Exsolution Intergrowths Research Articles

Some Mineralogical Characteristics of the Egyptian Black Sand Beach Ilmenite Part IΙ: Rutile-Ilmenite and the Various Titanhematite Grains

In addition to the grains of homogeneous ilmenite, ferriilmenite, hematite-ilmenite exsolved intergrowths, and the partially altered ilmenite grains, other textures are detected in the separated ilmenite concentrate. The grains of rutile-ilmenite exsolved intergrowth represent 0.8% of the detected ilmenite grains. The ilmenite component of this intergrowth is detected to be ferriilmenite associated with geikielite, pyrophanite, and rutile, with Cr2O3 content ranging between 0 and 0.5%. The exsolved rutile is ferriforrous rutile composed of rutile, hematite, geikielite, and pyrophanite, its Cr2O3 content ranging between 0 and 0.4%. The detected individual titanhematite grains represent 4.4% and include 3 textures arranged, in a decreasing order of abundance, as: ilmenite-hematite, rutile-hematite, and rutile-ilmenite-hematite exsolution intergrowths. MgO and MnO have minimum values and they do not follow Fe2O3. In some homogeneous titanhematite or exsolved rutile-hematite, Fe2O3 content may be replaced with SiO2. In all titanhematite intergrown textures, the Cr2O3 content ranges between 0 and 0.1%. Only in the case of the titanhematite host with exsolved rutile, the contained MgO ranges between 1.2 and 5.3%. Some ferromagnetic titanhematite grains separated with the fraction of magnetite are detected. In these grains, the Cr2O3, MgO, and MnO contents range between 0-0.2, 0-3, and 0-1.4% respectively. Several varieties of chromite and chromspinel mineral grains are found and represent 1.1% of the detected bulk ilmenite grains. In these grains, the Cr2O3, MgO, V2O3, and Al2O3 contents range between 16.69-56.72%, 0.54-17.33%, 0.14-0.58%, and 1.33-38.79% respectively. Although they are rarely met in the ilmenite concentrate, the relatively finer grain sizes could lead to the separation of some with ilmenite fraction rather than with the ferromagnetic one. It is concluded that the problem of high Cr2O3 and Fe2O3 contents of the Egyptian beach ilmenite concentrate is not only a mineralogical problem, but also an ore-dressing one.

Exsolution intergrowth of cpx-opx and pseudosection modelling of two-pyroxene mafic granulite from Daltonganj of Chhotanagpur Granite Gneiss Complex, Eastern India

In this study, we reconstruct for the first time the evolution of a two-pyroxene mafic granulite from the Daltonganj of Chhotanagpur Granite Gneiss Complex (CGGC). Transmission electron microscopy (TEM) revealed that some of the clinopyroxene have exsolution texture, where orthopyroxene occurs as thin lamellae within the porphyroblast of clinopyroxene. To tightly constrain the P-T conditions at which exsolution lamellae developed after the metamorphic peak, we have applied both conventional and multi-equilibrium thermobarometry, as well as forward thermodynamic modelling. Results from multi-equilibrium thermobarometry, using the software THERMOCALC, suggest peak conditions of the mafic granulite at average pressure-temperature (PTav) conditions of 6.7 ± 1.19 kbar/814 ± 60 °C. In contrast, exsolution bearing opx-cpx minerals crystallised at relatively lower temperature (772 ± 14 °C), determined by the conventional geothermometers. The peak to retrograde evolution of these mafic granulites is constrained through phase equilibrium modelling in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O) model system using the software Perple_X. Phase equilibria results of peak conditions (i.e. 6.0–6.78 kbar and 775–808 °C) are consistent with respect to those obtained through multi-equilibrium and conventional thermobarometry, while the retrograde path is defined down to ~4.5 kbar and ~540 °C. Our results have twofold implications: (i) they show how the integrating of different geothermobarometric methods is the best proxy to tightly constrain the evolution of high-grade metamorphic rocks, and (ii) they pavement to new constraints on the Paleoproterozoic to Neoproterozoic evolution of the CGGC.

