The solubilities of HCl-treated samples of corundum, gibbsite, boehmite, and diaspore in aqueous solutions at 298 K and one atmosphere pressure were determined from undersaturated and supersaturated initial conditions. Solution characteristics at apparent equilibrium were measured and used to calculate equilibrium constants (Kr) for mineral dissolution reactions:

$$\begin{array}{l} A{l_2}{O_3}\left( {corundum} \right) + 3{H^ + } = A{l^3} + 1.5\,{H_2}O,\\ Al{\left( {OH} \right)_3}\left( {gibbsite} \right) + 3{H^ + } = A{l^3} + 3\,{H_2}O,\\ and\\ AlOOH\left( {boehmite\,or\,diaspore} \right) + 3{H^ + } = A{l^3} + 2{H_2}O, \end{array}$$

Least-squares analysis on the diffraction intensity values certified for NIST SRM676a and SRM1976c α-Al2O3 (corundum) have shown that the intensities of SRM1976c can be simulated by the March-Dollase preferred orientation model along the (001)-direction. Diffraction intensities of randomly oriented corundum crystallites have been calculated from electron density data obtained by conventional and density functional theory (DFT) calculations, on the assumption of independent and similar atomic displacements for Al and O atoms. The results of DFT calculations support that the strongest peak of randomly oriented α-Al2O3 crystalline powder should be 113-reflection, though the intensities simulated by DFT calculations are not closer to NIST SRM676a intensities than those expected for a fully ionized model ${\rm Al}_2^{3 + } {\rm O}_3^{2-}$. Diffraction data of two types of relatively fine (nominally 2–3 μm and ca 0.3 μm) α-Al2O3 powder have been collected and processed by a deconvolutional treatment (DCT). Integrated peak intensities extracted from the DCT data by an individual peak profile fitting method also support that the 113-reflection is the strongest reflection of randomly oriented α-Al2O3 crystalline powder.

The structure of corundum is not amenable to accommodating beryllium, yet Be is commonly added to corundum for purposes of colour modification. Where the main sites in corundum are octahedrally coordinated, Be is tetrahedrally coordinated in all known Be minerals. To attempt to explain this discrepancy, the configurations of Be in the Be–Al–O±H system were determined using density functional theory (DFT). In the Be–Al–O system, the most likely Be configuration is in triangular coordination, replacing Al and bonded to three oxygen on the basal plane (large triangle) of the Al-vacant octahedral site. When H is added, OH groups form at the opposite face of the same octahedron, pointing either into or out of the site. Be–H (beryllium hydride) defects are also stable at the DFT level, with Be bonded to three O and one H. As some defects contain O–H or Be–H bonds, the DFT calculations can be corroborated by FTIR spectroscopy. Spectra recorded from 1000 corundum crystals with elevated Be confirm that bands centred at 3062 and 2490 cm–1 correspond to Be-associated defects. Polarised spectra show that the band at 2490 cm–1 resolves into two, one centred at 2491 cm–1, with a shoulder at 2523 cm–1, one of which is probably associated with a Be–H bond. The 3062 cm–1 band resolves into at least three, including a major band at 3058 cm–1, a shoulder at 3140 cm–1 and a small, sharp band at 3082 cm–1. Whilst there are caveats relating to curve fitting and baseline subtraction, the 3058 cm–1 band can probably be attributed to the defect where H points into the Al-vacant octahedral site, and the 3140 cm–1 band to the defect where the H points out. To our knowledge, this is the first occurrence of Be in triangular coordination proposed for a naturally occurring mineral. This substitution mechanism may also explain the high diffusivity of Be, as Be is effectively associated with its own Al vacancy.

The interaction of NCC-A (no cement content, with tabular corundum) refractory castable with corrosive refining steel slag CaO–Al2O3–SiO2–MgO–(FeO, MnO) was studied with static crucible corrosion tests. The corrosion process was evaluated using chemical and phase analyses, and the macro- and microstructures on the corrosion boundary of slag–refractory materials were investigated. We describe the corrosion progress and explain the NCC-A corrosion mechanism by oxide metallurgical slags. The alumina refractory material surface was wetted with molten oxide slag (contact angle θ1390°C from 10° to 20°). The slag penetrated through dissolution of the matrix (fine alumina cement), and the Ca–Si–Al slag, enriched with alumina, attacked the corundum grog. Marked changes in the chemical composition of the slag had an impact on the primary crystallization temperature. As the concentration of Al–O complexes increased in the slag, the binding parameters in Si–O complexes changed and complex tetrahedral units (Al–O–Si) and (Al–O–Mg) were created. Analyses of the corrosion interface grog show that calcium ions diffuse through structural defects in the corundum but Mg2+ diffusion was not observed. A sharp interface is observed between the slag and the corroded corundum layer. Differences in the chemical composition of the slag in the corrosion zone affected the convention stream, and formed the concave corrosion profile of the crucible.

Unusual corundum-bearing rocks occur in the Bond Range, northern Tasmania, in a hydrothermally altered Cambrian quartz porphyry. The assemblage exhibits quartz and corundum in mutual contact, a rare phenomenon, in association with andalusite, pyrophyllite, diaspore and other minerals. This metastable assemblage apparently resulted from advanced argillic alteration at moderate temperature and low pressure, followed by rapid depressurisation accompanying boiling of hydrothermal, granite-derived fluids. This corundum occurrence appears to be unrelated to the sapphires in placer deposits in Tasmania.

We report in this paper the first occurrence of the rare assemblage corundum + quartz in the high-grade metamorphic zone in the core of the Namaqualand Metamorphic Complex (NMC) (South Africa). The magnetite-bearing rocks hosting the corundum-quartz assemblage are very rare, and occur discontinuous bands a few cm wide, associated with diorite, tonalite and anorthosite within quartzfeldspar ± sillimanite ± biotite ± spinel supracrustal rocks extensively intruded by a foliated granite gneiss. The magnetite-bearing rocks are characterized by the presence of abundant crystals of magnetite together with ilmenite, spinel and large (∽1 cm in diameter) euhedral crystals of corundum both displaying a sharp contact with magnetite and quartz. However, in some micro-domains, spinel and quartz, as well as corundum and quartz, are separated by a thin corona of sillimanite with or without magnetite. These textural features suggest that the following reactions have taken place: quartz + corundum → sillimanite, and quartz + spinel + O2 → sillimanite + magnetite. These reactions are interpreted as occurring at higher, as yet unknown, P-T conditions than previously estimated for the NMC. Whether corundum and quartz were once in equilibrium is worth investigating, as it could constitute a geothermobarometric assemblage for ultra-high T granulites.

An inclusion of corundum (ruby) was found in a clinopyroxene xenocryst in alkali basalt from the late-Cenozoic Chanthaburi-Trat volcanics of eastern Thailand. The clinopyroxene is fairly sodic, highly aluminous and magnesian (0.12–0.14 Na, 0.31–0.33 AlIV and 0.36–0.40 AlVI per 6(O), and Mg/(Mg+Fe2+) > 0.9)) and is chemically similar to clinopyroxene inclusions in rubies from nearby alluvial gem deposits, suggesting a common origin for the two types of occurrence. Sapphirine (Mg/(Mg+Fe2+) = 0.91–0.94) and garnet (py56–67alm11–18grs18–23) also occur as inclusions in alluvial rubies. Thermodynamic calculation of the equilibrium 2 di + 2 crn = 2 cats + en constrains the temperatures of clinopyroxene + corundum crystallization to between 800 and 1150 ± 100°C. Use of other equilibria as stability limits places the pressures of crystallization between 10 and 25 kbar, implying depths of between 35 and 88 km. The most Fe-rich clinopyroxene crystallized at a pressure in the lower part of the range. The pyropic garnet inclusions in corundum crystallized at pressures of >18 kbar (i.e. at depths > ~63 km).

The xenocrystic clinopyroxene could have coexisted in equilibrium with garnet of similar composition to the observed inclusions at the deduced temperatures of crystallization. The rubies probably crystallized in rocks of mafic composition, i.e. garnet-clinopyroxenites or garnet-pyriclasites, within the upper mantle.

