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One hundred and five specimens were collected from 40 different sites in northwest-striking metabasalts and metagabbros of Lower Proterozoic (Aphebian) age in the northeastern part of the Labrador Trough (longitude: 69 °W, latitude: 57 °N). The radiogenic age determinations of surrounding rocks of the same age yielded 1860 m.y. A petrological and chemical description of the rocks is included. The chemical results for total Fe, MgO, and Na2O + K2O combined, drawn on a ternary diagram show that the metabasalts and metagabbros fall in the tholeiite field. Magnetite (with some occasional 5% ulvospinel) is the most frequent magnetic memory carrier; pyrrhotite is less commonly found. In order to obtain some pertinent information relative to the stability of the NRM component, step-wise alternating field demagnetization was conducted on 40% of the specimens and the others were demagnetized at a specific AF intensity. In 35% of the specimens, the AF demagnetization process induced an anhysteretic magnetization component above 250 Oe. A secondary isothermal remanent magnetization of lesser stability was often detected in the 50–150 Oe range. Tilting of the formations sometimes increased and sometimes decreased the dispersion of the NRM vectors. After tilting the lithological units that were AF demagnetized, the mean paleomagnetic pole position is 152 °E, 20° S (dm = 14°,dp = 07°, K = 34), with at least one reversal.

The ilmenite-rutile-ironequilibriaand ulvospinel-ilmenite-iron equilibria have been studied in the temperature range 990–1320 and 1050–1355 K, respectively, using an electrochemical technique. Effects on experimental results from deviations from the ideal stoichiometries FeTiO 3 , TiO 2 and Fe 2 TiO 4 have been modelled from available data in the literature. For ilmenite-rutile-iron (IRI), μO 2 ( ± 140) = −570745 + 97.278 T + 50278 T In T (800 T T (1042 T T (1184 T μO 2 ( ± 100) = −505563 + 4.990 T + 13.315 T In T (900 T T (1042 T T - 54.3800 T In T (1184 T μO 2 = joules per mole ; T = Kelvins ; reference pressure = 10 5 Pa (1 bar); quoted uncertainties are one standard deviation). Results from the IRI equilibrium are in good agreement with most of the previous phase equilibrium studies. In detail, however, there is evidence of possible error in the calorimetric data for TiO 2 or FeTiO 3 , or both. For the UII equilibrium, results indicate Fe 2 TiO 4 probably has the theoretically ideal zero point entropy of 2 R In 2, caused by mixing of Fe 2+ and Ti on the octahedral site of this inverse spinel.

The microscopic characters and chemical compositions of coexisting magnetites and ilmenites in a suite of differentiated gabbros are discussed. Textures of the minerals suggest that some discrete crystals of ilmenite owe their origin to complete exsolution from an originally homogeneous high-temperature solid solution. The magnetites contain exsolution bodies of ilmenite and sometimes of ulvospinel, Fe2TiO4, those of rocks rich in opaque oxides carrying much more ulvospinel than those of average rocks at the same horizons in the intrusion; leading to the conclusion that the magnetite compositions are largely determined by oxidation-reduction equilibria between ferrous and ferric oxides and oxygen in the crystallizing magma. The variation in contents of Mg, Co, Mn, Zn, Cu, Al, Cr and V in both magnetites and ilmenites is discussed. In the magnetites these vary much less than FeO, Fe2O3 and TiO2 between melanocratic and average bands at the same horizon and reflect both their ease of entry into the minerals and their availability in the magma at each stage.

The partitioning of zirconium between FeTiO 3 and Fe 2 TiO 4 , in the presence of ZrO 2 + Fe, was determined experimentally in the Fe-Ti-Zr-O system. The ratio of Zr il /Zr usp is strongly temperature dependent. The zirconium concentrations of coexisting ilmenite and ulvospinel were measured in several Apollo 14 and 15 rocks. These minerals in Apollo 15 rocks contain up to 0.42 and 0.23 wt% Zr, respectively, which are the highest zirconium contents yet reported for these minerals. Although the absolute zirconium concentrations in these phases vary within a given rock, the partitioning ratio is relatively constant. These ratios show that whereas some rocks have ‘quenched in’ the initial zirconium contents of the oxides, other rocks have reequilibrated to subsolidus temperatures as low as 850°C. The zirconium partitioning between coexisting ilmenite and ulvospinel appears to be a sensitive indicator of differences in cooling histories of lunar rocks.