Provenance Studies of Ilmenite from Red Sediments, Bhimunipatnam Coast, East Coast of India

Abstract Mineral chemistry of the ilmenites, drawn from the red sediments along the Bhimunipatnam coast, Andhra Pradesh, India was determined by EPMA technique. The major constituents are TiO2 and FeO, and the value of Ti/(Ti + Fe) < 0.5 of these ilmenites indicate that they are relatively fresh ‘ferrian ilmenites’ and were contributed recently to the red sediments in the study area. Using the mineral chemical data, the end-member composition of the analyzed ilmenites was calculated, which reveals that they are of ‘ilmenite - hematite’ and ‘ilmenite - geikielite - hematite’ composition, derived from calc-alkaline magma-sourced charnockites and metapelitic khondalite suite of rocks respectively of the Eastern Ghats granulite belt. The TiO2 content and Mn/Mg ratio of the study ilmenites indicate that these rocks constitute their provenance in the Gosthani river basin. Their back scattered electron (BSE) and scanning electron (SEM) images show their prominent sub-angular to sub-rounded shape, corroborating the study under microscope, with some preserving signatures of exsolution intergrowth, earlier deformation and rounded shape, all of which are attributed to their transportation by the Gosthani river, after their liberation from their provenance rocks.

The Crystal Chemistry of Rathite Based on New Electron-Microprobe Data and Single-Crystal Structure Refinements: The Role of Thallium

Crystal-structure refinements in space group P21/c were performed on five grains of rathite with different types and degrees of thallium, silver, and antimony substitutions, as well as quantitative electron-microprobe analyses of more than 800 different rathite samples. The results of these studies both enlarged and clarified the complex spectrum of cation substitutions and the crystal chemistry of rathite. The [Tl+ + As3+] ↔ 2Pb2+ scheme of substitution acts at the structural sites Pb1, Pb2, and Me6, the [Ag+ + As3+] ↔ 2Pb2+ substitution at Me5, and the Sb-for-As substitution at the Me3 site only. The homogeneity range of rathite was determined to be unusually large, ranging from very Tl-poor compositions (0.16 wt%; refined single-crystal unit-cell parameters: a = 8.471(2), b = 7.926(2), c = 25.186(5) Å, β = 100.58(3)°, V = 1662.4(6) Å3) to very Tl-rich compositions (11.78 wt%; a = 8.521(2), b = 8.005(2), c = 25.031(5) Å, β = 100.56(3)°, V = 1678.4(6) Å3). The Ag content is only slightly variable (3.1 wt%–4.1 wt%) with a mean value of 3.6 wt%. The Sb content is strongly variable (0.20 wt%–7.71 wt%) and not correlated with the Tl content. With increasing Tl content (0.16 wt%–11.78 wt%), a clear increase of the unit-cell parameters a, b, and V, and a slight decrease of c is observed, although this is somewhat masked by the randomly variable Sb content. The revised general formula of rathite may be written as AgxTlyPb16−2(x+y)As16+x+y−zSbzS40 (with 1.6 < x < 2, 0 < y < 3, 0 < z < 3.5). Based on Pb–S bond lengths, polyhedral characteristics and Pb-site bond-valence sums, we conclude that the Pb1 site is more affected by Tl substitution than the Pb2 site. When Tl substitution reaches values above 13 wt% (or 3 apfu), a new phase (“SR”), belonging to the rahite group, appears as lamellar exsolution intergrowths with Tl-rich rathite (11.78 wt%). Rathite is found only in the Lengenbach and Reckibach deposits, Binntal, Canton Wallis, Switzerland.

Post-collisional carbonatite-hosted rare earth element mineralization in the Hongcheon area, central Gyeonggi massif, Korea: Ion microprobe monazite U‐Th‐Pb geochronology and Nd‐Sr isotope geochemistry

