethite is post-diagenetic in origin, resulting from the ferruginization of the kaolinitic precursor. Crandallite-gorxeicite-goyazite, bolivarite and boehmite are also post-diagenetic in origin. Hematite was formed from the dehydration of goethite, whereas gibbsite (restricted to the upper part of the deposit) is of recent and in situ lateritic origin. The presence of newly formed authigenic pyrite and siderite (now replaced by hematite and goethite) are indicators of a reducing environment during diagenesis. The absence of diagenetic chamositic clay minerals, evidently caused by a low Mg concentration, suggests that fully marine conditions were not established during sedimentation. This is supported by the lack of fossils, brecciated shell materials and bioturbation features in the deposit. Reworking and redeposition of the primary constituents are inferred from broken pisoids, nuclei of pisoidal/ooidal fragments in pisoids and high iron concentrations present in the pisoids and ooids compared to that of the groundmass. These observations indicate that the Agbaja ironstone deposits of the Lokoja study area exhibit some environmental and mineralogical characteristics that are markedly different from other known deposits of Minette-type, where primary chamositic clay minerals generally form the protore for the ironstones. The recognition of kaolinite as the precursor constituent and the occurrence of similar deposits of the same age (Late Cretaceous) in Nigeria, Sudan and Egypt have implications for the paleoenvironmental interpretations of Phanerozoic ironstone deposits.

Removal of titanium impurities from feldspar ores by new flotation collectors

The principal impurities in some feldspar ores are titanium and iron, which impart color and in turn degrade the quality of the ore. Mineralogical investigations on the majority of Turkish feldspar ores indicate that rutile and, scarcely, sphene are the major titanium minerals and iron mainly originates from mica minerals. Conventionally, fatty acids, and particularly sodium oleate, are extensively used to float discoloring minerals from feldspar ores with reasonable success. In this study, new collectors, oleoyl sarcosine and hydroxamate, reported for the first time in the literature, have been used to float titanium impurities. It is found that compared to fatty acids these reagents achieve superior results. The mechanism of the action is elaborated on the basis of experimental data.

Temperature dependence of Kr ion-induced amorphization of mica minerals

Four different mica compositions, i.e., muscovite [KAl 2(AlSi 3O 10)(OH,F) 2], phlogopite [KMg 3(AlSi 3O 10)(OH,F) 2], biotite [K(Mg,Fe) 3(AlSi 3O 10)(OH,F) 2] and lepidolite [K(Li,Al) 3(Al,Si) 4O 10(OH,F) 2], were irradiated with 1.5 MeV Kr + ions in the temperature range between 20 and 1023 K using the HVEM-Tandem Facility at Argonne National Laboratory. The irradiation effects were characterized by in situ and high resolution transmission electron microscopy (TEM). All four micas are susceptible to irradiation-induced amorphization with relatively low critical amorphization doses ( D c) (1–2 × 10 14 ions/cm 2, fractions of a dpa) below or at room temperature. D c increased with irradiation temperature for all phases above 400 K. The predicted critical temperatures ( T c), above which amorphization will not occur, for the four micas are all at or above 1000 K. Lepidolite is the most easily amorphized among the four micas at all irradiated temperatures with a predicted T c of 1300 K. Large gas bubbles (1–2 μm in diameter) were observed in a 800 keV Kr 2+ irradiated biotite after 1 × 10 15 ions/cm 2.

Isotopic Evaluation on the Genesis of the Kanamaru Pegmatite Deposit, Niigata Prefecture, Japan

Abstract: The Kanamaru pegmatite is one of the two largest pegmatite deposits in Japan and is rich in K–feldspar, which is maximum microcline. The mica minerals and microcline from this ore deposit were studied for K‐Ar ages and δ18O ratios. Biotite gives K‐Ar age of 78. 5 Ma, but microcline is much younger in age of 66. 0 and 58. 3 Ma. Oxygen isotopic equilibrium temperature of the biotite‐quartz pair is 625°C. Using closure temperature of the K‐Ar system for biotite and microcline, cooling rate of the pegmatite is calculated to be 88°C/m. y. at the beginning but 12°C/m. y. in the later period of the cooling history. The microcline is slightly depleted in 18O, yielding apparent equilibrium temperatures of 230°C and 155°C on quartz‐microcline pairs. It is suggested that the fluid phase, magmatic and partly meteoric in origin, contained in the solidifying pegmatite, was interacted with the original orthoclase and converted it to microcline during the subsolidus stage.

Some metallic spherules in calcareous-marly sediments of the Romanoro Flysch, Sestola-Vidiciatico tectonic unit (Modena district, Northern Apennines, Italy)

Some metallic microspherules were found in a calcareous-marly sample from the Romanoro Flysch (Modena district, Northern Apennines, Italy). The outcrop is a tectonic scale, part of the Sestola-Vidiciatico Unit, without stratigraphic contacts with other formations. The age of the setting stratum was attributed at the middle Miocene, but this datum contradicts the previous authors, who dated the outcrop at the Eocene (Zanotti, 1988), or Cretaceous (Reutter, 1969; Rentz, 1971; Daniele et al. 1996). Morphological, mineralogical and chemical studies were carried out on the microspherules and on some grains. The crust of microspherules is essentially made of iron (from 66% to 48%, like FeO), with other minor elements (Al, Ti, Mn, Ni). The core is made of an FeNi alloy. There are other grains of: quartz, calcite, pyrite, mica and black minerals, with edges. The black ones are polymetallic aggregates of: Fe, Mn, Ti, Al, Cu. About the origin of these microspherules four hypotheses: cosmic origin (exceptional extraterrestrial event), volcanic origin, diagenetic origin, wartime origin (Extrema Ratio). The first one has been confirmed.