Optical examination of the opaque oxide minerals in specimens of diabase, intermediate granophyric diabase, granophyre, and diabase pegmatite from Triassic intrusions of Pennsylvania revealed distinct changes through the diabase-granophyre differentiation sequence. Qualitative electron microprobe scans across complex mineral grains supplemented and confirmed the optical interpretations. The opaque oxide minerals consist almost entirely of ilmenite, magnetite, and ulvospinel, and they comprise between 1 and 10 percent of the rocks in the diabase-granophyre sequence. Magnetite occurs in two forms: the first contains exsolved ulvospinel and is intimately associated with ilmenite; the second is Ti-poor and it does not contain microscopically visible Ti-bearing phases. The second form occurs as rims on the first or as separate crystals in differentiated rocks. Ilmenite occurs as intergrowths, as skeletal growths, and as individual crystals. Accompanying the igneous differentiation sequence there is a gradation from titaniferous magnetite with ilmenite, to skeletal ilmenite retaining an original spinel outline. We suggest that during the stage of iron-enrichment in the igneous differentiation sequence an aqueous fluid developed within the crystallizing intermediate magma and was capable of dissolving iron from the titaniferous magnetite intergrowths, leaving skeletal crystals of ilmenite. Some or all of this iron was redeposited as a second generation of Ti-poor magnetite within the source rock. It is possible that iron-bearing solutions so derived could escape to higher levels where they would be a potential source of magnetite deposits of Cornwall type.

The magnetic properties of crystalline rock and fines returned by the Apollo 12 mission have been measured. The fines contain at least 0.7% by mass of native iron, mainly in the super- paramagnetic form at room temperature. Native iron is present in the crystalline rock but in a much lower concentration. The paramagnetic minerals, ilmenite and ulvöspinel, are present. Some evidence has been obtained that magnetite is present in the crystalline rock, less than 0.02% by mass. It has not yet been established whether it is indigenous to the rock. Two samples exhibit a change in n.r.m. at low temperature consistent with this n.r.m. being carried by magnetite, but these investigations are not yet complete. The crystalline rocks possess an n.r.m. of intermediate stability: there is evidence for two components of magnetization, a weak and rather stable one, and a less stable but stronger component. The former indicates the presence of a magnetic field on the moon at the time of formation of the rocks. An external origin seems unlikely: thus the Moon possessed a magnetic field of internal origin 3400 Ma§ ago which has since decayed.

It is found that magnetite can acquire stable and intense remanent magnetization by heating it from −150°C to room temperature in a magnetic field. Because of the inversed direction of the temperature change for acquiring the remanent magnetization, the remanent magnetization may be called «inverse type of TRM (ITRM)». The production of ITRM is most effective near the zero point of the crystalline anisotropy energy of magnetite, namely, at about −125°C for stoichiometric magnetite and at about −165°C for 8% ulvospinel titanomagnetite. Experimental results show that the crystalline anisotropy energy is a key factor in the acquisition of ITRM. A theory is presented to account for the production of ITRM quantitatively. Assuming that a multidomain magnetite sample consists of numerous elementary domains characterized by the microscopic coercivity and the distribution of the microscopic coercivity is of a Gaussian type, a simple formula. $$(ITRM)_{T_1 ,H}^{T_2 } = (IRM)_{T_2 ,H} \times \frac{{(H_c (T_2 ))_{bulk} }}{{(H_c (T_1 ))_{bulk} }}$$ is derived as a relation between IRM and ITRM, where $$(ITRM)_{T_1 ,H}^{T_2 }$$ is ITRM produced by heating the sample fromT 1 toT 2 inH and(H c (T 1)) bulk and(H c (T 2)) bulk are the values of the coercivity atT 1 andT 2 respectively. This formula explains the general feature of ITRM production, the value of ITRM calculated based on this formula agreeing well with the observed one.

Basalt samples containing ore grains of homogeneous, stoichiometric titanomagnetite (45 mol% ulvospinel) were used to measure the diffusion coefficient and activation energy of titanium in titanomagnetite. The values were obtained by means of combined ore microscopic observations, electron microprobe studies and magnetic measurements. The samples were heated in air at 600 °C and 1000 °C and the time necessary to form lamellae of haemo-ilmenite in titanomagnetite was measured. The value for the activation energy of titanium in titanomagnetite thus obtained is 0.5 eV.

One hundred seventy-six oxide mineral grains in the Luna 20 samples were analyzed by electron microprobe. Spinel is the most abundant oxide, occurring in troctolite fragments. Next most abundant is ilmenite, which occurs in all rock types except those containing spinel. Chromite also occurs in all rock types except those containing spinel. Minor amounts of ulvospinel, armalcolite, zirkelite, baddeleyite and an unidentified TiO2-rich phase were also found. Spinel grains are predominantly spinel-hercynite solid solutions, commonly with very minor chromite. The Fe/(Fe + Mg) ratio is generally lower than in spinel from Apollo 14 rocks. Chromites in non-mare rocks are similar to those from mare rocks. Ilmenite of mare origin is Mg-poor and Zr-rich compared to non-mare ilmenite; these elements may therefore be useful in determining the origin of ilmenite grains. Phase equilibria considerations suggest that spinel troctolite crystallized from a melt high in alumina; a likely candidate is the high-alumina basalt of Prinz et al. (1973).