The Hongcheon area in the central Gyeonggi massif is a unique carbonatite locality in South Korea. The age and petrogenesis of this uncommon rock type and associated rare earth element (REE) mineralization still remain uncertain. The NNE trending, 20–50m wide and ~2km long Fe-REE ore bodies are hosted within a swarm of carbonatite dykes intruding Precambrian basement gneisses. The intrusive nature of the dykes, fenite alteration halos, exsolution intergrowths of constituent minerals and stable isotope data in the literature collectively attest to the ore formation by crystallization of carbonatite magma. The carbonatites are composed primarily of dolomite, ankerite, siderite, magnetite, monazite, apatite, strontianite and pyrite with subordinate quartz, barite, columbite, fergusonite and calcite. The principal carrier phase of REEs is monazite. The REE contents of monazite vary narrowly (TREO=66.1–69.4wt.%) irrespective of the textural occurrence. Although the monazite shows a sample-to-sample variation in La/Nd ratio, the textural varieties from each rock sample are similar with respect to this ratio. Thorium contents in monazite are consistently low (average=ca. 2500ppm) with unusually high (average=ca. 2200) Th/U ratios. Sensitive high-resolution ion microprobe (SHRIMP) dating of monazite yielded a weighted mean 208Pb/232Th age of 232.9±1.6Ma, which agrees with a weighted mean 206Pb/238U age of 227.2±8.3Ma within uncertainties. This age, coupled with comparable intrusion ages documented for kimberlites and monzonite-syenite-gabbro-mangerite suite from central Korea, demonstrates the occurrence of mantle-derived alkaline igneous activities and associated REE mineralization following the North and South China collision. The intrusion of the Hongcheon carbonatite and potassic or ultrapotassic suite in central Korea may have resulted from the post-collisional detachment of the subducted slab and consequent upwelling of hot asthenosphere and melting of the overriding lithospheric mantle. Initial Nd‐Sr isotopic ranges of the Hongcheon carbonatite (εNd=ca. −26, 87Sr/86Sr=0.703–0.706) and previous trace element data deny a petrogenetic linkage with the coeval silicate magmas. The metasomatism in the lithospheric mantle source of the Hongcheon carbonatite must have occurred in the distant past (>1.7Ga) to generate significantly negative εNd values.

Pyrite–sulfosalt reactions and semimetal fractionation in the Chinkuashih, Taiwan, copper–gold deposit: a 1 Ma paleo‐fumarole

AbstractThe mineralized fracture system that underlay paleo‐fumarole field at Chinkuashih, Taiwan has been exposed by copper–gold mining to depths of about 550 m below the paleo‐surface. Its mineralogy and systematic variations in metal and semimetal (Fe, Cu, As, Sb, Bi, Hg, Cd, Sn, Zn, Pb, Se, Te, Au, Ag) concentrations provide insights into the chemical responses of a magmatic‐vapor phase as it expands through fracture arrays to the surface and discharges as fumaroles associated with more extensive solfatara. At Chinkuashih, following initial sealing of the fractures by silica‐alunite alteration, brittle failure reestablished discharge from an underlying reservoir of magmatic vapor. Crystalline pyrite was deposited first in the fractures and was succeeded and replaced by ‘enargite’ (Cu3(As,Sb)S4) as sulfosalt encrustations (‘sublimate’) on fracture surfaces and in extensional cracks. Subsequent recrystallization resulted in complex exsolution intergrowths with antimony fractionation to the evolving crystal–vapor interface. Heavy metal fractionation between sulfosalt and vapor enriched the vapor phase in heavy metals that subsequently precipitated as complex Bi–Hg–Sn sulfosalts in discrete areas (paleo‐fumaroles) close to the paleo‐surface in a manner analogous to modern‐day fumaroles on active volcanoes such as Vulcano, Italy. As in similar paleo‐fumaroles (e.g., El Indio, Chile and Lepanto, Philippines), the most characteristic reaction sequence is the partial replacement of the early pyrite by enargite and Fe‐tennantite. It is proposed that this reaction tracks the decrease in the pressure of the underlying magmatic‐vapor reservoir because of the sustained discharge of vapor to the surface.

티탄철석-적철석 용출시료의 구조분석과 상태방정식 연구

용출구조를 보이는 티탄철석-적철석 광석시료에 대한 구조분석을 리트벨트법을 이용하여 시행하였다. 구조유사체인 두 광물의 분석결과, 기본구조인 팔면체의 형태는 티탄철석의 Ti를 중심으로 한 팔면체(M2)가 정팔면체에 가장 가까운 형태를 보여주며, 다음은 티탄철석의 Fe를 중심으로 한 팔면체(M1)이다. 적철석 팔면체의 경우 M1과 M2 중간정도이다. 고압실험은 두 광물의 회절선이 중첩되는 5.8 GPa까지 시행하였다. 이 압력구간에서 티탄철석은 정상적인 압축성을 보이나, 적철석의 압축은 미미하게 발생하는 비정상적인 거동을 보인다. 이러한 이상거동은 두 광물의 압축성 차이에 의한 차등대응에 의한 것으로 판단된다. Exsolution intergrowth of ilmenite and hematite was studied by the Rietveld refinement method. According to the analysis on these two structural analog minerals, it was found that octahedron (M2) of Ti in ilmenite is in the least deformation, then that (M1) of Fe in ilmenite is deformed next, and octaheron deformation of Fe in hematite is between M1 and M2. High pressure compression experiment was performed up to 5.8 GPa, where two minerals' XRD peaks merged completely. Ilmenite shows normal compression behavior, whereas hematite shrinks in very small amount. This kind of abnormal behavior might be due to the differential response to the applied pressure corresponding to the different compressibilities of the minerals each other.

