MgO Ratios Research Articles

Spinels in Mid-Atlantic Ridge basalts: Chemistry and occurrence

Three groups of spinels have been identified in dredged basalts from the Mid-Atlantic Ridge in the Azores region (30–40°N): (1) magnesiochromites with 0.4–0.5 Cr/(Cr + Al) are most common and characteristic of olivine tholeiites of the region; (2) titaniferous magnesiochromites are found in an olivine basalt with alkali affinities, of local occurrence and evolved in relatively high fugacity of oxygen; (3) chromian spinels with 0.23 Cr/(Cr + Al) occur in unusual high-Al picrites of local occurrence and possible high-pressure origin. Spinels are restricted in occurrence to the least fractionated lavas, with FeO */FeO * + MgO ratio less than 0.575 and with Cr content greater than 350 ppm. A close relationship between Al content of liquidus spinel and Al content of magma has been observed for basaltic types. High-Al spinels deviating from this relationship, such as those found in picritic lavas from the Mid-Atlantic Ridge, may have crystallized at high pressure. The use of spinels as geobarometers in magmas of a restricted compositional range seems a promising prospect. There is no evidence of systematic variation in spinel chemistry of occurrence along the Mid-Atlantic Ridge, such as could be related to different mantle sources of the basalts, plume versus non-plume or binary mantle mixing.

Elemental and isotopic geochemistry of nonhydrated quartz latite glasses from the Eureka Valley Tuff, east-central California

The Eureka Valley Tuff, erupted from the late Miocene Little Walker volcanic center, east-central California, includes two major ash-flow sheets: the more voluminous, widespread, and more silicic Tollhouse Flat Member and the overlying By-Day Member. Nonhydrated quartz latitic glasses from the Tollhouse Flat and By-Day Members have, respectively, 65.5 and 63.3 wt percent SiO 2 and are extremely rich in large-ion lithophile elements. The K 2 O contents and K/Rb ratios are 5.3 wt percent and 195 for the Tollhouse Flat glass and 5.5 wt percent and 190 for glasses from the By-Day Member. FeO*/MgO (FeO* = total Fe as FeO) ratios are transitional between values characteristic of basaltic and calc-alkalic suites. Initial 87 Sr/ 86 Sr ratios of 0.7054 to 0.7055 are consistent with the pattern of northwestward decrease in 87 Sr/ 86 Sr observed for Mesozoic plutonic and Cenozoic volcanic rocks in the western Great Basin and Sierra Nevada provinces. The glasses are higher in Rb than are Mesozoic granitic rocks of the Sierra Nevada batholith that have comparable Sr contents and initial 87 Sr/ 86 Sr ratios. Pb isotopic compositions fall at the upper end of arrays found for upper Cenozoic calc-alkalic rocks from the Cascade Mountains and nearly on trends found for rocks of the Sierra Nevada batholith. The moderately high FeO*/MgO ratios, very low Co, Cr, and Ni contents, high Ba/Sr and K/Sr ratios, and approximately 25 percent negative Eu anomalies demonstrate that the glasses are the products of appreciable crystal fractionation. This in turn strongly suggests that the Eureka Valley Tuff was derived from a hypersthene-normative or only slightly quartz-normative parent magma. A model is outlined in which a potassic mafic parent magma was produced by a moderate degree of partial melting of diapirically upwelling, undepleted ultramafic mantle material followed by zone refining of upwelling mantle and other material within the asthenosphere and lithosphere. Extensive fractional crystallization and crystal separation subsequently enriched the melt further in large-ion lithophile elements. The evidence for appreciable plagioclase separation demonstrates that much fractionation took place well within the Earth9s crust. The Tollhouse Flat and By-Day Members were erupted from discrete — although closely related — bodies of magma. Lower water and halogen contents of the By-Day magma may have prevented the intratelluric crystallization of biotite and thereby allowed the magmatic liquid to become richer in K and Rb than that of the more highly fractionated Tollhouse Flat Member.

