Oriented rutile/hematite intergrowths from Mwinilunga in Zambia were investigated by electron microscopy methods in order to resolve the complex sequence of topotaxial reactions. The specimens are composed of up to several-centimeter-large euhedral hematite crystals covered by epitaxially grown reticulated rutile networks. Following a top-down analytical approach, the samples were studied from their macroscopic crystallographic features down to subnanometer-scale analysis of phase compositions and occurring interfaces. Already, a simple morphological analysis indicates that rutile and hematite are met near the <010>R{101}R||<001>H{110}H orientation relationship. However, a more detailed structural analysis of rutile/hematite interfaces using electron diffraction and high-resolution transmission electron microscopy (HRTEM) has shown that the actual relationship between the rutile and hosting hematite is in fact <010>R{401}R||<001>H{170}H. The intergrowth is dictated by the formation of {170}H|{401}R equilibrium interfaces leading to 12 possible directions of rutile exsolution within a hematite matrix and 144 different incidences between the intergrown rutile crystals. Analyzing the potential rutile–rutile interfaces, these could be classified into four classes: (1) non-crystallographic contacts at 60° and 120°, (2) {101} twins with incidence angles of 114.44° and their complementaries at 65.56°, (3) {301} twins at 54.44° with complementaries at 125.56° and (4) low-angle tilt boundaries at 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga samples. Using HRTEM and high-angle annular dark-field scanning TEM methods combined with energy-dispersive X-ray spectroscopy, we identified remnants of ilmenite lamellae in the vicinity of rutile exsolutions, which were an important indication of the high-T formation of the primary ferrian-ilmenite crystals. Another type of exsolution process was observed in rutile crystals, where hematite precipitates topotaxially exsolved from Fe-rich parts of rutile through intermediate Guinier–Preston zones, characterized by tripling the {101} rutile reflections. Unlike rutile exsolutions in hematite, hematite exsolutions in rutile form {301}R|{030}H equilibrium interfaces. The overall composition of our samples indicates that the ratio between ilmenite and hematite in parent ferrian-ilmenite crystals was close to Ilm67–Hem33, typical for Fe–Ti rich differentiates of mafic magma. The presence of ilmenite lamellae indicates that the primary solid solution passed the miscibility gap at ~900 °C. Subsequent exsolution processes were triggered by surface oxidation of ferrous iron and remobilization of cations within the common oxygen sublattice. Based on nanostructural analysis of the samples, we identified three successive exsolution processes: (1) exsolution of ilmenite lamellae from the primary ferrian-ilmenite crystals, (2) exsolution of rutile lamellae from ilmenite and (3) exsolution of hematite precipitates from Fe-rich rutile lamellae. All observed topotaxial reactions appear to be a combined function of temperature and oxygen fugacity, fO2.
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... Penn & Banfield (1998) have shown that oriented attachment of polymorphic TiO 2 nanoparticles can result in twinning and oriented intergrowths. Another mechanism leading to rutile crystals/domains in twinned orientation is oriented (topotaxial) recrystallization and/or epitaxial growth of/on structurally related precursor minerals (Armbruster, 1981;Force et al., 1996) (Rečnik et al., 2015;Stanković et al., 2016). In principle, oriented recrystallization can result in the development of coplanar cyclic twins of rutile, which can be theoretically composed of up to six rutile domains, separated by five {101} TBs and an additional non-crystallographic contact (Hahn & Klapper, 2006;Padró n-Navarta et al., 2020). ...
... ). The possibilities are schematically presented in Fig. 7(b); a more detailed description of this process is given in the description of rutile exsolutions from matrix ilmenite, FeTiO 3 [see Table 1 and the corresponding explanation given by Rečnik et al. (2015)]. ...
Contact and multiple cyclic twins of cassiterite commonly form in SnO2-based ceramics when SnO2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO2 on CoNb2O6 and Co4Nb2O9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO2 grain growth; CoNb2O6 forms at ∼700°C and Co4Nb2O9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb2O6 grains can only result in the formation of contact twins, the Co4Nb2O9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb⁵⁺ and Co²⁺ ions into the SnO2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.