Subduction zone metamorphism – pioneering contributions from the Alps

Circum-Pacific-type high-pressure (HP) metamorphic belts occupy contractional sutures marked by intensely deformed, imbricated nappes that verge seaward. Mineral assemblages reflect recovered depths of ∼25–50 km for mafic and felsic crustal units carried down on an underlying oceanic plate; such units recrystallized prior to disengagement and exhumation of slices of chiefly low-density rock. Low-grade parts of Alpine-type subduction complexes are similar to regurgitated circum-Pacific-type HP belts, but coesite ± microdiamond inclusions in tough, unreactive host minerals show that lithospheric underflow and continental collision involve partial recovery of ultra-high-pressure (UHP) rocks from depths of 100–140 km. Associated garnet peridotites from the Central Alps and several other collision sutures display exsolution lamellae and intergrowths suggesting the former existence of majoritic garnet and additional phases requiring formation at >300 km, attesting to earlier, unrelated mantle upwelling, or to the resurrection of profoundly subducted and back-reacted complexes. HP and UHP terranes return to mid-crustal levels mainly driven by buoyancy. Times of deep-seated storage and rates of exhumation are short; mineral parageneses, intervals of HP–UHP recrystallization and nappe structures reflect the two-way, relatively rapid transport of now-exposed subduction complexes. More fully than any other areas, integrated mineralogic-geochemical-structural studies of the Alps have illuminated relationships between metamorphic petrology and convergent plate tectonics. HP–UHP transformations of mafic and quartzofeldspathic rocks initially formed at, or near, the Earth's surface, imbricate structures of subduction complexes including their prograde metamorphic zonations + retrograde parageneses, association with deep-seated UHP mantle rocks and speedy ascent were first documented in the Alps.

Hematite-Ilmenite Exsolution Intergrowth in Quartz Schist Closely Associated with the Cupriferous Pyritic Deposits of "Kieslager" Type at the Shirataki Mine, Kochi Prefecture

Hematite-Ilmenite Exsolution Intergrowth in Quartz Schist Closely Associated with the Cupriferous Pyritic Deposits of "Kieslager" Type at the Shirataki Mine, Kochi Prefecture

Heavy Mineral Placer Sand Deposits of Kontiagarh Area, Ganjam District, Orissa, India

AbstractThe Kontiagarh placer deposit in the Ganjam district, Orissa, India extends in northeast direction having a width of 700–1000 m. A total of 187 samples were collected meterwise from 55 bore holes in a grid pattern from beach, frontal, intermediate and back dunes covering an area of approximately 1 km2. Light minerals decrease in size from the beach to the back dunes, whereas the size distribution of heavy minerals in the beach and dunes is more or less uniform. The average heavy mineral content in the beach and dunes vary from 9.38% to 24.20%. The heavy minerals are ilmenite, garnet, sillimanite, rutile, monazite, and zircon with trace amounts of magnetite, hornblende, diopside, sphene, tourmaline, and epidote. Heavy minerals are mostly less than 350 µm in size, with a peak distribution in the range between 180 and 125 µm. Ilmenite shows exsolution intergrowth with hematite. Mineral chemistry of ilmenite, hematite, leucoxene, magnetite, monazite and sillimanite are examined by EPMA. Leucoxene is lower in Fe and higher in Ti, Al, Cr and V than ilmenite. The litho‐units of the Precambrian Eastern Ghats Mobile Belt, comprising primarily khondalite, charnockite, calc‐silicate granulite and gneiss, are the source of heavy minerals for this deposit. The bulk sample has 7.30% ilmenite, 5.24% sillimanite, 9.16% garnet, 0.18% rutile, 0.14% monazite, 0.06% zircon and 0.52% other heavy minerals. The deposit has good potential for economic exploitation of ilmenite, rutile, sillimanite, monazite, zircon and garnet.

