Titanium in ureyite: A substitution with vacancy

Solubility of Ti⁴⁺ in ureyite (cosmochlor, NaCrSi₂O₆) was experimentally studied at 1 atmosphere and ≈1000°C, using sodium disilicate as flux. Microprobe analyses indicate that at low titanium concentrations the substitution of titanium in ureyite is almost exclusively in the M1 site, coupled with a vacancy in the M2 site. At higher TiO₂ contents, a small additional amount enters the tetrahedral site. If the solubility of titanium is similar in jadeite and acmite, the □TiSi₂O₆ substitution could contribute significantly to the vacancy content of natural titanium bearing omphacites.     

Synthesis of kosmochlor and phase equilibria in the join CaMgSi2O6-NaCrSi2O6

Kosmochlor (NaCrSi₂O₆) was synthesized by the flux method from melts along the join Na₂O·2 SiO₂-Na₂O·Cr₂O₃·4 SiO₂ at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi₂O₆-NaCrSi₂O₆ was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr₂O₃) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.     

New experimental data for the system CaO-MgO-SiO2-H2O and a synthesis of inferred phase relations

Subsolidus and vapor-saturated liquidus phase relations for a portion of the system CaO-MgO-SiO₂-H₂O, as inferred from experimental data for the composition regions CaMgSi₂O₆-Mg₂SiO₄-SiO₂-H₂O and CaMgSi₂O₆-Mg₂SiO₄-Ca₃MgSi₂O₈ (merwinite)-H₂O, are presented in pressure-temperature projection. Sixteen invariant points and 39 univariant reactions are defined on the basis of the 1 atm and 10 kbar (vapor-saturated) liquidus diagrams. Lack of experimental control over many of the reactions makes the depicted relations schematic in part.An invariant point involving orthoenstatite, protoenstatite, pigeonite, and diopside (all solid solutions) occurs at low pressure (probably between 1 and 2 kbar). At pressures below this invariant point, orthoenstatite breaks down at high temperature to the assemblage diopside + protoenstatite; with increasing temperature, the latter assemblage reacts to form pigeonite. At pressures above the invariant point, pigeonite forms according to the reaction diopside + orthoenstatite = pigeonite, and the assemblage diopside + protoenstatite is not stable. At 1 atm, both pigeonite and protoenstatite occur as primary liquidus phases, but at pressures above 6–7 kbar orthoenstatite is the only Ca-poor pyroxene polymorph which appears on the vapor-saturated liquidus surface.At pressures above approximately 10.8 kbar, only diopside, forsterite, and merwinite occur as primary liquidus phases in the system CaMgSi₂O₆-Mg₂SiO₄-Ca₃MgSi₂O₈-H₂O, in the presence of an aqueous vapor phase. At pressures between 1 atm and 10.2 kbar, both akermanite and monticellite also occur as primary liquidus phases. Comparison of the 1 atm and 10 kbar vapor-saturated liquidus diagrams suggests that melilite basalt bears a low pressure, or shallow depth, relationship to monticellite-bearing ultrabasites.     

Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma

In the system CaO-MgO-Al₂O₃-SiO₂, the tetrahedron CaMgSi₂O₆(di)-Mg₂SiO₄(fo)-SiO₂-CaAl₂ SiO₆(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl₂Si₂O₈(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO₂-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.Department of Geosciences, University of Texas at Dallas Contribution No. 327. Hawaii Institute of Geophysics Contribution No. 814.     

Discrete diopside in alnöite from Alnö Island

The first occurrence of a discrete diopside nodule in alnöite from Alnö Island is reported. The diopside differs from all other Alnö clinopyroxenes by its subcalcic composition, low titanium content (0.5% TiO₂) and elevated chromium content (0.2% Cr₂O₂). An upper mantle origin for the diopside is suggested. The conditions of equilibration are estimated to 975°C at 15–20 kb. Pressure estimates have been deduced from the geotherm obtained for the Fen (Norway) lherzolites. This geotherm (oceanic type) is regarded as a maximum geotherm for the Alnö intrusion.     

