MgO Ratios Research Articles

Evaluation of Microwave Dielectric Properties of MgO-TiO2System by Dielectric Mixing Rules

Effects of compositions on the microwave dielectric properties of the MgO-TiO₂ system were investigated as a function of the molar ratio of MgO to TiO₂ (0.9≤MgO/TiO₂ (x)≤1.2). With the compositional changes, secondary phases of MgTi₂O? and Mg₂TiO₄ were also detected along with MgTiO₃. Microwave dielectric properties of the specimens were dependent on the types of phases developed in the sintered specimens. A single phase of MgTiO₃ showed a dielectric constant (K) of 18.2, a quality factor (Qf) of 198,000 ㎓, and a temperature coefficient of resonant frequency (TCF) of ?51 ppm/℃. However, the dielectric properties of the specimens with a secondary phase of MgTi₂O? (K=19.9, Qf=48,000 ㎓) and/or Mg₂TiO₄ (K=15.6, Qf=56,000 ㎓) were worsened. Dependence of the microwave dielectric properties on the secondary phase of the MgO-TiO₂ system was also discussed in terms of dielectric mixing rules.

Analysis of ordinary chondrites using powder X-ray diffraction: 2. Applications to ordinary chondrite parent-body processes

Abstract– We evaluate the chemical and physical conditions of metamorphism in ordinary chondrite parent bodies using X-ray diffraction (XRD)-measured modal mineral abundances and geochemical analyses of 48 type 4–6 ordinary chondrites. Several observations indicate that oxidation may have occurred during progressive metamorphism of equilibrated chondrites, including systematic changes with petrologic type in XRD-derived olivine and low-Ca pyroxene abundances, increasing ratios of MgO/(MgO+FeO) in olivine and pyroxene, mean Ni/Fe and Co/Fe ratios in bulk metal with increasing metamorphic grade, and linear Fe addition trends in molar Fe/Mn and Fe/Mg plots. An aqueous fluid, likely incorporated as hydrous silicates and distributed homogeneously throughout the parent body, was responsible for oxidation. Based on mass balance calculations, a minimum of 0.3–0.4 wt% H2O reacted with metal to produce oxidized Fe. Prior to oxidation the parent body underwent a period of reduction, as evidenced by the unequilibrated chondrites. Unlike olivine and pyroxene, average plagioclase abundances do not show any systematic changes with increasing petrologic type. Based on this observation and a comparison of modal and normative plagioclase abundances, we suggest that plagioclase completely crystallized from glass by type 4 temperature conditions in the H and L chondrites and by type 5 in the LL chondrites. Because the validity of using the plagioclase thermometer to determine peak temperatures rests on the assumption that plagioclase continued to crystallize through type 6 conditions, we suggest that temperatures calculated using pyroxene goethermometry provide more accurate estimates of the peak temperatures reached in ordinary chondrite parent bodies.

A study on the effects of Fe x/Ni y/MgO (1− x− y) catalysts on the volumetric and electrochemical hydrogen storage of multi-walled carbon nanotubes

The effects of various ratios of Fe/Ni/MgO and growth temperatures on yield, diameter and quality of multi-walled carbon nanotubes (MWCNTs) were studied. Thermal gravimetric analysis (TGA) confirmed that the MWCNT yield depends on Fe/Ni ratio with the following order; Fe 0.5 Ni 0.5 > Fe > Fe 0.75 Ni 0.25 > Fe 0.25 Ni 0.75 > Ni. The results indicated that there is an optimum temperature (940 °C) for the MWCNT growth both from quality and quantity (yield) aspects as compared to other temperatures. Moreover, the changes on Fe/Ni to MgO ratio for the MWCNT growth revealed that Fe/Ni/MgO with the ratio of 17.5/17.5/65 had the highest quality and surface area as compared to the other ratios. The hydrogen storage capacities of MWCNTs grown on Fe/Ni/MgO with various ratios obtained by using volumetric technique were in ascending order as 17.5/17.5/65, 15/15/70, 12.5/12.5/75, 10/10/80, 20/20/60, 22.5/22.5/55 and 25/25/50. In addition, the defective sites and mean diameter of the MWCNTs influenced the desorption temperature of stored hydrogen. Hydrogen storage by using electrochemical technique showed that Fe/Ni/MgO with the ratio of 17.5/17.5/65 had the highest hydrogen storage capacity compared with the other ratios. Based on electrochemical analysis, there are two regimes for hydrogen adsorption on the MWCNTs, one at about 0.8 V and the other at 0.15 V. The study on two kinds of adsorption region showed that the ratio of hydrogen storage capacity at 0.8 V to hydrogen storage capacity at 0.15 V increased with the increasing of the mean diameter of MWCNTs. The ratio reached to maximum value for the MWCNTs grown on Fe/Ni/MgO with the ratio of 20/20/60 as compared to the other ratios.