Soil samples 74220 ('orange soil'), 74241 and 75081 were sized, and the compositions of the opaque and silicate phases were determined. The ilmenites, particularly in 74241, contain up to 7.8 wt % MgO and display higher bireflectance than low-Mg ilmenites. They commonly contain exsolution-like chromite and rutile and occasionally are in association with native Fe in an assemblage probably resulting from reduction. The chromian ulvospinels are similar to Apollo 11 spinels in that they contain near-equal amounts of chromite and ulvospinel molecules. No primary chromites were observed. Most native Fe has No and Co contents of less than 1 wt %; some in 74220 contained 5-6% Ni and less than 1% Co in association with schreibersite.

Alteration of magnetite-ulvoespinel to turbid sphene, and olivine to Ia chlorite in differentiated Precambrian basic intrusive rocks of southeastern Missouri involves significant mobilization of Fe as well as other elements. This type of alteration also causes mineralogic and chemical changes in the rocks that affect the density, magnetic susceptibility and possibly the thermo-remanent magnetic properties of basic rocks. Because the sphene that replaced magnetite-ulvoespinel may be virtually opaque in thin sections, it may be overlooked. This paper describes fresh and altered basic rocks and suggests the possibility of large scale Fe mobilization and concentration to form ore deposits of magnetite and/or hematite.

Three titanomagnetite samples, of composition x=0.7, 0.9, and 1.0 (where x is the ulvospinel molecular fraction), were oxidized according to the method of Sakamoto et al., namely, wetgrinding followed by heating to 200–300°C. Like the samples of Sakamoto et al., our samples showed reversal of saturation magnetization during alteration of titanomagnetite to titanomaghemite. Our experimental results seem to be compatible neither with the ionic model of Verhoogen nor with the model of O'Reilly and Banerjee. The Curie point gradually increased and the lattice parameter decreased during low-temperature oxidation (titanomaghemitization). On heating above 300°C, the γ-titanomaghemites underwent high-temperature oxidation (unmixing into magnetite, pseudobrookite, which is formed only during heating to 600°C, and rutile phases). The unmixing results in a sudden increase in Curie point and decrease in lattice parameter and saturation magnetization. On further heating, the saturation magnetization reverses for a second time. Therefore, double self-reversal of remanent magnetization in naturally oxidized rocks is considered a real possibility.

SummarySix titanomagnetites from a differentiated alkaline intrusion have been studied using microscopie, chemical, and X-ray techniques. The titanomagnetites are homogeneous; with differentiation their FeO and TiO2contents decrease, and Fe2O3increases. The titanomagnetite course of crystallization extends from close to the ulvöspinel-magnetite join, near to ulvöspinel, towards magnetite. The recalculated normative compositions of three titanomagnetites indicate that they are essentially Fe3O4-(γ-FeTiO3)-(γ-Fe2O3)solid solutions. The changing chemistry of the titanomagnetites is ascribed primarily to variations in the chemistry of the magma during differentiation.

A visually homogeneous intrusive titanium magnetite is shown microscopically to consist of two phases: a) solid solution of magnetite in ulvospinel and b) solid solution of ulvospinel in magnetite; the phase ratio is about 1 : 2. On heating in air two new phases are produced in titanium magnetite, as oriented plates forming the mineral's characteristic reticulate microstructure: ilmenite and hematite. Ilmenite disappears on further heating and the end-phase becomes a solid solution of ilmenite in hematite. It is believed that natural oxidation in depths also yields to ilmenite plates in titanium magnetite, if ulvospinel is present. Oriented platelets of hematite may be similarly produced in TiO2 -free magnetite and in magnesium ferrite, although MgO retards their development in the latter. -- V. P. Sokoloff.

Ultrabasic rocks with Niggli values for si between 26 and 100, and k between 0.6 and 1 have been chosen for the study of the distribution of 23 major and minor constituents between phlogopite and the host rock. The rocks include kimberlites, carbonatites and lamprophyres which contain abundant zoned micas and a few zoned spinels. Chemical variations in zoned mica and spinel were determined by electron probe microanalysis, and distribution coefficients for Al, Ti, Fe, Cr, Mg, Mn, and Ni were calculated between mica and spinel for the average composition of the mica and spinel, and for chemically-related and adjacent zones. The spinel changes from chromite in the centre to chromian ulvospinel in the rim, and the mica from chromian phlogopite, through titaniferous biotite to chromium-poor phlogopite. Distribution coefficients for Cr and Fe between adjacent zones in spinel (0.8 and 0.02 for Cr; 1.1 and 3.1 for Fe) and in mica (0.4 and 0.1 for Cr; 2 and 0.3 for Fe) show more variation than the distribution coefficients between the cogenetic phlogopite and chromite (0.04 for Cr, 0.2 for Fe), and biotite and ulvospinel (0.08 for Cr; 0.2 for Fe). It is concluded that distribution coefficients for major and minor constituents between related zones are more meaningful than those calculated from the average chemical composition of zoned minerals.