Experimental determination of partition coefficients of ten rare earth elements and barium between clinopyroxene and liquid in the synthetic silicate system at 20 kilobar pressure

Two experiments were carried out at 20 kb pressure for the measurement of partition coefficients of ten lanthanides and barium between diopside clinopyroxene and liquid in the synthetic system diopside-enstatite-silica-H₂O. Starting materials are glass of two different compositions CaMgSi₂O₆ 56, MgSiO₃ 14, SiO₂ 30 weight % and CaMgSi₂O₆ 80, MgSiO₃ 20 weight %.In the experiment on the former composition, the partition coefficient increases regularly with decrease of ionic radii from La to Dy and decreases also regularly from Dy to Lu. In the experiment on the latter composition, stepwise shift of partition coefficient was observed between Dy and Er.The results suggest that the lanthanide cations occupy the site of 8-fold coordination in the clinopyroxene and that the partition coefficients depend on the ionic radii of lanthanide elements and also on the chemical composition of the starting material or heating process during the run.     

Effect of silica concentration on the diopsidic pyroxenes in the system diopside-CaTiAl2O6SiO2

The effect of silica concentration on the solubility of Al and Ti in diopsidic pyroxenes has been investigated at one atmosphere in the system diopside-CaTiAl₂O₆-SiO₂ at temperatures between 1150–1420° C. The composition of pyroxene in the system diopside-CaTiAl₂O₆-SiO₂ is influenced by the total SiO₂ content. Near the join diopside-CaTiAl₂O₆, the pyroxene forms a solid solution with the CaTiAl₂O₆ molecule, and co-exists with perovskite for compositions greater than 11 weight percent CaTiAl₂O₆. Anorthite is an accompanying phase. With increasing total SiO₂ content a series of mineralogical changes involving Ti-bearing phases occur. Sphene solid solution co-exists with diopside solid solution, anorthite, and perovskite in a small compositional range near the diopside-CaTiAl₂O₆ join. Additional total SiO₂ results in the elimination of perovskite and a decrease of solid solution in the pyroxenes. With further increase in SiO₂ content, tridymite appears and the pyroxene is approximately pure diopside. Rutile joins diopside, anorthite, sphene solid solution and tridymite over a broad compositional range in the Ti, Si-rich part of the system. These results demonstrate that increased silica concentration decreases the solubility of Al and Ti in diopsidic pyroxenes and controls the stability of co-existing Ti-bearing phases.     

Effect of chromium on phase relations in the join forsterite-anorthite-diopside in air at 1 Atm

The effect of Cr on the silicate system has been studied in air at 1 atm by adding a small amount of MgCr₂O₄ (0.2–0.5 wt.%) to the join Mg₂SiO₄ (forsterite) — CaAl₂Si₂O₈ (anorthite) — CaMgSi₂O₆ (diopside), which has been considered to form a thermal divide in the system CaO-MgO-Al₂O₃-SiO₂. The spinel primary field is enlarged compared with that in the Cr-free join at the expense of the anorthite primary field. The piercing points forsterite+anorthite+diopside+liquid and forsterite+anorthite+spinel+liquid approach each other with increasing MgCr₂O₄, meet at the join with 0.25 wt.% MgCr₂O₄ (0.20 wt.% Cr₂O₃) to form the ‘isobaric quaternary invariant point’ forsterite+anorthite+diopside+spinel+liquid, and then separate again as new ‘piercing points’ of diopside+spinel+anorthite+liquid and forsterite+diopside+ spinel+liquid. This process indicates that the join Mg₂SiO₄-CaAl₂Si₂O₈-CaMgSi₂O₆ containing more than 0.2 wt.% Cr₂O₃ cannot be a thermal divide in the basalt tetrahedron. The results of the present study show that the presence of a minor amount of Cr causes a significant effect on the phase relations and therefore, the role of Cr must be taken into account in the formulation of a petrologic model.     

Electron microprobe measurement of pyroxenes coexisting with H2O-undersaturated liquid in the join CaMgSi2O6-Mg2Si2O6-H2O at 30 kilobars,with applications to geothermometry

Mixtures of synthetic crystalline enstatite and diopside were reacted with small water contents in sealed capsules in piston-cylinder apparatus at 30 kb between 1000° C and 1700° C. The compositions of coexisting enstatite and diopside solid solutions were measured with an ARL-EMX electron microprobe between 1000° C and 1500° C. Between 1100° C and 1500° C the pyroxenes coexisted with H₂O-undersaturated liquid which quenched to inhomogeneous pyroxene crystals. The presence of liquid facilitated growth of pyroxene crystals suitable for microprobe determinations. The solvus of Davis and Boyd (1966) is generally used in geothermometry; our enstatite solvus limb is a few mol-% richer in Mg₂Si₂O₆ in the temperature range 1000–1400° C; our diopside solvus limb is a few mol-% richer in Mg₂Si₂O₆ below 1100°C, in close agreement between 1100° C and 1200° C, but richer in CaMgSi₂O₆ between 1200° C and 1500° C. Estimated equilibration temperatures for a diopside with composition 78.7% Di is 1300° C according to our results compared with 1210° C for the Davis and Boyd solvus.     