Physical and Microwave Dielectric Properties of the MgO-SiO2 System

Unreported dielectrics based on the binary system of MgO-SiO<TEX>$_2$</TEX> were investigated as potential candidates for microwave dielectric applications, particularly those demanding a high fired density and high quality factors. Extensive dielectric compositions having different molar ratios of MgO to SiO<TEX>$_2$</TEX>, such as 2:1, 3:1, 4:1, and 5:1, were prepared by conventional solid state reactions between MgO and SiO<TEX>$_2$</TEX>. 1 mol% of V<TEX>$_2$</TEX>O<TEX>$_5$</TEX> was added to aid sintering for improved densification. The dielectric compositions were found to consist of two distinguishable phases of Mg<TEX>$_2$</TEX>SiO<TEX>$_4$</TEX> and MgO beyond the 2:1 compositional ratio, which determined the final physical and dielectric properties of the corresponding composite samples. The increase of the ratio of MgO to SiO<TEX>$_2$</TEX> tended to improve fired density and quality factor (Q) without increasing grain size. As a promising composition, the 5MgO.SiO<TEX>$_2$</TEX> sample sintered at 1400 <TEX>$^{\circ}C$</TEX> exhibited a low dielectric constant of 7.9 and a high Q <TEX>$\times$</TEX> f (frequency) value of <TEX>$\sim$</TEX>99,600 at 13.7 GHz.

Steam reforming of biomass tar producing H 2-rich gases over Ni/MgO x/CaO 1−x catalyst

Series nickel catalysts Ni/MgO x /CaO 1− x ( x = 0.3, 0.5, 0.7, Ni: 5 wt%) were prepared and tested in fixed-bed reactor for biomass tar steam reforming, toluene as tar destruction model compound. Different ratios of MgO and CaO were mixed to simulate dolomite as Ni support. Two preparation methods: solid mixing with (SMW) and without water (SM) were used, the preparation methods and concentration of MgO had an important influence on toluene conversion and products. Catalysts prepared by SM method exhibited higher performance on toluene conversion, resulted in higher H 2 yield, and also, higher CO 2 and lower CO selectivity with higher temperature. For the same preparation method, higher concentration of MgO resulted in higher toluene conversion, and also influence on CO, CO 2 selectivity, but no obvious influence on the H 2 yield. Catalysts were characterized by BET, X-ray diffraction (XRD), SEM.

SYNTHESIS OF NANO-SIZE MgO POWDER BY CHEMICAL DEPOSITION OF LOW COST RAW MATERIALS

Nanostructured MgO powders have been synthesized using chemical methods. Ammonium oxalate and Magnesium sulphate were used as the precursor materials. The weight ratios of the raw materials (ammonium oxalate/Magnesium sulphate) were 0.9, 1, 1.1, 1.2, 1.3, and 1.4. As a result of chemical reaction (between them), Magnesium oxalate was synthesized. Produced samples were analyzed by XRD and SEM. The results show that the best ratio (for ammonium oxalate/Magnesium sulphate) is 1.4. Produced Magnesium oxalate powder was heated at 450 and 550°C. The final product was MgO nanopowder. XRD studies indicate that the highest ratio of MgO was observed in the specimen heated at 450°C.

Hydrothermal Synthesis Nano Mg-Al LDHs and its Formation Mechanism

Nano Mg-Al LDHs were prepared by a hydrothermal method in one step.Effects of pre- cursors,alkalinity of the starting solution,temperature and time on phase purity and crystallite sizes of Mg-Al LDHs were systematically investigated.Reaction products were characterized by powder XRD and SEM.Crystallite sizes of Mg-Al LDHs were determined by the X-ray whole powder pat- tern fitting method.The optimal parameters for preparing Mg-Al LDHs are keeping the ratio of MgO:Al(OH)_3:Na_2CO_3=2:1:1 at 140℃for 4h to 10h.The formation mechanism proposed for Mg- A1 LDHs is dissociation-precipitiation-crystal growth.The reactant type and hydrothermal conditions may influence the dissociation rate of the precursors,nucleation and growth rates of Mg-Al LDHs,as well as phase combination and crystallite sizes of the reaction products.

In vitro reactivity of Na 2O–MgO–SiO 2 glasses

A variety of bioactive glasses have been investigated over the last two decades as substitute material for diseased or damaged tissues in a human body. In this investigation, three different melt derived bioactive glasses, each having 55% by mole SiO 2 and ratio of MgO to Na 2O varying from 1:8 to 8:1, were prepared by melting various oxides at temperature >1250 °C. After microstructure evolution, vitro reactivity of these glasses was examined by keeping them in simulated body fluid (trans buffered pH 7.25 at 25 cc). The surface reactivity of these glasses gradually increased with increasing Na 2O/MgO ratio.

Hydrothermal formation and characterization of magnesium hydroxide chloride hydrate nanowires

Magnesium hydroxide chloride hydrate (MHCH), Mg 2(OH) 3Cl·4H 2O, nanowires have been synthesized via a facile hydrothermal approach using magnesium oxide and magnesium chloride as reactants. A series of experiments were conducted to define the key parameters, which determine the shape and phase of the nanowires. It was found that the concentration of MgCl 2, and the hydrothermal treatment conditions (temperature and time) are important for the control of the morphology evolution of Mg 2(OH) 3Cl·4H 2O nanocrystals: at lower MgCl 2 concentrations and lower temperatures (from room temperature to 100 °C), MHCH are prone to display needle-like morphology. With the further increase of MgCl 2 concentration and hydrothermal temperatures, the needle-like crystallites become the preferred nanowires morphology, with average diameter of about 20 nm. By carefully adjusting the molar ratio of MgO to MgCl 2, the other two types of MHCH, i.e., Mg 3(OH) 5Cl·4H 2O- and Mg 10(OH) 18Cl 2·5H 2O-phased nanowires can also be controlled synthesized. Our capability of obtaining MHCH nanowires not only benefits the wider applications based on MHCH as a brand new class of nanofiller materials in polymer composite engineering with very high aspect ratio, but also it serves a promising basis for systemically studying magnesium hydroxide carbonate hydrates and magnesium hydroxide sulfate hydrates nanostructures.

Shrimp U–Pb zircon geochronology, geochemistry, and Nd–Sr isotopic study of contrasting granites in the Emeishan large igneous province, SW China