Classification, Characteristics, and Origin of Ophiolites

Tholeiitic (TH) series rocks occur in almost all types of tectonic settings, whereas calc-alkalic (CA) series rocks occur characteristically in island arcs and continental regions. Abyssal tholeiites and associated gabbros in mid-oceanic ridges show much narrower ranges of $$SiO_{2}$$ contents and FeO* (total iron as FeO)/MgO ratios than volcanic rocks in oceanic islands and island arcs. Abyssal tholeiites and oceanic island volcanics tend to show higher $$TiO_{2}$$ contents than island arc volcanics. These are among the diagnostic features of volcanic rock series. In terms of volcanic rock series, the ophiolitic complexes may be classified into a number of distinct classes, three of which are discussed here. The ophiolitic complexes of Class I are characterized by the presence of volcanic rocks of both CA and TH series. A typical example is the Troodos massif in Cyprus, which was formed probably in an island arc. The ophiolitic complexes of Class II are characterized by the presence of TH series volcanics. Some of them were formed probably in island arcs and others in mid-oceanic ridges. The ophiolitic complexes of Class III are characterized by the presence of TH and alkalic series volcanics. Examples are ophiolites in high-pressure (glaucophanitic) metamorphic terranes. The volcanic rocks in such ophiolites show a marked chemical resemblance to the volcanics in some hot spot islands as well as to some volcanic rocks on stable continents. A few more classes may exist.

Chemical and isotopic constraints on the origin of low-silica latite and andesite from the Andes of central Peru

Low-silica latite highly enriched in large-ion-lithophile elements and moderately potassic low-Si andesite were erupted in central Peru during late Cenozoic time. FeO*/MgO ratios of 0.93 to 1.25 at 53 wt percent SiO/sub 2/ indicate a definitely calcalkalic character. The combination of low FeO*/MgO ratios, low SiO/sub 2/, and high Cr, Co, and Ni with large-ion-lithophile and light rare-earth elements makes it very unlikely that the parent magmas were produced by high-pressure partial melting of subducted ocean-floor basalt (eclogite). The data are more compatible with small degrees of partial melting of ultramafic material or mixtures of basalt and ultramafic mantle. The initial melts probably contained 52 to 53 percent SiO/sub 2/ and had a content of large-ion-lithophile elements nearly as high as that of the rocks. /sup 87/Sr//sup 86/Sr ratios of from 0.7042 to 0.7051 and low to very low Rb/Sr ratios indicate an isotopically variable source region that, at least in part, had earlier been depleted in Rb relative to Sr.

Lavas from Niuafo'ou Island, Tonga, Resemble Ocean–Floor Basalts

Major element, trace element, and mineral analyses of rocks from the island of Niuafo9ou in the western Pacific Ocean show that Niuafo9ou rocks have a marked similarity to ocean-floor basalt, especially with basalt of relatively high FeO*/MgO ratios.

Phase Relation in the System MgO-B<sub>2</sub>O<sub>3</sub> and Effects of Boric Oxide on Grain Growth of Magnesia

An extremely pure MgO as one of the starting materials was prepared by decomposing magnesium nitrate solution of 99.9% magnesium. No boric compound in magnesia obtained was recognized by the emission spectral analysis. Many mixtures of magnesium nitrate solution and boric acid corresponding to the various mixing ratio of MgO and B2O3 were preheated at around 600°C for 10hrs and then heat-treated at 910°, 1050°, 1300° 1500°, 1600°, 1700° and 1750°±5°C for 24 to 72hrs.The lattice constant of MgO and the crystallite size of MgO were measured with the internal standard of 99.999% Si and phases presented in the heat-treated specimens were identified with a X-ray diffractometer using the radiation of Cu Kα.Particle size of periclase was observed with a scanning electron microscope. The shrinkage, apparent porosity of magnesia body after heat-treatment and evaporation of B2O3 were also measured for evaluating the effects of the addition of B2O3 on the physical properties of sintered magnesia.The compound MgO⋅B2O3 was not detected, but the compounds 3MgO⋅B2O3, 2MgO⋅B2O3 and MgO⋅2B2O3 were recognized. It was confirmed that there was no solid solubility of B2O3 in MgO up to the temperature of 1750°C, and the grain growth of periclase was extremely accelerated by the addition of B2O3 particularly at the heat treatments of 1500°C. The shrinkage of magnesia body was maximum with the addition of B2O3, 2 to 3%, and decreased rapidly with the associate of bubbling of B2O3 when the temperature was higher than 1500°C in specimens containing more than 5wt% B2O3. Apparent porosity of magnesia body after heat-treatment at 1750°C for 24hrs was minimum with the addition of 0.1wt% B2O3 to be about 8%. The porosity increased with the increase in the addition of B2O3 for the sintered bodies containing higher than 0.1wt% B2O3.

Sierra Blanca Igneous Complex, New Mexico

The Sierra Blanca igneous complex of south-central New Mexico consists of the Oligocene Sierra Blanca Volcanics which have been intruded by Oligocene monzonite to quartz syenite stocks, sills, and dikes. The Sierra Blanca Volcanics are composed of four formations: Walker Andesite Breccia, Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte. The Walker Andesite Breccia comprises the major volume of the eruptive rocks. It is characterized by monolithic blocks transported and frozen in a matrix of plagioclase microlites, enstatite, hornblende, and hematite. A few K-Ar dates indicate that volcanism began approximately 35 m.y. ago and continued until about 25 m.y. ago. The Walker Andesite Breccia is succeeded by flows, tuff layers, and welded ash flows of the Nogal Peak Trachyte, Church Mountain Latite, and Godfrey Hills Trachyte, which are preserved as widely separated remnants. The younger volcanic formations give evidence of a more explosive period of volcanism compared with the Walker Andesite Breccia stage. Four hypabyssal stocks intrude the Sierra Blanca Volcanics: Rialto stock, Chaves Mountain stock, Bonito Lake stock, and Three Rivers stock. Three are multiple intrusives. Alkali enrichment is pronounced in one of the stocks. K-Ar dates of the stock range from 34.4 m.y. to 25.8 m.y. The volcanic and intrusive rocks show progressive mineralogical and chemical changes typical of alkali-calcic series. The oldest volcanic rocks contain zoned plagioclase (An 77 to An 47 ) and enstatite, but in successive volcanic rocks plagioclase becomes more sodic (An 32 ) and hornblende becomes the dominant ferromagnesian mineral. FeO(total Fe)/MgO ratios increased from 1.45 to >5 during magma differentiation. Fractional crystallization with removal of calcic plagioclase and enstatite during volcanism appears to have been responsible for differentiation and generation of the late-stage silicic intrusive phases. The large volume of andesitic to trachytic volcanic rocks and intrusive syenitic rocks along with the lack of cogenetic basalt suggest that the parent magma at Sierra Blanca was andesitic. Major structural planes of weakness, which intersect in the Sierra Blanca area, would allow magma generated in the mantle to move into the crust and erupt on the surface. Similar controls appear to have been operational along the Rocky Mountain trend and parallel to the Rio Grande depression. Other andesite breccia sheets present along the trend include the Absaroka volcanic field, Wyoming, the Thirtynine Mile volcanic field, Colorado, and the Red River volcanic-intrusive complex, New Mexico. They are Oligocene in age and have many similar characteristics.

Water Attack on Na2O—MgO—SiO2Glasses in Relation to their Structure

Glasses in the system Na2O—MgO—SiO2 containing various amounts of Na2O and MgO were subjected to the action of water and the amounts of the constituents extracted were analysed chemically. The effect of MgO in increasing the resistance towards water attack was strong only in glasses having 20 mole per cent or more of Na2O.The molar ratio SiO2: Na2O in the extract was not the same as in the glass. Generally it increased with an increase of the SiO2: Na2O ratio in the glass composition. The molar ratio of MgO to SiO2 in the extract decreased with increasing value of the same in the glass and was nil at about 20 to 31 mole per cent of alkali in the glass.Values of the equilibrium pH were determined for all the glasses. At a constant Na2O content, the pH values passed through a minimum with the replacement of SiO2 by MgO. The results were interpreted in relation to the oxygen activity of the glass. Equilibrium pH also appeared to have linear relationship with logarithm of alkali extractions, with slopes varyi...

Density Distribution in Earth

Earth models selected by a Monte Carlo procedure were tested against geophysical data; 5 million models were examined and six have passed all tests. Common features of successful models are an increased core radius and a chemically inhomogeneous core consistent with Fe-Ni alloy (20 to 50 percent Fe) for the solid portion and Fe-Si alloy (15 to 25 percent Fe) for the fluid core. The inhomogeneous mantle is consistent with an increase in the FeO:FeO + MgO ratio by a factor of 2 in the deep mantle. The transition zone is a region of not only phase change but also composition change; this condition would inhibit mantlewide convection. The upper-mantle solutions show large fluctuations in density; this state implies insufficient constraint on solutions for this region, or lateral variations in mantle composition ranging from pyrolite to eclogite.

Highly refractory' material based on magnesia spinel

An increase in the content of commercial alumina in magnesite refractory body to a ratio of MgO∶Al2O3 =1.3, corresponds to an increase in the refractoriness-under-load and 4% compression, increases the spalling resistance with a certain reduction in refractoriness.

Olivine composition in chondrites

Olivine is almost omnipresent in chondrites, being absent only in the enstatite chondrites and a few carbonaceous chondrites. In most chondrites it is the dominant mineral. Since it shows a considerable range of composition, which is directly related to the FeO MgO ratio in the individual meteorite, and since its composition is readily determined either by X-ray diffraction or by refractive index measurement, its composition provides an excellent criterion for the classification of chondrites. This paper reports the olivine composition in about 800 of the approximately one thousand chondrites known; of the chondrites examined, 14 are enstatite chondrites, 364 are olivine-bronzite chondrites, 427 are olivine-hypersthene chondrites, 12 are olivine-pigeonite chondrites, and 17 are carbonaceous chondrites. The significance of these data for theories of the origin of chondrites is briefly discussed, and it is suggested that the different types of chondrites may represent samples from different regions of the pre-planetary solar system.

Effect of Kaolin Addition on Properties of Forsterite Porcelain

The stability of the forsterite porcelian in hydrogen at high temperatures, which is required in the application of the forsterite porcelain to a high frequency insulator, is governed by additives used to promote sintering. Among several additives examined kaolin has the effect to make the porcelain stable in hydrogen at high temperatures, so giving it favorable properties for Mo-Mn metallizing processed under such conditions.In this paper 0-10% of kaolin was added to forsterite porcelains which were prepared from Fukushima siliceous stone and sea-water magnesia in the molar ratio of MgO: SiO2=2:1.By the addition of kaolin the temperature range for sintering was remarkably extended and the firing shrinkage was almost constant within this range. The coefficient of linear thermal expansion decreased with the increase of added kaolin, and no considerable change was found by repeated heat treatments. With the increase of kaolin content from 0 to 10%, the softening temperature under load lowered to 1160°C from 1280°C. The electric resistance at elevated temperatures lowered gradually, but was still sufficiently high for practical use. The flexural strength decreased and the modulus of elasticity lowered gradually in the temperature range from room temperature to 300°C with the increase of kaolin content. The modulus of elasticity, however, became larger with the rise of temperature in this temperature range.Within 5% of kaolin added, the dielectric constant and dielectric loss were unvaried and these values were excellent for the high frequency insulator.The result of X-ray diffraction gave only the pattern of the forsterite. By microscopic observation, forsterite crystal grain changed from allotriomorphic into idiomorphic and glass matrix increased with the kaolin content.It is concluded that the addition of 2.5-5.0% of kaolin was most suitable for preparing the forsterite porcelain for this purpose.

Almandine in Thermal Aureoles

The concept that almandine-rich garnet is an anomalous phase in thermal metamorphic assemblages is based partly upon the rarity of almandine in hornfelses, and partly upon evidence of breakdown of ‘regionally’ formed garnets when their host-rocks have been thermally metamorphosed. Where amphibolite facies regional gneisses have been re-metamorphosed within the pyroxene-hornfels facies conditions of the late Caledonian Lochnagar aureole, garnet of composition Alm80Py11p6 (Gr+And)4 has reacted with biotite, sillimanite, and muscovite to give cordierite-rich pseudomorphs; where armoured from reaction by immersion in quartz or feldspar, no signs of dissolution are seen. In some totally reconstituted hornfelses, almandine garnet of composition AIm80P13Sp4 (Gr+And)3 appears to have coexisted stably with cordierite and orthoclase. It is concluded that the ‘unstable’ garnet was breaking down not because its P/T stability field had been exceeded, but because the alman-dine-bearing rock-composition field in the thermal (pyroxene-hornfels) facies was more restricted than that in the regional (amphibolite) facies. Chemical data from the hornfelses suggest that this restriction was mainly due to the stability of cordierite—the plane spinel-cordierite-quartz restricts garnet to those rocks with effective mol. (FeO+MgO-t-MnO) A12O3>1, while within that range cordierite-biotite tie-lines restrict garnet to rocks of high (FeO+MnO)/MgO ratio (Fig. 4b). Recorded instances of garnet ‘instability’ in thermal aureoles show similar features to the Lochnagar aureole—garnet breaks down by reaction; unequivocal instances of isochemical breakdown are rare. This, combined with the widespread occurrence of almandine in volcanic and plutonic igneous rocks, suggests that almandine is a physically stable phase not only in the hornblende-hornfels facies, but also in the pyroxene-hornfels facies and possibly in portion of the sanidinite facies as well. The rarity of almandine in thermal aureoles is the result of its very narrow rock composition field under the P/T conditions of such environments. Comparison of thermal and granulite facies garnet-cordierite assemblages suggests that P and T modify the almandine-bearing rock composition field mainly by modifying the limiting Mg/Fe ratio of garnet and the limiting Fe/Mg ratio of cordierite (Fig. 10). The wide rock-composition field of almandine in the amphibolite facies may be contingent upon the inhibition of cordierite by hydrous minerals under relatively high partial pressures of water.

Sapphire-bearing rocks from MacRobertson Land, Antarctica

SummaryA small isolated outcrop in a large area of charnockites in the Mawson area, MacRobertson Land, Antarctica, consists of enstatite-sapphirine-cordierite rocks. Other highly magnesian outcrops occur in the district and these rocks may be completely recrystallized xenoliths. Analyses of the country rock, the two sapphirine-bearing rocks, and the sapphirine are given; the country rock is very rich in iron and has a high FeO : MgO ratio, while the sapphirine-bearing rocks are very low in iron and high in MgO, approximating to ternary mixtures of MgO, Al2O3, and SiO2.

Solid Solution and Chromium Oxide Loss in Part of the System MgO‐Al2O3‐Cr2O3‐SiO2

Thermal and X‐ray studies show that there is complete solid solution between MgO.Cr2O3 and MgO.Al2O3 and that the spinel solid solutions are stable with no exsolution down to temperatures as low as 510°C. There is no solid solution of excess Cr2O3 in MgO.Cr2O3 nor of MgO.Cr2O3 in Cr2O3. The join MgO.Cr2O3–Al2O3 is found to be nonbinary; compositions along that join yield mixtures of a chromium oxide‐alumina solid solution and a spinel solid solution on firing to temperatures high enough to promote solid‐state reaction. Chromium oxide loss by volatilization increases at higher temperature. At a given temperature, chromium oxide loss is found to vary directly with the partial pressure of oxygen in the furnace atmosphere and with the ratio of MgO to SiO2 in the charges heated.

Optically positive cordierite from the khondalites of Pachipenta, Vizagapatam District

An optically positive cordierite from the khondalites in Pachipenta of Vizagapatam District was chemically analysed and its optical properties determined. It has a hardness of 7·5 and Sp. Gr. 2·71 and its 2V as determined by Fedorov’s Stage is 74°. In contrast to the negative optical sign of a normal cordierite, this cordierite gives a positive optical sign. The tentative suggestion put forward by Krishnan and others that the optically positive character of cordierites is probably due to the ratio of MgO to FeO does not find support from the present studies. As shown by Winchell it appears that there is no close correspondence between chemical composition and optical characters in cordierites. The possibilities of traces of alkalies contributing to the change in optical sign as suggested by Folinsbee, is worth consideration but must await the availability of analyses giving these constituents.

BRUCITE DEPOSITS IN THE RUTHERGLEN DISTRICT, ONTARIO

Some deposits of brucitic limestone with a ratio of MgO: CaO higher than that of dolomite are described. Evidence is given to show that the excess magnesia derived from dissociation of dolomite by the heat of the adjacent granite. Selective transfer of the products of dissociation into the carbonate rocks is attributed to igneous emanations. The bulk of the brucite is believed to have been deposited by direct replacement of carbonates, rather than by hydration of previously formed periclase, as has been suggested for similar occurrences elsewhere.

Preliminary chemical correlation between chromites and the containing rocks

It is proposed that chemical analyses of chromites be expressed in terms of normative chromite, (Mg, Fe)Cr204 , spinel, (Mg, Pe)Al204, and magnetite, Fe3O4, with the ratio of MgO to total FeO, thus: Cr60Al35Mt5 (Mg61Fe39). Since the magnetite and FeO can be readily calculated by difference, the formula in most instances can be shortened to Cr60Al35(Mg61). All spinel analyses must be reduced to this form for plotting on the triangular prism of composition described by Stevens, and for ready comparison with the silicates associated with them.

Sediments of Great Salt Lake, Utah

Sediments of Great Salt Lake, Utah