Concentrations and behavior of oxygen and oxide ion in melts of composition CaO · MgO · xSiO2

The concentrations and behavior of oxygen and oxide ion were studied in silicate melts of composition CaO · MgO · xSiO₂ (1.25 ≤ x ≤ 3) in the temperature range 1425 to 1575°C by cyclic voltammetry and chronopotentiometry. Electroreduction of oxygen is a reversible, 2 electron process involving dissociated oxygen atoms. The Henry's Law constant for O₂ in molten diopside (CaO · MgO · 2SiO₂) is 0.023 ± 0.004 mole/l atm at 1450°C. The diffusion coefficient for molecular oxygen in diopside melt is 4.5 ± .5 × 10^?6 cm²/sec at 1450°C and the activation energy of diffusion is 80 ± 2 kcal/mole. Oxide ions produced by electroreduction of oxygen, rapidly dissociate silicate polymers, causing the concentration of free oxide ions in diopside melt to be buffered at a low level (4.7 ± .8 × 10^?5 mole/l). The concentration of free oxide ion increases at higher proportions of metal oxides but remains at this value in more silicic melts. The rate of formation of oxide ions by polymerization in diopside melt is 0.021 ± .007 mole/l sec. Thermodynamic parameters (the standard free energy, enthalpy and entropy) for the oxidation of Ni, Co, and Zn in diopside melt in equilibrium with gaseous oxygen agree with those for solid oxide systems. The platinum reference electrode in molten diopside is a reversible, oxygen electrode.     

The bearing of the system CaMgSi2O6CaAl2SiO6CaFeAlSiO6 on fassaitic pyroxene

The system CaMgSi₂O₆CaAl₂SiO₆CaFeAlSiO₆ has been studied in air at 1 atm. The phase assemblage at subsolidus temperatures in the CaMgSi₂O₆-rich portion is Cpx + An + Mel and that in the CaMgSi₂O₆-poor portion Cpx + An + Mel + Sp. At subsolidus temperatures the sigle-phase field of clinopyroxene increases with an increase in the CaFeAlSiO₆ component of the system. The Al₂O₃ content of clinopyroxene, however, continues to increase beyond the single-phase field and attains at least 16.04 wt.% Al₂O₃ with 3.9 wt.% Fe₂O₃. The stability field of fassaite in the system over a range of pressures and oxygen fugacities has been estimated from data in the literature as well as the present data. The CaFeAlSiO₆ content of fassaite is dependent on oxygen fugacity, but is not influenced by pressure. The stability field is strongly influenced by oxygen fugacity at low and high pressure, and decreases with decreasing oxygen fugacity. Clinopyroxenes in both volcanic and metamorphic rocks from various localities, when plotted on the CaMgSi₂O₆CaAl₂SiO₆CaFeAlSiO₆ triangle, show that there is no compositional gap between diopside and fassaitic pyroxene in metamorphic rocks, and that the fassaitic pyroxene in alkalic rocks becomes richer in both CaAl₂SiO₆ and CaFeAlSiO₅ components as crystallization proceeds. These results agree with those obtained in the experimental study.     

Reversed pyroxene phase equilibria in CaO-MgO-SiO2 from 925° to 1,175° C at one atmosphere pressure

Experiments using V₂O₅ as a high-temperature solvent have produced compositional reversals defining the miscibility gap between enstatite and diopside on the join Mg₂Si₂O₆-CaMgSi₂O₆ between 925° and 1,175° C at atmospheric pressure. These experiments locate an equilibrium near 1,000° C among diopside, protoenstatite, and orthoenstatite; they verify the stable coexistence of diopside and protoenstatite above 1,000° C and disprove the hypothesis that orthoenstatite has a stability field which is continuous from temperatures below 1,000° C to the solidus. The phase relations suggest that the orthorhombic low-Ca pyroxene on the solidus in this system (formerly identified as orthoenstatite) is a phase distinct from the orthoenstatite stable with diopside at low subsolidus temperatures. Data locating the orthoenstatite-diopside miscibility gap validate the use at low pressures of symmetric orthopyroxene and asymmetric clinopyroxene solution models in this system.     