Characterization of Mica Dissolution in the Presence of Bulk and Confined Solutions: Implications for Natural Weathering Processes

A significant portion of silicate weathering reactions taking place in soils and rocks occurs in the presence of thin films of water, for example within microcracks and micropores, and along grain boundaries. Here, the properties of aqueous solutions (e.g. atomic structure, solvent properties, viscosity, dielectric constant) can be quite different than they are in bulk solution, and the dominant transport mechanism shifts from advection to diffusion (Hochella and Banfield, 1995). It is unclear how these differences may influence dissolution and precipitation reactions. In this study we examine the characteristics and rates of mica dissolution at room temperature both in the presence of bulk solutions and thin, confined aqueous films. Mica minerals are ideally suited for this study because they can be cleaved to yield atomically smooth surfaces to produce films of uniform thickness. both the open and closed experiments, the samples were removed, thoroughly rinsed, and blown dry with N2. The same regions of the sample were then reimaged using SFM. Several scans of each region were taken to ensure that observed changes were not caused by tip induced artifacts. The thickness of fluid films formed between mica surfaces can accurately be measured over a range of a few nanometers to tens of micrometers with an optical interferometry technique known as FECO (fringes of equal chromatic order). Measurements using this technique have been made in our lab to measure film thicknesses between 800 nm and 25 ~tm, with an accuracy of ___ 80 nm. Our implementation of the technique is currently being refined so that it will achieve sub-nanometer resolution down to film thicknesses in the nanometer range as previously demonstrated by Israelachvili (1973).

Quantitative lithology: open and cased hole application derived from integrated core chemistry and mineralogy database

Abstract A new quantitative lithology interpretation is based on elemental concentrations of silicon, iron, calcium and sulfur available from logs. The lithology interpretation is founded on an integrated chemistry-mineralogy core database comprising over 400 samples from many wells of predominantly sand and shaly sand composition located on four continents. The lithological components include ‘clay’, which is the sum of all clay minerals; ‘carbonate’, which is the sum of calcite and dolomite; ‘anhydrite’, which is the sum of anhydrite plus gypsum; and ‘sand’ or ‘quartz-feldspar-mica’, which is the remainder of the formation essentially constituting the sand fraction. The new interpretation demonstrates that the elements aluminium alone or a combination of silicon, calcium, and iron provide a much more accurate estimation of clay than either gamma ray or its individual components potassium, thorium and uranium. Calcium alone or calcium and magnesium are used to determine carbonate concentrations. Calcium and sulfur can be used to estimate the anhydrite fraction. Having estimated the total clay, carbonate, and anhydrite fractions, the remainder of the formation is assumed to be composed quartz, feldspar, and mica minerals. Examples of the new lithology interpretation are provided for core data and also for geochemical log data from both open and cased hole environments.

Petrographic and paragenetic studies of the Agbaja Ironstone Formation, Nupe Basin, Nigeria

The Agbaja Ironstone Formation of the Lokoja district of Central Nigeria occurs within the Upper Cretaceous sedimentary sequence of the Nupe Basin which trends northwest-southeast. Three lithostratigraphical units occur in the basin: the basal Lokoja Formation, overlain by the Patti Formation and the uppermost Agbaja Ironstone Formation. The Lokoja Formation rests unconformably on the Precambrian basement complex and grades upwards from conglomerate to arkosic sandstone. The Patti Formation consists of interbedded sandstone, siltstone and carbonaceous mudstone units. The Agbaja Ironstone Formation is made up of oolitic, pisolitic and argillaceous ironstones. Four petrographic facies are identified within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, detrital mud-ironstone and breccia mudironstone. Kaolinite ooids are spherical and fragmented, usually with pseudomorphs of goethite after pyrite at the core. The goethite pisoids are cemented in a kaolinitic to goethitec matrix, are elliptical to subspherical in shape, and are composite in nature. Constituent minerals of the mud-ironstone are kaolinite, quartz and heavy minerals. Paragenetic studies indicate that pyrite, siderite, kaolinite, quartz, mica and heavy minerals; were the primary minerals of the ironstone deposit, whereas secondary minerals; were goethite, hematite, goyazite-crandallite, bolivarite and boehmite. Mineralogical studies provided evidence for a kaolinitic precursor for the ironstone deposit, contrary to the earlier proposed chamositic precursor. In addition, the presence of pseudomorphs of pyrite (in the nuclei and incorporated into the cortex of the ooids) are reliable indicators of a possible accretionary model for the formation of ooids and pisoids prior to ferruginisation. Two ferruginisation periods unrelated to lateritisation are indicated for the ironstone deposit.