Optical, magnetic, and X–ray data are presented, together with chemical analyses, for selected North Island ironsand concentrates.Titanomagnetite in the form of magnetite–ulvospinel solid solutions is the predominant constituent, containing minor amounts of aluminous spinels, mainly in solid solution.The magnetic data indicate appreciable variation in composition within each sample. Ulvospinel and aluminous spinels in solid solution combine to depress the Curie temperature of the magnetite, but act in opposition on the lattice dimension, which therefore remains close to that of pure magnetite Progressive oxidation of the titanomagnetite in some specimens has caused exsolution of rhombohedral FeoCX–FeTiOn solid solutions. The end product of such oxidation is titanhematite, which constitutes the whole of some grains, and in others is accompanied by pseudobrookite.Oxidation of titanomagnetite grains with an exsolved aluminous spinel phase has been accompanied by re-solution of the latter and accommo...

The opaque minerals in the lunar rocks 10047, 10050, 10057, 10059, 10068 and in the dust were studied. Rock 10047 contains ilmenite as a main component besides Al-bearing chromian ulvöspinel, a new Ti, Fe, Zr, Y, Ca silicate, troilite, cobaltian alpha Fe, minor hafnian baddeleyite and dysanalyte. Dysanalyte contains Ca, Ti, and Fe as major elements and shows concentrations (rare earth elements together approximately 10 percent) of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Hf, Y, and Zr plus traces of Nb, Ba, and Na. The chemistry of ilmenite in the igneous rocks shows no appreciable variations. A new mineral of the solid solution series MgTi(2)O(5)-FeTi(2)O(5) has also been encountered in the polymict breccias. The microbreccias and the fines contain meteorite debris indicating an impact origin.

SummaryChemically analysed samples of magnetites carrying exsolved ulvöspinel and ilmenite have been heated in vacuo at various temperatures without change in overall chemical composition and the changes occurring studied microscopically, magnetically, and by X-rays.Magnetite with exsolution bodies of ulvöncl can be homogenized probably below 600°C.; thermomagnetic curves and microscopic textures suggest that the solvus curve is very steep at the Fe3O4 end of the Fe3O4-Fe2TiO4 series.On heating magnetite containing ilmenite lamellae up to 950°C., some interchange of ions between the phases occurs (Fe3O4+FeTiO3 → Fe2TiO4+Fe2O3, the ilmenite lamellae taking Fe2O3 into solid solution. The accompanying host magnetite not only gains Fe2TiO4, but at the same time dissolves some FeTiO3. Above 950°C. extensive decomposition takes place, with the production of pseudobrookite, and homogenization has not been achieved. It is suggested that under magmatic conditions extensive solid solution exists in which the FeTiO3 component may be regarded as having a spinel (γ) structure; after exsolution, monotropic inversion may occur to yield a rhombohedral ferri-ilmenite (Chevallier et al., 1955), which on slow cooling changes in composition towards FeTiO3.Magnetite with exsolution bodies of both ulvöspinel and il menite shows on heating the features of both the above binary types.

Texturally, the two hundred basic igneous rock fragments we have examined from soil samples 12001, 12032, 12033, 12037 and 12070 fall in a smoothly continuous sequence from vitrophyres through variolitic and subophitic basalts to coarse equigranular gabbros exhibiting cumulate texture. Spinel compositional data from a variety of these fragments, and from four NASA Apollo 12 crystalline rocks are consistent with the textural suggestion that most of this variety has resulted from differentiation that followed extrusion onto the lunar surface. As in earlier studies, spinels from all rock types indicate a complete solid solution between aluminous chromite and ulvospinel. Early crystallizing spinels are Mg and Cr rich, and are often enclosed wholly or in part by olivine and pigeonite phenocrysts. Ulvospinel occurs as overgrowths over chromite, and as discrete anhedral interstitial grains. On plots of Ti/(Cr + Al + 2Ti) vs. Fe/(Fe + Mn + Mg), spinels from the finer grained basalts fall on nearly identical curves formed essentially of two straight line segments with strongly different slope. Compositional variation in early chromites is largely in Fe/Mg reflecting the change in the melt's composition caused by the early crystallization of olivine and pigeonite. Later spinels show variation dominated by the substitution of Ti + (Mg, Fe) for 2 (Cr + Al). Spinels in the coarse gabbros fall on straight lines parallel to, and generally more Mg-rich than the portions of the curves for the fine grained basalts where Ti/Cr is the dominant variation. The results are consistent with the idea that the coarse cumulate gabbros have formed by early crystallization from a flow whose surface, and near-surface layers are represented by the vitrophyres and fine grained basalts, respectively.

Electron microprobe analyses of eight optically isotropic chrome spinels (gray and blue) and seven optically anisotropic chrome ulvospinels (tan, pink, khaki) have been carried out on four unshocked microgabbros from the Apollo 12 lunar samples. These analyses fall into two compositionally distinct groups along the join Fe2TiO4 (ulvospinel)-FeCr2O4 (chromite) in the system (Fe,Mg)O-TiO2-(Cr,Al)2O3; compositionally the isotropic minerals range from Usp9Cr91 to Usp25Cr75, whereas the anisotropic phases range from Usp74Cr26 to Usp90Cr10 and hence lie outside the compositional limits of the Apollo 11 samples (Usp53Cr47). Up to 4 percent by weight of Fe2O3 is inferred to be present in the spinels. A khaki spinel containing 90 percent of the ulvospinel molecule is the richest Fe2TiO4 phase yet found in nature. Chrome ulvospinels are considered to be a new series of nonisometric phases along the join Fe2TiO4-FeCr2O4. Based on coexisting Usp90 and Ilm100, estimates for the temperature and ƒO2 of crystallization do not provide a unique solution, but at 1200°C the ƒO2 is 10−11.5.

SummaryThe cell dimensions of powder samples of some Bushveld titanomagnetites and ilmenites were determined using a 114·6 mm AEG Guinier camera. The average cell dimension of the titanomagnetite is 8·400±0·005 Å and there is no definite relationship between the cell edge of the samples and their position in the layered sequence. The average cell parameters of coarse-grained ilmenite are a 5·085±0·005 Å and c 14·09±0·02 Å. Ilmenite that has replaced ulvöspinel has on average a value of some 0·003 Å greater than that of associated coarse ilmenite grains.Chemical analyses indicate that titanomagnetite from pegmatoidal plugs and from the top of layered sequence was generally subject to late-stage magmatic alteration and hence lies in the Titanomagnetite II field. In the Bushveld Complex there is an increase in the TiO2 content of titanomagnetite from some 12 % at the base to 18 % at the top of the Upper Zone. However, Bushveld titanomagnetite tends to be poorer in TiO2 than similar samples from the Skaergaard intrusion.A discrepancy between modal and normative hercynite in the Bushveld samples indicates that a considerable amount of hercynite is probably still in solid solution in the titanomagnetite.Chemical analyses of Bushveld ilmenites reveal that they lie in the same tholeiitic field as those of the Skaergaard intrusion.

The opaque minerals in rock 14310, a high-alumina fine grained basalt consist of ilmenite, and optically anisotropic chromium ulvospinel, native FeNi metal, baddeleyite, tranquillityite, schreibersite, and a Fe, Ni sulfide (pentlandite or chalcopyrrhotite?). The opaque assemblage of rock 14053, a high-Fe medium grained basalt with subophitic texture consists of ilmenite, a titanium-chromite, chromium ulvospinel, troilite, FeNi metal, fayalite with breakdown products including pure Fe, tridymite and silica glass. In rock 14310 only one spinel was found; it is a chromium ulvospinel which occurs in pyroxene and the groundmass and is similar in composition to the ulvospinels formed during late-stage crystallization of Apollo 12 rocks. Spinels encountered in 14053 are a titanium aluminium rich chromite, chromium ulvospinel and titanium picotite. Their composition is similar to spinels found in the Apollo 12 microgabbro 12018. The Fe Ni metal in 14310 contains 1.5 to 37 wt % Ni and 0.5 to 1.9 wt % Co and occurs middle to late in the paragenetic sequence. These Ni contents cannot be correlated with the mineral assemblage in which the metal phase occurs. Two coexisting iron phosphide minerals,schreibersite, one a Ni-free variety and the other containing 23 wt % Ni, were found in FeNi metal and troilite. This is the first reported occurrence of schreibersite as anindigenous mineral in the lunar rocks and casts suspicion on the criterion for meteoritic contamination used with the Apollo 11 and 12 samples. The zirconium dioxide, baddeleyite, is commonly associated with the opaque minerals, mainly ilmenite. Electron microprobe analyses indicate the presence of minor Zr in both ilmenite and ulvospinel (0.2 up to 0.57 wt % ZrO2 in ilmenite versus 0.03 to 0.19 in ulvospinel). i.e. Zr is preferentially concentrated in the ilmenite structure. The presence of schreibersite in 14310 and the late stage breakdown of fayalite to pure Fe + silica are strong evidences supporting crystallization under extremely low ƒ:02.

Compositional data for spinels in the chromite-hercynite-ulvospinel system from three large basalts and for spinels in 48 basalt particles in four soil samples show: (1) that the range of compositions are comparable to those of spinels in Apollo 12 basalts; (2) that the spinel series is complete between 0.75 FeCr2O4 0.25 FeAl2O4 and Fe2TiO4; (3) that the Apollo 12 bimodal distribution is present insofar as chromite-rich and ulvospinel-rich phases are more commonly present than compositions intermediate between these two end members; (4) well defined ionic relationships exist for those cations displaying high octahedral site preference energies in the spinel structure (viz. Cr vs Al and Ti vs Cr+ Al); (5) cations displaying low site preference energies (Fe2+, Mg) show poor correlation; and (6) the absence of a simple relationship between the divalent ions (Fe2+ and Mg), and the paucity (10% of 356 analyses) of intermediate compositions suggests a strong dependence on the structural stability of intermediate members of a normal (hercynite, chromite) — inverse (ulvospinel) solid solution series

Equations for the configurational entropy and homogeneous equilibria in Ca2MgSi2O7-Ca2SiAl2O7 melilites are derived for a site constraint that does not permit Mg on the smaller T2 tetrahedral sites. This constraint leads to one ordering parameter and one composition parameter. The maximum configurational entropy for perfectly ordered crystalline solutions is 3.795 cal K−1 mol−1, corresponding to the structural formula Ca2(Mg0.33Al0.67) (Si0.67Al0.33)2O7 and not the equimolar composition Ca2(Mg0.5Al0.5)(Si0.75Al0.25)2O7. Similarly, the configuration Ca2(Si0.33Al0.67)(Si0.33Al0.67)2O7 has the maximum entropy for the gehlenite end-member composition. The tabulated entropy of end-member gehlenite at 298.15 K must be corrected by at least 2Rln2, which corresponds to a substantial correction to its Gibbs energy at high temperature. The same corrections are applicable to other minerals having two crystallographically distinct sites in a 2∶1 ratio and where the same configurational entropy equation applies: MgFe2O4 (magnesioferrite), NiFe2O4 (trevorite), TiFe2O4 (ulvospinel), TiMg2O4, TiZn2O4, CuFe2O4, and TiFe2O5 (pseudobrookite) inasmuch as these substances prefer the same inverse ordered state as gehlenite at low temperatures. The effects of the ideal enthalpy and entropy on temperatures of homogeneous equilibria in gehlenite and MgTi2O5 are evaluated. Geological-thermometer phase diagrams of the long-range ordering parameter plotted against temperature have a sigmoidal shape for these minerals owing to the fact that this type of disordering is non-convergent.

Iron-titanium oxides in the compositional range magnetite-ulvospinel-ilmenite are likely to undergo oxidation during cooling in the presence of magmatic gases or air. The mineralogy and magnetic properties of igneous rocks depend on the initial composition of the oxide phases, the composition (particularly with regard to sulfur) and amount of gas phase, and the relative rates of cooling, unmixing, and oxidation. Tlfe oxides in an igneous rock may have a more complicated chemical and magnetic history than a deceptively simple mineralogy might lead one to suspect. Several mechanisms for self-reversal of the remanent magnetization are described. Approximate free energies of formation have been calculated for ulvospinel and maghemite. Partial pressures of oxygen have been calculated for some oxidation reactions involving ulvospinel. The equilibrium between members of the ilmenite-hematite and ulvospinel-magnetite series has been investigated.

An optical (reflected light) and electron probe study has been made of the opaque minerals in an Apollo 12 rock (12052) with particular attention to the spinels. These occur mainly as grains with cores of aluminian chromite and rims of chromian ulvospinel. Both cores (grey) and rims (pink) show zoning following the substitution (Fe, Mg)+2 +Ti+4 ⇌ 2(Cr,Al)+3, and also show an increase in Fe/Mg ratio towards their outer margins. The variations in composition taken together with those reported for Apollo 11 and other Apollo 12 samples, suggest a continuous range of solid solution between chromite and ulvospinel. Textural and chemical evidence lead to the conclusion that the aluminian chromite crystallised early along with or before pyroxene phenocrysts, and that chromium depletion led to the formation of the ulvospinel rims. Ilmenites, plagioclase and groundmass pyroxenes crystallised later followed by troilite and native iron. The ilmenites are stoichiometric FeTiO3, and the irons contain approximately 2% Ni and 1.5% Co.

The solid solutions in the system FeO-Fe2O3-TiO2 were synthesized under hydrothermal conditions up to 700°C and 2.5 kb (PH2O). The results of X-ray diffraction, chemical and thermomagnetic analyses indicated that the products always consisted of magnetite-ulvospinel or hematite-ilmenite solid solutions and, in some cases, rutile was also formed. The presence of the solid solutions of magnetite and ulvospinel indicates a reducing atmosphere during the treatment. When PH2O increased from 1 kb to 2.5 kb, the FeO contents in the products decreased.

SummaryA detailed study of ilmenite concentrates from Chavara and Manaalakurichi deposits has brought to light several interesting textures traceable to exsolution, eutectic crystallization, and replacement and alteration. Magnetite-ilmenite, magnetite-ilmenite-picotite, ilmenite-picotite, ilmenite-hematite, and ilmenite-rutile exhibit exsolution intergrowths. Part of the ilmenite seems to have been formed either by oxidation of ulvöspinel or from a cubic phase of ilmenite by monotropic inversion. Some of the ilmenite-hematite intergrowths display either an emulsoid or a triangular type of texture. Subgraphic intergrowth is observed between ilmenite-magnetite and ilmenite-rutile, suggesting an eutectic relationship between them.Magnetite-hematite, magnetite-goethite, and magnetite-maghemite intergrowths are the result of alteration of magnetite. Sphene also partly replaces some ilmenite grains.The proportion of ilmenite grains with and without intergrowths and the probable origin of the intergrowths and their effect on the marketability of ilmenite concentrates are discussed in the paper.