SIMULTANEOUS REFINEMENT OF TWO COMPONENTS OF AN EXSOLUTION INTERGROWTH: CRYSTAL STRUCTURES OF THE LINDSTROMITE - KRUPKAITE PAIR

Crystal structures of intergrown lindstromite and krupkaite were determined in two distinct samples by a simultaneous crystal-structure refinement using a modified jana program package. The exsolution intergrowths have a bulk composition, determined by means of electron microprobe, equal to bd 60.1 and bd 64.1 , respectively. The subscript indicates a percentage of the aikinite component in this bismuthinite derivative . The crystal structure of lindstromite contains two aikinite-like ribbons Cu 2 Pb 2 Bi 2 S 6 and eight krupkaite-like ribbons (ideally CuPbBi 3 S 6 ) in a unit cell. The latter are oversubstituted; the ideally empty tetrahedral sites associated with them refined to the occupancy values of 0.04–0.09 Cu and 0.14–0.18 Cu, respectively, for the two above bulk compositions. The refined phase-compositions are Cu 6.26 Pb 6.26 Bi 13.74 S 30 (bd 62.6 ) associated with CuPbBi 3 S 6 (bd 50 ) for the sample ‘bd 60 ’ and Cu 6.64 Pb 6.64 Bi 13.36 S 30 (bd 66.4 ) associated with oversubstituted krupkaite, the composition of which could not been refined, for the sample ‘bd 64 ’. From stoichiometry calculations, the latter is ~bd 60. Both latter values may be somewhat overestimated as a result of cross-correlations between the two closely related structures during the refinement.

Magnetization of exsolution intergrowths of hematite and ilmenite: Mineral chemistry, phase relations, and magnetic properties of hemo‐ilmenite ores with micron‐ to nanometer‐scale lamellae from Allard Lake, Quebec

Hemo‐ilmenite ores from Allard Lake, Quebec, were first studied over 50 years ago. Interest was renewed in these coarsely exsolved oxides, based on the theory of lamellar magnetism as an explanation for the high and stable natural remanent magnetizations (NRMs), 32 to 120 A/m, reported here. To understand the magnetism and evolution of the exsolution lamellae, the microstructures and nanostructures were studied using scanning electron microscopy and transmission electron microscopy (TEM), phase chemistry, and relations between mineral chemistry and the hematite‐ilmenite phase diagram. Cycles of exsolution during slow cooling resulted in lamellae down to 1–2 nm thick. Combined electron microprobe, TEM, and X‐ray diffraction (XRD) results indicate that hematite hosts reached a composition approximately ilmenite (Ilm) 14.4, and ilmenite hosts ∼Ilm 98. The bulk of the very stable NRM, which shows thermal unblocking ∼595–620°C, was acquired during final exsolution in the two‐phase region canted antiferromagnetic Rc hematite + R ilmenite. Hysteresis measurements show a very strong anisotropy, with a stronger coercivity normal to, than parallel to, the basal plane orientation of the lamellae. Magnetic saturation (Ms) values are up to 914 A/m, compared to 564 A/m predicted for a modally equivalent spin‐canted hematite corrected for ∼15% R2+TiO3 substitution. Low‐temperature hysteresis, AC‐susceptibility measurements, and Mössbauer results indicate a Néel temperature (TN) of the geikielite‐substituted ilmenite at ∼43 K. The low‐temperature hysteresis and AC‐susceptibility measurements also show a cluster‐spin‐glass‐like transition near 20 K. Below TN of ilmenite an exchange bias occurs with a 40 mT shift at 10 K.

Progressive or continual exsolution in pyroxenes: an indicator of polybaric igneous crystallization for the Perinthatta anorthositic gabbro, northern Kerala, southwestern India