... In this study, we investigate the nanoscale relationships between dislocations and trace element segregation in rutile. Rutile is known to exhibit twinning (Daneu et al., 2014(Daneu et al., , 2007Penn and Banfield, 1998;Recnik et al., 2015;Reece and Morrell, 1991), therefore instead of targeting deformation-related lowangle boundaries, which may show variable dislocation types, dislocation density and trace element enrichment (Tacchetto et al., 2021), we investigate systematic dislocations associated twin boundaries. ...
... EBSD and TKD data across the twin boundaries yield misorientation axes parallel to <100> and this direction should define the dislocation geometry on the {101} interface (Fig. 3a, Fig. C.1). Observations by TEM show linear features that are consistent with the <100> direction, confirming these features as dislocations. The presence of dislocations on the twin interface is consistent with formation via attachment of two opposing growing rutile crystals from a precursor phase (Daneu et al., 2014;Recnik et al., 2015). Consequently, these dislocations are stationary and intrinsic to the twin interface. ...
Element mobility is a critical component in all geological processes and understanding the mechanisms responsible for element mobility in minerals is a fundamental requirement for many geochemical and geochronological applications. Volume diffusion of elements is a commonly assumed process. However, linear defects (dislocations) are an essential component of the high-temperature creep of minerals. These defects are commonly inferred to form fast-diffusion pathways along which trace elements can more rapidly migrate. In contrast, dislocations in minerals are also energetically favourable sites of trace element segregation, which counters the notion that they enhance bulk diffusion rates by a pipe diffusion mechanism. In this paper we characterize the trace-element composition of dislocations on twin boundaries in rutile by combining atom probe tomography with transmission electron microscopy. First, morphology and correlative microstructural data are used to demonstrate that the linear compositional features in the atom probe tomography dataset represent dislocations. Assessment of dislocation composition indicates that segregation is trace element specific. The data show that dislocations in rutile act as both, fast-diffusion pathway and trace-element traps which potentially leads to erroneous estimations of the composition.
... Hematite-ilmenite exsolution patterns formed from initial ferrian ilmenite (e.g. McEnroe, et al. 2005, Rečnik et al. 2015 and exsolutions of rutile in these assemblages (Rečnik et al. 2015) are used to estimate temperature of phase transformation and to study oxygen activity on cooling of ferrian ilmenite. In the cited papers, as well as in a large number of other works, the usage of the composition and exsolution textures of Fe-Ti oxide in the determination of physico-chemical conditions in magma was mainly applied to slowly cooled mafic igneous rocks. ...
... Hematite-ilmenite exsolution patterns formed from initial ferrian ilmenite (e.g. McEnroe, et al. 2005, Rečnik et al. 2015 and exsolutions of rutile in these assemblages (Rečnik et al. 2015) are used to estimate temperature of phase transformation and to study oxygen activity on cooling of ferrian ilmenite. In the cited papers, as well as in a large number of other works, the usage of the composition and exsolution textures of Fe-Ti oxide in the determination of physico-chemical conditions in magma was mainly applied to slowly cooled mafic igneous rocks. ...
Ferrian ilmenite, representing an intermediate member of the hematite-ilmenite solid solution, is frequently present as an acessory mineral in hornblende andesites of the Timok Magmatic Complex (Serbia), which host world-class porphyry copper systems of the Bor Metallogenic Zone. In Krivelj, a type locality of this rock, ferrian ilmenite occurs in homogenous crystals without exsolutions present. Its unit cell parameters are a = 5.062(1) Å, c = 13.895(4) Å, V = 308.4(1) Å3 and according to the dependence of unit volume with composition, studied ferrian ilmenite corresponds to an intermediate member of the series with 53 mole% ilmenite (Xilm = 0.53). Transmission electron microscopy investigations show that this mineral is homogenous in terms of hematite-ilmenite solid solution and presenceof exsolutions, even at the nanoscale level. Ferrian ilmenite from other localities (Majdanpek and Nikoličevo) shows various patterns of exsolution, even at the nanoscale level. Thus, ferrian ilmenite from Majdanpek shows very fine exsolutions of hematite-ilmenite phases. X-ray diffraction analysis confirms that initial ferrian ilmenite is broken down to two phases: a predominant host phase (ilmenohematite) with Xilm = 0.31 and a second exsolved phase (hemoilmenite) with Xilm = 0.78. Average grain compositions of this decomposed ferrian ilmenite correspond to a large degree to the previous homogenous ferrian ilmenite from Krivelj. However, due to slower cooling of andesite from the Majdanpek locality, a subsequent breakdown of the initial ferrian ilmenite was played out resulting the formation of exsolution textures, that also caused changes in the magnetic properties of this mineral. In general, differences in ferrian ilmenite textures can imply the local variability in the conditions of the formation of the host volcanic rock and/or indicate that subsequent alteration processes have occurred.