TWO principal compositional groups of spinels have so far been recognized in the lunar material, first the compositions that extend from ulvospinel (Fe2TiO4) to chromite (FeCr2O4) and that are described variously as chromian–ulvospinel, titanian–chromite and aluminian–chromite1 and, second, compositions that cluster midway between spinel (MgAl2O4) and hercynite (FeAl2O4). These spinels are slightly enriched in chromium and are termed chromian–pleonasts. Chromian–pleonasts are confined to the Apollo 14 samples (this study) whereas the ulvospinel–chromite series is present in Apollo 11 (ref. 1), 12 (refs. 2–13) and 14 (this study).

The ulvospinel, Fe 2 TiO 4 , contains two kinds of ferrous ions; one is in the tetrahedral site and the other in the octahedral site. The electronic structures of both ferrous ions were studied by observing temperature dependence of the Mossbauer spectra in the temperature range 800°K–4.2°K. The behavior of the spontaneous magnetization which has maximum at about 100°K was also explained qualitatively by the obtained electronic structures.

Microscope study and electron microprobe analysis of lunar rocks and soil show that ilmenite, troilite, and native iron are accompanied by trace amounts of ulvöspinel, titanochromite (new mineral name), an unidentified Ti-Fe oxide, and a complex Zr-Y silicate. The assemblage requires a strongly reducing environment. Textures and modal proportions show that the rocks present are not a differentiation series. The restricted nature of the opaque mineral assemblage suggests a narrow range of composition for the materials from which the parent liquids of the rocks were generated. Textural variety mnust reflect differences in cooling rates, probably related to depths of formation.

Spinellids showing unmixed intergrowths of chromite or chromian spinel (sensu stricto) and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks. They do occur within metamorphosed rocks that attained temperatures sufficiently high (upper amphibolite facies) for the formation of homogeneous Al-Cr-Fe3+-Ti spinel phases with compositions not matched in slowly cooled igneous rocks. In the Tugela Rand intrusion complex intergrowths of chromian spinel, chromian magnetite, ulvospinel, ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks. Compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed Tugela Rand and Mambulu Complexes confirm the extension of the magnetite-hercynite solvus (Turnock and Eugster 1962) towards magnesium- and chromium-rich compositions. The Tugela Rand spinellids are compared with those from the Carr Boyd Complex (Purvis et al. 1972) and the ultramafic rocks of the Giant Nickel Mine (Muir and Naldrett 1973) and the Red Lodge district (Loferski and Lipin 1983). Significant differences between the spinels from the Red Lodge district compared to the other three occurrences may reflect the different metamorphic histories of these areas.

The unmixing of titanomagnetite contains many problems. Experimental studies always show first an unmixing of ulvospinel, in spite of the fact that natural occurrences carry mostly only unmixing of ilmenite. Rocks of the well-known “larvikite” show in the immediate neighbourhood four or more types of unmixing: old unmixing of ilmenite followed by ulvospinel, unmixing of ulvospinel alone, breakdown to ilmenite alone, unmixing of contemporaneous ulvospinel and ilmenite, and further variability — that is in a rather acid rock.

The Fe-Ti deposits of the Suwalki District (North-East Poland) are magmatic in origin and were formed as a consequence of differentiation of magma melt that served as a parent for mineralization in the host and ore rocks. Ore mineralization in the host rocks is of accessory and segregational type and was formed in conditions characterized by low content of interprecipitable liquid, relatively high oxygen fugacity and slow, continuous cooling. Nearly all Ti was removed from titanomagnetite above the solvus; hence, the true exsolution of ulvospinel was of minor importance in the host. Ore mineralization is characterized there by low content of impurities and low Fe/Ti ratio. High content of admixtures, extreme mineralogical and compositional variations, lack of geochemical cyclicity patterns in particular sections are typical of ore rocks. Both an increased amount of interprecipitable liquid and higher temperatures, stimulated by intermediate fO2, controlled the formation of ore rocks. The fO2 here remained credibly intermediate from early to late stages of mineral evolution. The formation of magma and minerals enriched in Fe-Ti, settling in a bottom part of the magma chamber, governed the formation of large ore bodies. Some other mechanisms such as filterpressing or immiscible segregation may be responsible for the formation of some discordant and concordant smaller ore bodies in the host. The disclosed partitioning pattern of major and minor elements in ore minerals of host and ore rocks suggests that titanum-free magnetite concentrate should be not practicable.

Ilmenite, chrome-titanium spinel, ulvöspinel, troilite, native iron, iron-nickel alloy, and native copper are present. In addition mackinawite is provisionally identified. Three additional opaque phases are present but not identified. Modal analysis indicates that the breccia is enriched in iron relative to ilmenite and troilite, and the high nickel content of this iron suggests that it is largely of meteoritic origin. The bulk composition of liquids corresponding to iron/troilite droplets in the rocks indicates an oxygen fugacity no greater than 10(-15.5) and a sulfur fugacity of 10(-6) bar. Complete melting of rocks produced a glass containing complex iron/troilite globules and skeletal ilmenites.

The magnetic microstructure of a natural, finely exsolved intergrowth of submicron magnetite blocks in an ulvöspinel matrix is characterized by using off-axis electron holography in the transmission electron microscope. Single-domain and vortex states in individual blocks, as well as magnetostatic interaction fields between them, are imaged at a spatial resolution approaching the nanometer scale. The images reveal an extremely complicated magnetic structure dominated by the shapes of the blocks and magnetostatic interactions. Magnetic superstates, in which clusters of magnetite blocks act collectively to form vortex and multidomain states that have zero net magnetization, are observed directly.

Four groups of spinels have been identified in Madeira Island alkaline basalts: 1) Al-rich chromian-spinels (Cr# 31; 30

The Flemington intrusion outcrops over an area of 65 sq. km about 10 km northwest of Fifield. It is part of the Tout Complex of Silurian age; complex and extensive aeromagnetic anomalies, over 4000 gammas, occur in this region. Rock types were sampled near Flemington homestead. (Refer to Figure 1 in other paper by E.E. & C. this issue for regional location). In Table 1 are presented the results of chemical and transmitted light microscopy studies on typicsl samples. These rocks are monzonites (FIB is strictly a quartz monzonite). The sample FM5B is typical of an outcrop 750 m east of Flemington homestead; sample FIB was collected 1.4 km east of FM 5B ? it is typical of a large outcrop that lies in the centre of the intrusion 10 km north west of Fifield. The rocks have a medium grain size (+1 mm). It should be noted that there are other rock types in the intrusion ? these have not yet been sampled. Laboratory tests on the monzonite rocks (see Table 2) showed the rocks to be characterised by medium density, high susceptibility and a very pronounced susceptibility anisotropy. Natural magnetic remanence is high with Koenigsberger ratios in the range 1.6 to 65.4 indicating that induction makes only a minor contribution to the magnetic anomalies. Any quantitative interpretation and modelling of causative bodies must take into account the magnitude and direction of remanence and the effect of susceptibility anisotropy. Reflected light microscopy studies on the iron oxide minerals showed them to be mainly titaniferous magnetites with some prominent lamellae of ilmenite and exsolved lamellae of iron spinel ? hercynite. The titanomagnetites contain abundant small inclusions of hercynite and ulvospinel. The titanomagnetites constitute about 3% of the monzonites, they are randomly distributed in the form of equant, anhedral grains with an average size of 0.15 mm with some grains up to 0.5 mm. Small inclusions (few microns size) of pyrite, chalcopyrite and gold were noted in association with the non opaque (pyroxene?) phases. Figure 1 illustrates the character and type of magnetite in the monzonites.

A method is described for producing single crystals of ulvospinel from the melt. The melting is done in a depression pressed in a platinum strip which is electrically heated. The subsequent cooling of the ulvospinel through its melting point commences at a cold spot produced at the base of the depression by a sliver of platinum welded to the external surface. The whole system is enclosed in a carbon dioxide-hydrogen atmosphere whose partial oxygen pressure is in equilibrium with the ulvospinel at the melting point.

Titanomagnetites of basalts and diorites from an off-axis volcanic episode in the Pacific Ocean (Leg 61, DSDP) were studied for their ulvospinel composition and for changes in the iron-titanium ratio with progressive low-temperature oxidation. Thermomagnetic analysis and microscopy indicate that these Fe/Ti spinels show relatively little low-temperature oxidation, despite their Cretaceous age. Microprobe investigation shows that the Fe/Ti spinels have a wide range of ulvospinel content directly reflecting the range of parent rock chemistries. This is an important demonstration that ulvospinel contents are not constant but vary within a restricted range of compositions, namely at least x =0.5 to x =0.75 (Fe 2− x Ti x O 4 ). Recognition of this fact is important in attempting to elucidate changes in cation ratio as a result of progressive low-temperature oxidation. Examination of samples of different oxidation states for cation ratio changes as a result of low-temperature oxidation revealed lower titanium in the more oxidized samples and Ti enrichment in the less oxidized ones. This is probably a reflection of changing x values with progressive crystallization of individual sills, and thus unrelated to cation ratio changes with oxidation. In more than 100 grains analyzed, no evidence was found for a more oxidized shell on the exteriors of grains, suggesting that the oxidation process did not proceed uniformly with respect to grain geometry.

RECENTLY we have considered it of interest to examine several spinel oxides containing iron, using the Mossbauer effect in the isotope iron-57, in order to determine the cationic oxidation state and site distribution1. A case in which such information might be valuable is the oxide Fe2TiO4. Since the original X-ray intensity investigation of Barth and Posnjak2 this spinel has been considered to possess the inverse arrangement Fe[TiFe]O4. However, the determination of cation distribution for an oxide with cations of such similar scattering coefficients is not easy. In addition the calculated intensity value for the 135 reflexion given by those authors for the normal arrangement is in error by several orders of magnitude. The inverse arrangement is favoured for a 2–2–4 spinel on the basis of electrostatic considerations3, and the known ferrimagnetic properties of Fe2TiO4 (Curie point 120° K (ref. 4)). The normal arrangement Ti[FeFe]O4 requires Ti4+ ions on tetrahedral A sites. This is uncommon, but has been observed by Gorter5 in the material: on the basis of saturation magnetization and g-factor determinations.