Alluvial rubies and sapphires are found in palaeodrainage deposits along the Cudgegong-Macquarie River system, central eastern New South Wales, Australia. A pink to red suite contains Cr (up to 0.6 wt.% Cr2O3) as the main chromophore, exceeding Fe (up to 0.5 wt.%Fe2O3). Corrosive etching suggests a prior xenocrystic Mesozoic-Cenozoic basaltic transport, while Cr2O3/Ga2O3 to Fe2O3/TiO2 ratios indicate an original metamorphic source. Syngenetic mineral inclusions include Al-rich diopside, meionite and anatase. The Al-rich diopside (‘fassaite’) contains extremely high Al2O3 (20–21 wt.%). A blue-green suite contains Fe (up to 0.8 wt.% Fe2O3) as a dominant chromophore, while a rare nepheline-anorthoclase composite inclusion supports a magmatic phonolitic origin. The Cudgegong- Macquarie ruby formation is compared with a garnet granulite origin proposed for Thailand rubies and a xenolith of corundum-bearing garnet granulite from Ruby Hill, Bingara, Australia. Clinopyroxenecorundum thermometry suggests the Cudgegong-Macquarie rubies formed at T >1000–1300ºC, a high equilibration T for proposed lithospheric granulites. These rubies form a distinctive suite compared to other rubies from Australian and SE Asian basalt fields, but have some similarities with eastern Thailand rubies.

Cobalt-rich spinel is found as a ∼200 µm inclusion, together with a glassy phase, in a gem-quality blue sapphire from Bo Ploi, Thailand. This is the first reported natural occurrence of such a spinel. Its composition is directly analogous with that of cochromite, a previously reported rare cobalt-rich chromite. The compositional ranges for the cobalt-rich spinel, obtained using electron microprobe and proton microprobe (methods described below), are Al2O3 48.18–61.27 %, CoO 19.7–22.84 %, Cr2O3 0–12.28 %, FeO 8.64–9.67 %, MgO 6.04–6.89, TiO2 0.49–0.73 %, Ni 2251–2532 p.p.m., Zn 335–371 p.p.m., Mn < 177–849 p.p.m., Ga 113–153 p.p.m., Nb 24–1252 p.p.m., Zr <4–167 p.p.m., Sn 22–428 p.p.m., As <4–56 p.p.m., Sr <4–59 p.p.m., Ag 13–64 p.p.m. Transition elements decrease in abundance from core to rim of the spinel while the other elements increase. Crystal chemical considerations suggest that a vacancy-creating substitution mechanism may be operative in the cobalt-rich spinel despite the small scale, i.e. 3Co2+ = 2Al3+ + [4]□. The glassy phase coexisting with the spinel is likely to be the product of heating by the host basaltic magma. The mode of occurrence of the cobalt-rich spinel prevents further physical investigation. This unusual spinel is considered to be the result of a complex magma mixing process in the lower crust.

Experimental breakdown of staurolite + inclusions of albite and muscovite (representing an aluminous Fe-rich metapelitic composition) occurs within the KNFMASH system at 800°C and 0.1 GPa under fluid-present conditions over periods of 2 days to 8 weeks. The overall melt-producing reaction is staurolite + muscovite + plagioclase + vapour = hercynite + corundum + mullite + quartz + liquid, and involves three reactions; staurolite + vapour = hercynite + corundum + quartz + liquid, albite + vapour = mullite + liquid, and muscovite + vapour = mullite + liquid. Melt (glass) compositions are siliceous (77.2–64.5 wt.% SiO2), peraluminous (A/CNK = 1.5–5.5), and have granite-trondjhemite compositions in terms of normative Ab-Ab-Or contents. Textures indicate that quartz formed later than corundum. The corundum + quartz + hercynite assemblage after staurolite is considered to be metastable and only likely to be found in nature where staurolite-bearing rocks have undergone rapid heating and melting followed by quenching.

A new computer code (MARVIN) has been developed for the simulation of surfaces and interfaces. The models and methodologies employed within the program are briefly discussed. One application of the code, calculating crystal morphologies, is explored using zircon, quartz and α-Al2O3 as examples. The new code enables the use of covalent type force fields and the effect of surface relaxation on the growth morphology to be calculated for the first time. It is found that relaxation does affect the attachment energy but not by a large enough amount to significantly change the growth morphology for the three examples discussed here. Finally, the calculated surface relaxation for the basal plane of α-Al2O3 is found to be in complete agreement with Hartree-Fock ab initio calculations, verifying that the potentials, which are derived from bulk properties, transfer well to this surface.

Mineral and fluid inclusions were studied in seven gem corundums from gravels of three areas in Sri Lanka. All fluid inclusions are pure CO2. Microthermometry results on primary fluid inclusions suggest formation of corundum under granulite facies metamorphism (>630°C, 5.5 kbar). Secondary fluid inclusions indicate different retrograde events of post-metamorphic cooling and uplift for different source areas.

Specimens collected from a small lens of phlogopite-rich rock in the granulite-facies terrain of the Arunta Block, central Australia, have unusual bulk compositions and mineral assemblages. One sample consists of phlogopite enclosing blue spinel (mg 96) with minute granules of corundum and sapphirine at the margins; a second of phlogopite enclosing porphyroblasts of corundum and peraluminous sapphirine. In the first the sapphirine is close to the 7 : 9 : 3 composition; in the other it is markedly peraluminous, e.g. (Mg1.628Fe0.028)Al4.714Si0.636O10, intermediate between the 7 : 9 : 3 and 3 : 5 : 1 members. The texture suggests that this sapphirine is a stable phase in equilibrium with eastonitic phlogopite and corundum. The very potassic, very magnesian bulk composition of the rocks is attributed to potassium metasomatism of a protolith consisting of magnesian chlorite and quartz.

In the aureole of the late Caledonian Omey granite corundum develops in the Dalradian country rocks in a zone up to 250 m from the granite contact. The distribution of andalusite and K-feldspar in pelites and calcite+wollastonite+grossularite in marbles is consistent with inner-aureole metamorphic conditions of 615±25°C at 2.5±0.25 kbar, and ⋍ 0.85. Corundum develops from the reaction muscovite → corundum + K-feldspar+H2O and first appears over 100 m further from the granite than the assemblage wollastonite + grossularite + anorthite. Experimentally determined equilibria can be satisfied only if for the corundum-producing reaction was less than 0.6 and perhaps as low as 0.4. Corundum always grows within large muscovite crystals;fH2O within the crystal lattice is unrelated to that in the grain-boundary fluid of the surrounding rock.

Although whole-rock oxidation ratios are irregularly distributed within the aureole they are uniformly low in corundum-bearing rocks. Reducing conditions probably resulted from localized flow of H2O-rich fluid away from the granite in a diffuse channelway that contains most of the corundum localities and also a distinctive skarn. Although corundum growth within muscovite is sealed off from the external water vapour conditions, it is suggested that movement of H2O down a thermal gradient (and hence down an fH2O gradient at constant pressure) promotes the escape of (OH)− from the muscovite lattice and so allows the corundum reaction to proceed.

Ca-poor and typically Na-rich feldspar megacrysts are common associates of spinel lherzolitic and pyroxenitic xenoliths in Scottish alkalic basalts. Associated megacrysts and composite megacrysts and salic xenoliths include apatite, magnetite, zircon, biotite, Fe-rich pyroxene(s) and corundum. The salic xenoliths and related megacrysts are referred to collectively as the ‘anorthoclasite suite': the majority of the samples are inferred to derive from the disaggregation of coarse-grained, typically Na-rich, syenitic protoliths at depth. Rare occurrences of euhedral anorthoclase megacrysts, together with zircon dating, imply that the suite crystallized at, or very shortly before, their entrainment by the basaltic host magmas. Some evidence suggests that the anorthoclasite suite protoliths lie within ultramafic (pyroxenitic) domains in the deep crust. The latter are inferred to be pegmatites, crystallized from carbonated trachytic magmas with widely variable Ca, Na, K, Ba and trace-element contents, and to have ranged from metaluminous to peraluminous. Crystal zonation and resorption textures within the salic xenoliths imply that the crystallization of the parent magmas was complex. Confirmation of this comes from cathodoluminescence studies of the feldspars showing that early ('primary’) anorthoclases and potassian albites exhibit partial replacement by a more potassic feldspar. A third generation of potassic feldspar (enriched in an assortment of trace elements and deduced to have crystallized from a carbonated high-K melt) forms transecting zoned veins in which carbonate fills the axial zone.

Whereas most of the anorthoclasite suite materials are inferred to have grown from metaluminous magmas, the occurrence of magmatic corundum in salic xenoliths indicates crystallization from magmas that were peraluminous. The corundum-bearing samples also contain Nb-rich oxide minerals and their associated feldspars have the highest rare-earth element (REE)contents. Accordingly, the peraluminous trachyte magmas are deduced to have been specifically enriched in high field-strength trace elements. It is proposed that formation of the anorthoclasite suite protoliths is a phenomenon closely related to that of salic glass ‘pockets', well known from spinel lherzolite xenoliths around the world. Not only are there compositional affinities, but both sets of phenomena appear to have closely pre-empted the ascent of alkali basalt (host) magmas. We propose that the two sets of phenomena are linked and that the anorthoclasite suite derived from coarse-grained sheets, generated by the aggregation of salic melt fractions rising from the shallow mantle and heralding the onset of basaltic magmatism.

We report an unusual occurrence of margarite, corundum, gahnite and zincohögbomite from the Raleigh terrane in the Piedmont Province of Virginia. The assemblage occurs in a chlorite-rich blackwall associated with a small metamorphosed ultramafic rock body. The blackwall is dominated by chlorite, but also contains distinctive clusters (1–4 mm across) that typically consist of ragged Zn-rich spinel grains surrounded by masses of randomly oriented margarite. Also spatially associated with spinel are smaller grains of corundum and högbomite. In most cases, spinel and högbomite are sufficiently enriched in Zn to be called gahnite and zincohögbomite, respectively. Some högbomite grains are distinctly banded in back-scattered electron images, primarily reflecting variations in Zn-content. Textures suggest that högbomite formed at the expense of spinel (although locally the reverse relationship holds), and högbomite compositions mimic those of spinel. Margarite appears to be a later phase, and textures imply formation via reactions with spinel, corundum and, possibly, chlorite. This occurrence of Zn-rich spinel and högbomite is clearly related to the bulk composition of the blackwall rock, which contains >2500 ppm Zn. The origin of this Zn-rich composition is unclear, but could be due to a small amount of sphalerite or zincian-staurolite in the protolith.

The intergrowths of Fe-rich corundum + Al-rich hematite + spinel + hibonite have been found as an assemblage in a plagioclase-clinopyroxene rock (paralava, former hornfels) at the Hatrurim Basin, Hatrurim combustion metamorphic Formation. Most spinels show oriented exsolution structures and vary from (Mg0.75${\rm Fe}_{{\rm 0}{\rm. 25}}^{2 +} $)(Al1.80${\rm Fe}_{{\rm 0}{\rm. 20}}^{3 +} $)O4 (with exsolutions) to (Mg0.77${\rm Fe}_{{\rm 0}{\rm. 23}}^{2 +} $)(Al1.95${\rm Fe}_{{\rm 0}{\rm. 05}}^{3 +} $)O4 (homogeneous) indicating a tendency towards magnesioferrite and magnetite, and enrichment in NiO (up to 1.9 wt.%) and ZnO (up to 1.4 wt.%). Hibonite is Ti rich (TiO2 > 8 wt.%) and close to CaAl9Fe3+(Mg,Fe2+)TiO19. Corundum varies in Fe2O3 (4.2–11.8 wt.%). Hematite is also inhomogeneous and contains oriented exsolution structures of corundum. It shows variable concentrations of TiO2 (0.7–5.6 wt.%), Al2O3 (0.7–8.6 wt.%), Cr2O3 (0.2–1.5 wt.%), V2O3 (0.1–1.0 wt.%) and MgO (0.3–2.0 wt.%). Crystallization of this specific assemblage is assumed to be at 1000–1200°C using evaluations for the corundum hematite pair with reference to published experimental data. The active role of superheated oxidised volatiles is suggested during both crystallisation of this corundum-bearing association and host-rock transformation (melting event for hornfels → paralava).

Significant uncertainty surrounds the processes involved in the formation of basalt-hosted corundum, particularly the role that the mantle plays in corundum generation. Some previous studies have suggested that trace-element ratios (namely, Cr/Ga and Ga/Mg) are useful for distinguishing two types of corundum: ‘magmatic’ and ‘metamorphic’, designations that include mantle and crustal processes. However, recent studies, including this one, have discovered transitional groups between these end-members that are difficult to classify.We used laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) to measure trace-element concentrations in sapphire and ruby crystals from eight alluvial deposits that span a significant length of the eastern Australian gemstone belt. Additionally, we collected LA–ICP–MS U–Pb and traceelement data from zircon megacrysts atWeldborough, Tasmania, which is also within the gemstone belt. Our sapphire and ruby results reveal a continuum in trace-element compositions, an observation that raises questions regarding previous classifications that ascribe corundum from basalt-hosted gemfields to either ‘magmatic’ or ‘metamorphic’ sources. The spatial association of basalt-related gemfields in eastern Australia with a long-lived convergent margin suggests a link between corundum formation and Al-enrichment of the mantle wedge during periods of subduction.