... In fact, co-orientation characterized by parallelism or near-parallelism of crystallographic planes and directions (rows of atoms) in (semi)-coherent mineral interfaces is the most energetically favorable (and frequent) scenario (Putnis 2002;Bunge et al. 2003;Zhong et al. 2011;Cayron et al. 2014;Awan and Khan 2017;Adegoke et al. 2022;Keller and Ague 2022). However, this exsolution model assumes ab initio the unmixing of a solid solution in a closed system, where an initially homogenous parent crystal is able to contain all the components that later become unmixed into a host matrix with similar or dissimilar crystal structure and daughter inclusion (e.g., Rečnick et al. 2015;Stanković et al. 2016;Keller and Ague 2022). This raises an immediate question: is laurite able to incorporate enough Cu, Fe and Ni to exsolve Cu-Fe-Ni sulfide inclusions upon cooling? ...
This paper provides a top-down nanoscale analysis of Cu-Ni-Fe sulfide inclusions in laurite from the Taitao ophiolite (Chile) and the Kevitsa mafic-ultramafic igneous intrusion (Finland). High-resolution transmission electron microscopy (HRTEM) reveal that Cu-Ni-Fe sulfide inclusions are euhedral to (sub)-anhedral (i.e., droplet-like) and form single, biphasic or polyphasic grains, made up of different polymorphs, polytypes and polysomes even within a single sulfide crystal. Tetragonal (I4d) and cubic (F3m) chalcopyrite (CuFeS2) host frequent fringes of bornite (Cu5FeS4; cubic F3m and/or orthorhombic Pbca) ± talnakhite (Cu9(Fe, Ni)8S16; cubic I3m) ± pyrrhotite (Fe1 − xS; monoclinic C2/c polytype 4C and orthorhombic Cmca polytype 11C) ± pentlandite ((Ni, Fe)9S8; cubic Fm3m). Pentlandite hosts fringes of pyrrhotite, bornite and/or talnakhite. Laurite and Cu-Fe-Ni sulfide inclusions display coherent, semi-coherent and incoherent crystallographic orientation relationships (COR), defined by perfect edge-to-edge matching, as well as slight (2–4º) to significant (45º) lattice misfit. These COR suggest diverse mechanisms of crystal growth of Cu-Fe-Ni sulfide melt mechanically trapped by growing laurite. Meanwhile, the mutual COR within the sulfide inclusions discloses: (1) Fe-Ni-S melt solidified into MSS re-equilibrated after cooling into pyrrhotite ± pentlandite, (2) Cu-Ni-Fe-S melts crystallized into the quaternary solid solution spanning the compositional range between heazlewoodite [(Ni, Fe)3±xS2] (Hzss) and ISS [(Cu1±x, Fe1±y)S2]. Additionally, nanocrystallites (50–100 nm) of Pt-S and iridarsenite (IrAsS) accompanying the sulfide inclusions spotlight the segregation of PGE-rich sulfide and arsenide melt earlier and/or contemporarily to laurite crystallization from the silicate magmas. Cobaltite (CoAsS)-gersdorffite (NiAsS) epitaxially overgrown on laurite further supports the segregation of arsenide melts at early stages of chromitite formation.
... In Lufilian arc, Copper sulphides are generally associated with Tioxides (Garrard, 1972;Richards et al., 1988;Unrug, 1988). Sample of hematite crystals in Mwinilunga, Zambia were found to be covered by rutile (Rečnik et al., 2015). This mineralization probably evolved from diagenetic alteration of ilmenite to rutile in a two-stage process (Sweeney et al., 1991). ...
Central and Southern Africa is home to major Archean Cratons, flanked by Proterozoic mobile belts. The region is characterized by anomalously high topography and shallow Curie depth (Zb). Factors controlling topography and Zb anomalies in old continental crusts remain poorly understood. In this study, we present high-resolution Moho undulation (Zm) derived with gravity spectral technique. Archean Cratons and Proterozoic belts are mostly characterized by deep Moho (>36 km), whereas the Moho depths in Mozambique belts is mostly shallow (~26 to 41 km). Extremely shallow Zb (<10 km) is dominant in Archean Cratons, whereas intermediate Zb (10–25 km) is prevalent in the mobile belts. The Curie points are mostly above the Moho. Low surface heat flow (Qs) in Archean Cratons indicate that Zb is not correlated with thermal perturbation. Shallow Zb are mostly found in areas with low Vp/Vs ratio, predominantly felsic lower crust and sharp Moho. Shallow Zb also correlate with intense metasomatism and mineral enrichment, as well as depletion in basaltic components and geochemical overprinting by multiple episodes of melt and fluid infiltration. Crustal composition is the main factor that controls shallow Zb in Central and Southern Africa Archean Cratons. The regional elevated topography is not due to thermal perturbation as mostly argued by mantle plume hypothesis, but may be caused by regional thermo-chemical mantle upwelling dominated by composition.
... A number of methods exist to measure crystallographic orientation but the most common are based on diffraction of electrons, X-rays or neutrons. Studies of twin formation and exsolution, which seek to quantify orientation relationships precisely, use transmission electron microscope (TEM)-based imaging and electron diffraction to measure the orientations of the adjacent crystals whilst also imaging the interface (Wang et al. 1990;Rečnik et al. 2015). Electron backscatter diffraction (EBSD) in the scanning electron microscope (SEM) is used to measure the crystallographic orientation across a much larger field of view than is possible in a TEM and can provide orientation relationships from a statistically representative number of grains but without the precision of a TEM. ...
Replacement reactions occur during metamorphism and metasomatism in response to changes in pressure, temperature and bulk rock and fluid compositions. To interpret the changes in conditions, it is necessary to understand what phases have previously been present in the rocks. During fluid-mediated replacement, the crystallography of the replacement phases is often controlled by the parent reactant phase. However, excessive fluid fluxing can also lead to extreme element mobility. Titanium is not mobile under a wide range of fluid compositions and so titanium-bearing phases present an opportunity to interpret conditions from the most extreme alteration. We map orientation relationships between titanium-bearing phases from ore deposits using EBSD and use symmetry arguments and existing relationships to show that completely consumed phases can be inferred in ore deposits.
An ilmenite single crystal from Junction gold deposit is replaced by titanite, rutile and dolomite. The rutile has the following well-documented orientation relationship to the ilmenite
[0001]ilmenite // < 100 > rutile and < 101¯0 > ilmenite // [001]rutile
The anatase is a single crystal and shows a potential orientation relationship
[0001]ilmenite = (0001)ilmenite // {211}anatase and < 101¯0 > ilmenite // < 01¯1 > anatase
The single crystal orientation and lack of symmetrical equivalent variants suggest nucleation dominates the anatase production. Dolomite grew epitaxially on the ilmenite despite only sharing oxygen atoms suggesting the surface structure is important in dolomite nucleation.
Titanite partially replaced ilmenite during metasomatism at Plutonic gold deposit. The titanite orientation is weakly related to the ilmenite orientation by the following relationship:
[0001]ilmenite // < 100 > titanite and {101¯0}ilmenite // (001)titanite
The prevalence of subgrain boundaries in the titanite suggests multiple nucleation points on an already deformed ilmenite needle leading to the formation of substructure in the absence of deformation.
Existing known topotaxial replacement relationship can be used to infer completely replaced phases using the misorientation distributions of the replacement polycrystals. Orientation modelling for a cubic phase replaced by rutile in a sample from Productora tourmaline breccia complex shows misorientation distributions consistent with
< 001 > Rutile // < 110 > cubic and < 100 > Rutile // < 111 > cubic
Combining this with volume constraints and assuming Ti is immobile, the composition of the cubic phase is constrained as titanomagnetite with 85% ulvospinel. Complex microstructures with domanial preferred orientations can also be used to document the microstructure of replaced phases. An aggregate of rutile grains with two parts that share a common < 100 > axis is interpreted as having replaced a twinned ilmenite grain. Modelling shows that the misorientation distribution for the aggregate is consistent with the above relationship replacing ilmenite with a {101¯2} twin.