The Cida A-type granitic stock (∼ 4 km 2) and Ailanghe I-type granite batholith (∼ 100 km 2) in the Pan-Xi (Panzhihua-Xichang) area, SW China, are two important examples of granites formed during an episode of magmatism associated with the Permian Emeishan mantle plume activity. This is a classic setting of plume-related, anorogenic magmatism exhibiting the typical association of mantle-derived mafic and alkaline rocks along with silicic units. SHRIMP zircon U–Pb data reveal that the Cida granitic pluton (261 ± 4 Ma) was emplaced shortly before the Ailanghe granites (251 ± 6 Ma). The Cida granitoids display mineralogical and geochemical characteristics of A-type granites including high FeO ⁎/MgO ratios, elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios, which are much higher than those of the Ailanghe granites. All the granitic rocks show significant negative Eu anomalies and demonstrate the characteristic negative anomalies in Ba, Sr, and Ti in the spidergrams. It can be concluded that the Cida granitic rocks are highly fractionated A-type granitoids whereas the Ailanghe granitic rocks belong to highly evolved I-type granites. The Cida granitoids and enclaves have Nd and Sr isotopic initial ratios ( ε Nd( t) = − 0.25 to + 1.35 and ( 87Sr/ 86Sr) i = 0.7023 to 0.7053) close to those of the associated mafic intrusions and Emeishan basalts, indicating the involvement of a major mantle plume component. The Ailanghe granites exhibit prominent negative Nb and Ta anomalies and weakly positive Pb anomalies in the spidergram and have nonradiogenic ε Nd( t) ratios (− 6.34 to − 6.26) and high ( 87Sr/ 86Sr) i values (0.7102 to 0.7111), which indicate a significant contribution from crustal material. These observations combined with geochemical modeling suggest that the Cida A-type granitoids were produced by extensive fractional crystallization from basaltic parental magmas. In contrast, the Ailanghe I-type granites most probably originated by partial melting of the mid-upper crustal, metasedimentary–metavolcanic rocks from the Paleo-Mesoproterozoic Huili group and newly underplated basaltic rocks. In the present study, it is proposed that petrogenetic distinctions between A-type and I-type granites may not be as clear-cut as previously supposed, and that many compositional and genetically different granites of the A- and I-types can be produced in the plume-related setting. Their ultimate nature depends more importantly on the type and proportion of mantle and crustal material involved and melting conditions. Significant melt production and possible underplating and/or intrusion into the lower crust, may play an important role in generating the juvenile mafic lower crust (average 20 km) in the central part of the Emeishan mantle plume.

Finger prints of the formation of geothermal springs on the granitoids: Beypazarı–Ankara, Turkey

The Beypazari granitoid in central Anatolia was emplaced during the Late Cretaceous and it is partly covered with Early Miocene–Quaternary sedimentary and volcanic units. Compression and tension regimes of the Beypazari granitoid emplacing along the Sakarya River were evaluated considering the kinematical characteristics of discontinuities on granitoids, petrographic/geochemical works, formation of geothermal spring and irregularities in topographic structure. It was determined that the Beypazari granitoid was witnessed with NE–SW compressional stress and NW–SE extensional stress which resulted in the formation of the Kapullu normal fault with N55–72°E, 78°SE crossing the Kapullu spring site. There are also several other probable normal faults around this fault in the Sakarya River basin. The results of the geochemical studies reveal that SiO2, Na2O, Al2O3, CaO and K2O as a major oxide and As, Rb, Sr, Th and U as trace elements are decreased away from the Kapullu bath spring. On the other hand, the ratio of Fe2O3 and MgO as major oxide and Y as a trace element with the lost on ignition (LOI) are increased away from the Kapullu fault. Regard as the regional plate tectonism, there is a consistency between the direction of compression in the Beypazari granitoid and that developed by the southwesterly movement of the Anatolian plate.

Environmentally Conscious Gas Mixtures for Magnesium Melt Protection

The candidate gases for the replacement of SF6 in magnesium melt protection were reviewed from the viewpoint of environmental effects, melt protection properties, and chemical properties. The surface films formed under fluorine bearing gases were analyzed by using XPS. The XPS results revealed that the surface films consist of MgO and MgF2. The fluorine content of the surface film increases with increasing gas concentration and exposure time. The Pilling- Bedworth ratio (PBR) of MgO was 0.73, and the porous magnesium oxide could not cover the whole melt surface. PBR of MgF2 was 1.29, and the surface film which contains MgF2 could cover a lager area of the melt surface. This higher PBR of composite surface film prevented further oxidation and evaporation of magnesium.

紀伊半島大峯花こう岩質岩から見いだされたＭｇに富む火成包有物

Magnesian igneous enclaves showing sub-angular shape and up to 2 m in the maximum dimension were found from granodiorite in the Dorogawa pluton, the northernmost pluton of the Ohmine granitic rocks in Kii peninsula, southwest Japan. Igneous enclaves are composed mainly of plagioclase and hornblende, as phenocrysts, and of plagioclase, hornblende, quartz, and biotite as groundmass minerals. They also contain hydrous mafic mineral aggregates consisting of chlorite, actinolite, and biotite with rare chromian spinel microcrystals. Their whole rock compositions were characterized by high MgO content (5.8-7.0 wt%), low FeO*/MgO ratio (0.80-0.92), high Cr (256-346 ppm), and Ni (81-138 ppm) contents. Patterns of trace element abundances are close to those of the high-Mg andesite (HMA) of the Setouchi volcanic rocks. The compositional range of microcrystals of chromian spinel within hydrous mafic mineral aggregates is similar to those of included in olivine or orthopyroxene phenocrysts of the Setouchi HMA. These geochemical and mineralogical characteristics of the igneous enclaves suggest the activity of HMA magma closely in space-time with the intrusion of the Dorogawa pluton.

A study on usability of magnesium oxide with titanium dioxide in PVC door and window profiles

In this study, the usability of magnesium oxide with titanium dioxide used in the manufacture of polyvinyl chloride door and window profiles is investigated in terms of discoloration or fading and some mechanical properties. By each compound associated with the different amount of MgO and TiO 2, five different compounds are used to produce PVC profiles of doors and windows. The most suitable profile in terms of discoloration is obtained from the compound containing MgO and TiO 2 with the ratio of MgO to TiO 2 of 1/3. It is observed that the addition of MgO into a compound dramatically decreases the elongation of material and thus, it improves the hardness of PVC-based material. It is concluded that the profiles fabricated from the compounds with the ratios of MgO to TiO 2 of 1/2 and 1/1 do not meet the standard in terms of brittleness obtained from the impact strength testing in low temperature (−10 °C). It is also concluded that although the brittleness of some profile associated with the ratios of MgO to TiO 2 of 1/2 and 1/1 is not in standard values, the addition of the certain amount of MgO into the compound provides some improvements on the discoloration of PVC-based door and window profiles. The obtained results are evaluated by considering the previous studies and thus, they qualitatively agree with the previous studies.

An Experimental Investigation of the Influence of Water and Oxygen Fugacity on Differentiation of MORB at 200 MPa

Crystallization experiments were performed at 200 MPa in the temperature range 1150–950°C at oxygen fugacities corresponding to the quartz–fayalite–magnetite (QFM) and MnO–Mn3O4 buffers to assess the role of water and fO2 on phase relations and differentiation trends in mid-ocean ridge basalt (MORB) systems. Starting from a primitive (MgO 9·8 wt %) and an evolved MORB (MgO 6·49 wt %), crystallization paths with four different water contents (0·35–4·7 wt % H2O) have been investigated. In primitive MORB, olivine is the liquidus phase followed by plagioclase + clinopyroxene. Amphibole is present only at water-saturated conditions below 1000°C, but not all fluid-saturated runs contain amphibole. Magnetite and orthopyroxene are not stable at low fO2 (QFM buffer). Residual liquids obtained at low fO2 show a tholeiitic differentiation trend. The crystallization of magnetite at high fO2 (MnO–Mn3O4 buffer) results in a decrease of melt FeO*/MgO ratio, causing a calc-alkaline differentiation trend. Because the magnetite crystallization temperature is nearly independent of the H2O content, in contrast to silicate minerals, the calc-alkaline differentiation trend is more pronounced at high water contents. Residual melts at 950°C in a primitive MORB system have compositions approaching those of oceanic plagiogranites in terms of SiO2 and K2O, but have Ca/Na ratios and FeO* contents that are too high compared with the natural rocks, implying that fractionation processes are necessary to reach typical compositions of natural oceanic plagiogranites.

Cenozoic lithospheric extension induced magmatism in Southwest Japan

Island chains off western Kyushu are the surface exposure in the northern margin of the Taiwan–Sinzi Folded Zone that spreads along the arc–trench system in the back-arc side from SW Japan to Taiwan. Intermittent igneous activity between the Middle Miocene and Holocene occurred on these islands and widely covered or intruded sedimentary rocks of Early–Middle Miocene. Geochemistry of the volcanic rocks from the Hirado, Ikitsuki and Takushima islands believed to relate to the back-arc opening along the East China and Japan Seas shows a temporal change in source material. Submarine to sub-aerial volcanism occurred on Hirado Island at 15 Ma during the final opening stage of the East China Sea producing tholeiitic basalt and associated andesite–dacite. These eruptives show low incompatible element contents and high FeO*/MgO ratios and reflect a tholeiitic differentiation trend. High Sr and Pb and low Nd isotopic ratios suggest the involvement of EM2-like lithospheric mantle and crustal material in the formation of these syn-opening volcanic rocks. Post-opening alkali basalt volcanism occurred at 9–6 Ma on the islands is characterized by OIB-like higher large ionic lithophile elements (LILE) and high field strength elements (HFSE) compared to 15 Ma basalts in this region and Quaternary basalts along the volcanic front. They have variable range of incompatible element concentrations and ratios along with variable Sr, Pb and Nd isotopic ratios suggesting the involvement of both lithospheric and asthenospheric sources at variable melting degrees (from 4% to less than 15%). The observation that the isotopic compositions of Quaternary alkali basalts south of the studied area are even more depleted suggests an increase in the involvement of asthenospheric source with time.

The Influence of Nickel Oxide Additives on the Phase Separation and Crystallization of Glasses in the MgO–Al2O3–SiO2–TiO2System

The changes observed in the structure and phase composition of titanium-containing glasses in the magnesium aluminosilicate system at the equimolar ratio of MgO and Al2O3 during their heat treatment are investigated using the small-angle X-ray scattering (SAXS) technique and X-ray powder diffraction analysis. Nickel oxide additives (from 0.5 to 5.0 mol %) are introduced into the glasses. It is shown that heat treatment of the studied glasses is accompanied by three-phase separation. As a result, two amorphous phases precipitate in the high-silica matrix. One phase (magnesium aluminotitanate) is enriched with magnesium, aluminum, and titanium oxides, whereas the other phase (magnesium aluminate) is enriched with aluminum and magnesium oxides. Nickel oxide is incorporated into both phases. Moreover, nickel oxide reduces the crystallization ability of the magnesium aluminotitanate phase and magnesium aluminotitanates do not crystallize in glasses containing 5 mol % NiO. During Subscriptsequent heat treatments, regions of the nickel magnesium aluminate phase serve as precipitation centers of the magnesium aluminate phase, and, consequently, nickel oxide affects the microinhomogeneous structure of glasses at the final stages of heat treatment. This leads in particular to a decrease in the size of spinel microcrystals. The temperature of the onset of crystallization in regions of existence of the nickel magnesium aluminate phase (spinel precipitation) is lower than that in glasses free from nickel oxide. It is assumed that spinel microcrystals are solid solutions of nickel aluminum and magnesium aluminum spinels.

Iron-Titanium Oxide Geothermometry and Petrogenesis of Lava Flows and Dykes from Mandla Lobe of the Eastern Deccan Volcanic Province, India