Clinopyroxene and orthopyroxene from the Perinthatta anorthositic gabbro in southwestern India exhibit exsolution intergrowths of pyroxenes (occurring in five stages involving pigeonite, orthopyroxene and/or augite) and Fe–Ti oxides. The thicker irregular pyroxene lamellae appear to be the result of heterogeneous nucleation, indicating that this was the first unmixing process to begin. This was followed by homogeneous nucleation of regularly spaced, quite uniform pyroxene lamellae and defect-free fine pyroxene platelets between early-formed exsolution lamellae, suggesting progressive or continual nature of exsolution during slow cooling. Pyroxene thermometric calculations indicate that subcalcic augite and pigeonite equilibrated at ∼935 °C, with the different lamellar generations formed between ∼914 and ∼748 °C. Pre-metamorphic anorthositic gabbro crystallization is considered to have taken place at deep crustal levels (∼30 km; ∼9–10 kbar) and later emplacement at shallower crustal levels (∼15 km; ∼5–6 kbar) suggesting polybaric conditions. The apparent difference between the pressure of crystallization and the emplacement of the Perinthatta anorthositic gabbro supports the common Proterozoic anorthosite petrogenetic model, involving initial crystallization at depth, followed by intrusion as a crystal-rich mush. Following emplacement, the igneous assemblage was affected by granulite facies metamorphism on cooling, as indicated by smaller granulite minerals along grain boundaries and the partial re-equilibration of exsolved phases.

AN INTEGRATED PROCESS FOR THE RECOVERY OF TUNGSTEN AND TIN FROM A COMBINED WOLFRAMITE–SCHEELITE–CASSITERITE CONCENTRATE

This article presents results of the studies on development of viable process route for quantitative recovery of tungsten and tin values occurring together in the form of a multimineral concentrate of wolframite, scheelite, and cassiterite. The investigations were carried out using a tin–tungsten multimineral ore from Kyrgyzstan (Central Asia). Tungsten minerals in this ore occur both as distinct liberated mineral phases and as composites, viz. ultrafine dissemination of wolframite in scheelite and vice versa. Cassiterite also has similar characteristics, albeit to a lesser extent, with few grains containing ex-solution intergrowth of columbite/tantalite and very fine dissemination of tungsten minerals in its lattice. The complex nature of occurrence of both tungsten and tin minerals necessitated an integrated process comprising physical beneficiation and chemical extraction. The process route that was developed gave a tungsten and tin recovery of about 80 and 90%, respectively, on treating a primary gravity preconcentrate analyzing 16.5% WO3 and 25.7% Sn.

Nanoscale haematite–ilmenite lamellae in massive ilmenite rock: an example of ‘lamellar magnetism’ with implications for planetary magnetic anomalies

Summary Massive, nearly ‘pure’, haemo-ilmenite layers from historic ore deposits in Rogaland, Norway contain very few silicates or other oxides and typically produce remanence-dominated magnetic anomalies. These rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains. A typical bulk composition, Ilm 84, exsolved at high temperature to produce host ilmenite Ilm 94 and micron-sized haematite lamellae Ilm 23 as measured by electron microprobe (EMP). Subsequent undercooling of the ilmenite and the micron-scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit-cell scale, leaving depleted hosts between lamellae with compositions of Ilm 98 and Ilm 15–13 as measured by TEM–EDX. Samples have high coercivities, and average NRM values of 25 A m−1, which typically show ∼2 per cent saturation in the NRM state. The amount of magnetization in these samples is too high to be solely accounted for by a spin-canted AF moment in the haematite. Based on Monte Carlo simulations of haematite–ilmenite interfaces at the atomic scale and on measured rock-magnetic properties, we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine-scale titanohaematite and ilmenite exsolution lamellae.

COMPOSITION RANGES AND EXSOLUTION PAIRS FOR THE MEMBERS OF THE BISMUTHINITE-AIKINITE SERIES FROM FELBERTAL, AUSTRIA

A combination of electron-microprobe data, single-crystal X-ray diffractometry and BSE-based image analysis of polished sections was used to evaluate the composition of single-phase grains, replacement aggregates and exsolution intergrowths of the members of the bismuthinite–aikinite series from the scheelite deposit at Felbertal, Austria. Composition ranges of the classical products and new members of the series have been established, and the mineral associations in products of replacement and exsolution defined. The deposit contains a nearly complete range of bismuthinite–aikinite derivatives, the proportion of the aikinite component in them ranging from 3 to 95%. Three independent events of sulfosalt replacement were recognized at Felbertal, respectively yielding krupkaite, gladite and Cu-bearing bismuthinite as replacement products of the older bismuthinite–aikinite derivatives. Exsolution in the bismuthinite–aikinite series proceeds at each stage toward a pair of phases that are compositionally closest to the parent phase, building up to three-stage exsolution-induced sequences not described before. Ample documentation on these, as well as on all the measured compositions and crystallography of bismuthinite–aikinite derivatives, form part of this paper. We attempt a geological interpretation of the observed spatial distribution of the observed phenomena over the scale of the deposit.

Valence state mapping and quantitative electron spectroscopic imaging of exsolution in titanohematite by energy-filtered TEM

The microstructure of hematite-ilmenite exsolution intergrowth of a natural titanohematite crystal from a granulite has been investigated in a transmission electron microscope equipped with an energy filter. Special emphasis is on quantitative compositional mapping at the nanometre scale using electron spectroscopic imaging, as well as mapping the Fe3+ and Fe2+ valence distribution in the intergrowth. Quantitative point analyses by energy-dispersive X-ray analysis have been compared with results from electron energy-loss spectroscopy and element-distribution mapping. The results indicate that the coexisting compositions of the two phases (Ilm88Hem12 and Ilm16Hem84) are independent of the size of the exsolution. The application of quantitative mapping to determining diffusion profiles around precipitates is demonstrated.

TEM study of franklinite-hetaerolite (ZnFe2O4-ZnMn2O4) exsolution intergrowths

The spinel structure-type is derived from ccp oxygen packing by cation occupation of one quarter of the tetrahedral and one half of the octahedral interstices with a general stoichiometry of AB2O4. Franklinite, ideally ZnFe2O4, was the dominant ore mineral of zinc at the great Franklin and Sterling Hill zinc deposits, New Jersey. Although the chemistry of franklinite conformed to the spinel formula, the composition showed a wide range in the proportions of iron, zinc, and manganese. The microscopic examination of polished sections of the Franklin spinel revealed oriented exsolution intergrowths of franklinite, magnetite (Fe2+Fe3+2O4), and gahnite (ideally ZnAl2O4). In this contribution, the exsolution intergrowths of franklinite and hetaerolite (ideally ZnMn2O4) are well characterized using JEM 2000FX and JEM 2010 electron microscopes operated at 200 keV.

Quantitative chemical relationships in a franklinite-magnetite exsolution intergrowth from Franklin, Sussex County, New Jersey

Quantitative chemical relationships in a franklinite-magnetite exsolution intergrowth from Franklin, Sussex County, New Jersey

Magnetism and transmission electron microscopy of Fe‐Ti oxides and pyroxenes in a granulite from Lofoten, Norway

Granulites from southwest Lofoten, Norway, often carry intense and stable components of natural remanent magnetization (NRM). Coercivities in excess of 100 mT and blocking temperatures up to 580°–645°C are common. Several phases in these rocks can carry stable components of NRM. Clinopyroxenes (augite with exsolved pigeonite and orthopyroxene) usually contain abundant Fe‐Ti oxides, the majority of which are too small to be studied conclusively with visible light. Additionally, these rocks typically contain coarse‐grained (∼100–1000 μm) exsolution intergrowths of rhombohedral Fe‐Ti oxides and coarse‐grained magnetite. We have studied the magnetism and mineralogy of the titanohematite and the inclusion‐bearing augites in one granulite sample using transmission electron microscopy (TEM) and magnetic experiments to understand better the remanence properties of naturally occurring Fe‐Ti oxides and their relationships to silicate minerals. The coarse‐grained titanohematite has a coercivity of 350 mT and a laboratory saturation remanence that unblocks at 570°–600°C. The hysteresis characteristics of this titanohematite are typical of much smaller ∼10μm‐sized single domains, which apparently reflects the presence of exsolved ilmenite lamellae, as small as <10 nm in thickness. These lamellae presumably inhibit (1) domain wall nucleation and (2) movement of walls that manage to nucleate. The inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560°C. Magnetite, hematite, and ilmenite are present in the augite, and intergrowth of the rhombohedral phases, revealed by TEM, persists down to the 20‐nm scale. The distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure, while that of the rhombohedral Fe‐Ti oxides is strongly controlled by silicate microstructure, with intergrowths of these oxides commonly decorating lamellae of pigeonite, orthopyroxene, and clinoamphibole within the augite. The NRM of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m.y. ago. Paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event. In addition to the included oxides, the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites: pigeonite lamellae are not parallel to the b‐crystallographic axis, pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur. Rare earth elements are concentrated within inclusions that reside in the augite.