In this paper preliminary mineralogical characterization of hematite from Alinci is presented. Several crystals of hematite were collected for research. The straight-forward identification of the studied mineral sam-ples was enabled by optical microscope, SEM-EDS, ICP-MS and XRPD methods. The use of these methods showed that they are very useful methods for rapid mineral analysis contributing important analytical information. With these methods was established that the investigated mineral is hematite. Hematite crystals occur in the syenites. The size of the crystals is up to 2 cm. Crystals of hematite included small idiomorphic to hypidiomorphic crystals of rutile and ilmenite. Idiomorphic to hypidiomorphic crystals of ilmenite are corroded, relictized and separated as a solid break-down solution in a homogeneous FeO–Fe2O hematite mass. Twinned idiomorphic aggregate of ilmenite partially cata-clased into a homogeneous Fe-oxide mass also appear. Relict and corroded hypidiomorphic to allotriomorphic forms of rutile appear in a compact and homogeneous Fe-oxide hematite mass. EDS analyses on samples from Alinci show that hematite matrix contains from 1.59 to 5.89 % of Ti, whereas the rutile domains may contain from 1.15 to 1.50% of Fe.
Geratiyon-ki-Dhani (GKD) area is located in Khetri sub-basin of North Delhi Fold Belt and exposes metasediments of Ajabgarh Group. The Ajabgarh Group comprises quartz-biotite schist which is intruded by albitite and granite. The primary uranium phases viz. davidite and brannerite occurring with calcite and /or quartz veins are common in albitite. Quartz and calcite from mineralised albitite have been selected for characterization of the ore-forming fluid, which makes a promising uranium deposit.
Fluid inclusion petrography indicated presence of three (03) types of fluid inclusions (FIs) based on phases present at room temperature and mode of occurrence; polyphase (type 1a & 1b) and biphase (type 2) inclusions. Type 1a FIs are primary in nature and are composed of vapour bubble, salt crystals with <50% liquid phase and are irregular or negative crystal in shape varying in size between 12–23µm and occasionally contain hematite grain. The type 1a inclusions homogenised mostly by dissolution of salt crystal between 243–382°C (mode=340°C, n=30) corresponding to salinity of 34.32–46.13 wt% NaCl-eq (mode=42wt% NaCl-eq, n=30). Type 1b FIs have vapour bubble, salt crystal with >50% liquid phase, occur as trans-granular trails and in clusters, having irregular shapes with size varying from <5–12µm. The inclusions homogenised by disappearance of salt crystals at 209 to 294°C (mode= 280°C, n=35) corresponding to salinity range from 29.03–38.59 wt% NaCl-eq (mode=36wt% NaCl-eq, n=35).
A wide range of overlap is noted in salinity (34.32–38.59 wt% NaCl-eq) and homogenization temperature (243–294°C), histograms of type 1a & 1b inclusions suggest them to be part of a common fluid activity. Polyphase inclusions show simple cooling and follow halite saturation curve in salinity-temperature diagram. Biphase inclusions are invariably associated with type 1b and also show a cooling trend. On the basis of fluid characteristics, it is concluded that magmatic albitite has provided single mineralising solution resulting in davidite enriched mineralisation. The gradual cooling of high temperature (243–382°C), high saline (34.32–46.13 wt% NaC-leq) hydrothermal fluids has ensued U-mineralisation in GKD. The evidences presented here are significant as magmatic albitite hosted mineralisation is found for the first time in North Delhi Fold Belt of Khetri sub-basin of northwest india. it has opened a new concept for exploration which will help more resource addition.
Nanotwin structures in materials engender fascinating exotic properties. However, twinning usually alter the crystal orientation, resulting in random orientation and limited performances. Here, we report a well-aligned rutile TiO2 nano-twin film with superior preferential orientation than its isostructural substrate. By means of the synchrotron X-ray Laue nano-diffraction technique, the crystal orientation, twin boundaries, and deviatoric stresses of the film were quantitatively imaged at unprecedented spatial resolution to unravel the underlying mechanism of this anomalous alignment. Massive {101}-type rutile nanotwins were observed and a crystallographic relationship of the heteroepitaxy was proposed. The rapid twinning and twin-controlled heteroepitaxy are responsible for the texture improvement. This work would open up opportunities for rational design of better twin-based functional materials, and implies the powerful capabilities of X-ray nanodiffraction technique for multidisciplinary applications.
High-resolution and energy-filtered TEM of the interface between hematite and ilmenite exsolution lamellae: Relevance to the origin of lamellar magnetism - Volume 9 Issue S03 - Takeshi Kasama, Ute Golla-Schindler, Andrew Putnis
The Bureau of Mines is conducting research into the behavior of phases in the Fe-0 system during reduction and oxidation reactions. Basic understanding of the reduction of hematite to magnetite or oxidation of magnetite to hematite must consider the atomic arrangement of crystal structure of each phase and the rearrangement of atoms resulting from the reaction. A nautral and a synthetic hematite crystal were investigated. The structure model of Pauling and Hendricks has been confirmed with essentially no change in the iron coordinates andAPPROXIMATELY A //5% change in the oxygen coordinates. Selected interatomic distances and angles were calculated as were anisotripic temperature factors.
Orientation relations between rutile, hematite, and spinel structure-types are based on parallel oxygen layers between host and guest crystals. Oxygen chains in rutile exist along <101> and <001> and cross under 57o15'. In spinel (111) or hematite (0001) those chains cross under 60o. Alignment of rutile <101> or <001> with corresponding directions in close-packed structures leads to two different orientation relations which can be observed in nature and laboratory experiments. (Author's abstract) -W.T.
Some aggregates of rutile, classically considered to be "elbow" twinned, instead are topotactic replacements of ilmenite or other hexagonal titaniferous precursors. Twinned rutile can be differentiated from the reticulated rutile of topotactic replacements by the angle of prism intersections, junction morphology, and the overall form of the aggregate. In a special case of topotactic replacement of ilmenite, rutile forms pseudomorphs of "trellis"-textured ilmenite lamellae in {111} of precursor magnetite. We trace the progress of rutile formation through the alteration of fine-grained magnetite-bearing host rocks. The sequential two-step topotaxy from magnetite through ilmenite to rutile requires rutile prisms to parallel the intersections of {111} planes in precursor magnetite. Some coarse reticulated rutile may result from the same paragenetic sequence.
High-resolution transmission electron microscopy has been used to characterize the defect microstructure of rutile from chlorite-, chloritoid-, garnet-, and staurolite-grade metapelites. Analytical electron microscopy (AEM) revealed that the rutile contained between 0.5 and 3 wt.% FeO and that the Fe content generally increased with metamorphic grade. High-resolution images, nanoprobe-AEM analyses, and electron diffraction patterns indicated that the Fe is contained within platelets generally less than 1 nm wide that have the hematite structure. The regularly spaced platelets are coherently intergrown parallel to (100) and (101) of rutile. They closely resemble platelets reported from experimental studies of Ti3+- and Fe-bearing rutile. It is proposed that the platelets in natural rutile are hematite and that apparent tripling of the {010} spacings results from dynamical diffraction. The hematite (or Ti2O3) structure represents an end-member arrangement of pairs of face-sharing octahedra. At higher temperatures in synthetic rutile these pairs are ordered to form crystallographic shear planes. The hematite platelets are interpreted to have formed by a precipitation mechanism. This origin is consistent with the defect distribution and orientation and is supported by related experimental studies. -Authors
The interfaces between fine-scale exsolution lamellae of hematite and ilmenite from an igneous rock in Rogaland, Norway have been studied by conventional transmission electron microscopy (TEM), high-resolution TEM, and energy-filtered TEM (EFTEM), to investigate the lamellar magnetism hypothesis for the origin of the unusual magnetic properties of this rock. Very fine hematite and ilmenite lamellae, less than 50 nm in length and parallel to (001) of their host, were abundant throughout the hemo-ilmenite sample. Dark-field and EFTEM observations indicated the hematite and ilmenite have very sharp structural and compositional interfaces with their hosts. The interfaces between the coarse hematite and ilmenite lamellae (length >1 μm) and their hosts have some interface dislocations to relieve elastic coherency strain. On the other hand, very fine lamellae (length <50 nm) have no interface dislocations and are perfectly coherent. The interface dislocations for lamellae on the order of 100 nm in length are distributed heterogeneously, and more than 80% of the length of the interfaces seen by TEM is dislocation free. Thus, most of the interfaces in the sample are coherent. These results are in accord with the predictions of Monte Carlo simulations of the exsolution process (Harrison and Becker 2001) and with the hypothesis that coherent and sharp structural and compositional interfaces are the origin of lamellar magnetism (Robinson et al. 2002).