Armalcolite stability as a function of pressure and oxygen fugacity

The stability of synthetic armalcolite of composition (Fe_0.5Mg_0.5Ti₂O₅ was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O₂) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down ($\text{dT}\text{dP}\text{= 20°C/}\text{kbar}$), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ($\text{dT}\text{dP}\text{= 30°C/}\text{kbar}$) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO₂ appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O₂ was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe_0.5Mg_0.5)-Ti₂O₅, is stable over a range ?O₂ from about 10^?9.5to 10^?10.5 atm. Below this range to at least 10^?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti³⁺. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.     

Crystallization sequences of Ca-Al-rich inclusions from Allende: An experimental study

The equilibrium crystallization sequence at 1 atmosphere in air of a melt corresponding in composition to the average composition of Type B Ca-Al-rich inclusions from the Allende meteorite is: spinel (1550°C) → melilite (1400°C; Åk22) → anorthite (1260°C) → Ti-Al-rich clinopyroxene (1230°C; “Ti-fassaite”). The melilite becomes increasingly åkermanitic with decreasing temperature. The pyroxene is similar in composition to fassaites from Type B inclusions. Preliminary results suggest that the crystallization sequence is similar at oxygen fugacities near the iron-wüstite buffer.The results of these experiments have been integrated with available phase equilibrium data in the system CaO-MgO-Al₂O₃-SiO₂TiO₂ and a phase diagram for predicting the crystallization sequences of liquids with compositions of coarse-grained Ca-Al-rich inclusions has been developed.Available bulk compositions of coarse-grained inclusions form a well-defined trend in terms of major elements, extending from Type A and Bl inclusions near the spinel-melilite join to more pyroxene-rich Type B2 inclusions. The trend deviates from the expected sequence of solid condensates from a nebular gas at P = 10^?3 atm if pure diopside is assumed to be the clinopyroxene that condenses. The Type A-B1 end of the trend is similar in composition to calculated equilibrium condensates at 1202–1227°C and the trend as a whole parallels the sequence of condensates expected from diopside condensation at ~ 1170°C. The trend is consistent to first order with the condensation of solid Ti-rich fassaite in place of pure diopside at higher temperatures than those at which pure diopside is predicted to condense. Partially molten condensates may be likely in this case or if the nebular pressure is higher than 10^?3 atm.     

Experimental study on the phase equilibria in the join CaMgSi2O6-CaCrCrSiO6 with special reference to the blue diopside

The behaviour of tetrahedrally coordinated and octahedrally coordinated Cr³⁺ ions in diopside is discussed from the study on the join CaMg-Si₂O₆-CaCrCrSiO₆. The molecule CaCrCrSiO₆ decomposes into uvarovite+eskolaite and its maximum solubility in diopside is 6.7 wt percent at 940 ° C. Crystalline phases are diopside ss (ss is abbreviation of solid solution), uvarovite ss, wollastonite ss, spinel and eskolaite. The diopside ss is blue in colour. Its optical spectra were measured in the wavelenght range of 325–2600 nm, and assigned after tetrahedral configuration Td and octahedral configuration Oh. It is estimated that octahedral Cr³⁺ ions are in high spin state, while tetrahedral Cr³⁺ ions may be probably in low spin state. The _t and B are 10,300–10,370 cm^–1 and 429–432 cm^–1. The CFSE for tetrahedral low spin Cr³⁺ ions is nearly the same as that for octahedral high spin Cr³⁺ ions. The ionic radii of tetrahedral low spin Cr³⁺ ions calculated are 0.47–0.53 Å, shrinked from the radius of octahedral high spin Cr³⁺ ion (0.615 Å) as much as 14–24 percent. Petrologic implications of the result are also discussed.The first half of the D. Sc. dissertation of K. Ikeda presented to Hokkaido University in June, 1976     