Compositional studies on different forms of magnetite, ulvospinel, ilmenite and hematite mineral phases occurring in 37 lava flows and 6 dykes of the Mandla lobe are presented in this paper. Ilmenite (0001) in equilibrium with titanomanetite show high values of temperature of equilibration, ranging from 1172–974°C, for high alumina quartz normative tholeiitic lava flows of Chemical Type - A; 1129–1229°C for low alumina quartz normative tholeiitic lava flows of Chemical Type - B; 1283–1124°C for tholeiitic lava flows of Chemical Type - F and 1243°C and 99O°C for two diopside olivine normative tholeiite flows of Chemical Type D. High olivine normative flows of Chemical Type - G and H show 1095°C and 1092°C respectively. Whereas, high hypersthene normative tholeiite flow of Chemical me C shows temperature of 1187°C. Data plots disposition over iron-titanium oxide equilibration temperature vs – logfo 2, diagram for Mandla lava flows and other parts of the Deccan (Igatpuri, Mahabaleshwer, Nagpur and Sagar areas) revealed that tholeiitic (evolved) basalt of the eastern Deccan volcanic province formed at high temperatures whereas, picritic (primitive) lavas of Igatpuri and tholeiitic basalt of Mahabaleshwar areas were formed at low temperatures. Mahabaleshwer basalts follow FMQ (fayalite-magnetite-quartz) buffer curve but, plots of the Mandla basalts lie above this curve indicating higher temperatures of crystallisation of ilmenite-titanomagnetite than that of the lava flows from other parts of Deccan 'Raps. The eastern Deccan Traps are most evolved types of lava as characterised by its low Mg-number and Ni content whereas, Igatpuri lava flows are picritic (primitive), having high Mg-number and Ni contents. Temperature vs FeO + Fe 2O 3 / FeO + Fe 2O 3 + MgO ratio data plots for Mandla and other Deccan lava flows and liquidus data for Hawaiian tholeiites, indicated that Igatpuri basalts lie parallel to the liquidus line of Hawaiian tholeiite but at lower temperatures. Large data plots of Mandla lava flows lie along the liquidus line of the Hawaiian lava. The highly vesicular nature of compound lava flows having large amount of volatile is responsible for low temperature values whereas, lava flows represented by high temperatures show high modal values of glass and opaque minerals.

Hydration of medium reactive industrial magnesium oxide with magnesium acetate

Medium reactive magnesium oxide reacts incompletely with available water to form magnesium hydroxide. To enhance the hydration of medium reactive magnesium oxide, the effect of magnesium acetate as hydrating agent was studied. The extent to which different parameters (concentration of magnesium acetate, solution temperature and solid to liquid ratio of MgO to magnesium acetate) influence the hydration rate of a medium reactive industrial sample of magnesium oxide were evaluated. The degree of rehydration measured as percentage Mg(OH)2 being formed, increases from approximately 56% using 0.5 M magnesium acetate solutions at 25°C to 64% at 50°C, to more than 70% at 70°C. The major part of rehydration of the medium reactive MgO sample occurs within the first few minutes of the reaction for all three temperatures studied.

Spatial and temporal evolution of basalts and magnesian andesites (“bajaites”) from Baja California, Mexico: the role of slab melts

Late Miocene to Quaternary basalts and associated magnesian basaltic andesites and andesites, locally referred to as “bajaites”, occur in the central part of the Baja California (BC) Peninsula. They form five volcanic fields (Jaraguay, San Borja, San Ignacio, Santa Rosalia, La Purisima) delineating a 600-km-long array parallel to the Gulf of California. They range in age from Late Miocene to Pleistocene, and display very specific geochemical characteristics: SiO 2=50% to 58%, high MgO contents, very low FeO*/MgO ratios usually less than 1.5, highly fractionated rare earth element patterns with low Y and heavy rare earth element, very high Sr (commonly between 2000 and 3000 ppm) and Ba (up to 2300 ppm) contents. The geochemical study and K–Ar dating of ca. 50 samples of these rocks allow us to show that most of their incompatible element ratios, which vary significantly in space and time, reflect source heterogeneities rather than partial melting, fractional crystallisation or crustal contamination effects. Their slab melt imprint increases from northwest to southeast and with time. It is best expressed in the geochemical signatures of Quaternary lavas from La Purisima volcanic field. These features reflect the origin of the “bajaites” by melting of mantle peridotites previously metasomatised by slab melts, in connection with the opening of an asthenospheric window below the Baja California Peninsula during Early and Middle Miocene in northern Baja California, and during Late Miocene in southern Baja California. Melting was initiated by the high thermal regime accompanying ridge subduction or slab tearing/breakoff, and later by Plio-Pleistocene thermal pulses linked to the opening of the Gulf of California. We show that the incongruent melting of metasomatic pargasitic amphibole, leaving a garnet-rich residue, accounts for most of the specific geochemical features of the magnesian andesite suite. This breakdown started at ca. 1000 °C at depths of 70–110 km, and amphibole was probably not entirely consumed during the melting process.