Crystal chemistry of Sc-bearing synthetic diopsides

Scandium substitution in the diopside structure was studied by single-crystal X-ray diffraction on a series of synthetic diopside samples. These diopsides were doped with increasing amounts of Sc³⁺ through a coupled substitution involving ^M1(Sc³⁺)₁ ^M2[] ₁ ^T (B)₁ ^M1(Mg²⁺)_?1 ^M2(Ca²⁺)_?1 ^T(Si⁴⁺)_?1 exchange, whereby charge compensation is achieved by vacancies at the M2 sites and B at the tetrahedral sites. The substitution of scandium for magnesium at the M1 site results in an increase in volume and distortion of the M1O₆ polyhedron. The accompanying creation of vacancies at the M2 sites causes an increase in the M2 polyhedral volume. The modifications of the M1 and M2 polyhedra result in an increase of the polyhedral strip along the b lattice direction and a straightening of the tetrahedral chain. The geometrical modification of the M1 polyhedron due to scandium incorporation is comparable to those observed when similar amounts of Ti³⁺/Ti⁴⁺ substitute for Mg in the diopside structure, suggesting a structural control on the solubility of Sc and Ti in diopside that may influence the extent of the solid solutions between the Sc and Ti end-members.     

High-pressure behaviour of Ca-rich C 2 /c clinopyroxenes along the join diopside-enstatite (CaMgSi2O6-Mg2Si2O6)

 An in situ high-pressure (HP) X-ray diffraction investigation of synthetic diopside and of the Ca_0.8Mg_1.2Si₂O₆ clinopyroxene (Di₈₀En₂₀) was performed up to respectively P=40.8 and 15.1 GPa, using high brilliance synchrotron radiation. The compression of the cell parameters is markedly anisotropic, with β_b ⋙ β_c > β_a > β_asinβ for any pressure range and for both diopside and Di₈₀En₂₀. The compressibility along the crystallographic axes decreases significantly with pressure and is higher in Di₈₀En₂₀ than in diopside. The β cell parameter decreases as well with pressure, at a higher rate in Di₈₀En₂₀. The cell volume decreases at almost the same rate for the two compositions, since in diopside a higher compression along a* occurs. A change in the mechanism of deformation at P higher than about 5–10 GPa is suggested for both compositions from the analysis of the strain induced by compression. In diopside at lower pressures, the deformation mainly occurs, at a similar rate, along the b axis and at a direction 145° from the c axis on the (0 1 0) plane. At higher pressures, instead, the deformation occurs mostly along the b axis. In Di₈₀En₂₀ the orientation of the strain axes is the same as in diopside. The substitution of Ca with Mg in the M2 site induces at a given pressure a higher deformation on (0 1 0) with respect to diopside, but a similar change in the compressional behaviour is found. Changes in the M2 polyhedron with pressure can explain the above compressional behaviour. A third-order Birch-Murnaghan equation of state was fit to the retrieved volumes, with K=105.1(9) GPa, K′=6.8(1) for diopside and K=107.3(1.4) GPa, K′=5.7(3) for Di₈₀En₂₀; the same equation can be applied for any pressure range. The elasticity of diopside is therefore not significantly affected by Mg substitution into the M2 site, in contrast to the significant stiffening occurring for Ca substitution into Mg-rich orthopyroxenes. Received: 3 January 2000 / Accepted: 21 May 2000     

Liquidus karrooite stability and composition at reducing conditions

We have determined the stability of rutile and karrooite on the liquidus of pseudobinary silicate melts of anorthite–diopside eutectic composition with, in addition, of up to 32 wt% TiO₂ at one atm total pressure and at wide range of oxygen fugacities. At 1,300°C and at an fO₂ below 10^–11.2 atm rutile (TiO₂) is replaced as liquidus phase by the pseudobrookite-type MgTi₂O₅–Ti₃O₅ solid solution with some Al in the crystal structure. The composition of karrooite was found to be strongly dependent on oxygen fugacity. Crystalline phases were identified by transmission electron microscopy (TEM). The results obtained in this study are relevant for understanding the chemistry of lunar armalcolites. Using excess rutile solubility at reducing conditions, we estimated the Ti⁴⁺ /Ti³⁺ ratio in silicate melts at 1,300°C as function of fO₂.     

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: the effect of Fe-Ti oxide minerals and the origin of basaltic andesites

One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO₂ in the liquid, and a notable increase in SiO₂ and Al₂O₃, and the minor oxides K₂O and P₂O₅, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation.