Fe2N-type SiO2 from shocked quartz

The modified niccolite structure (Fe₂N-type) of SiO₂, previously found in diamond anvil experiments at 35 to 40 GPa, was formed in a porous mixture of crystalline α-quartz and copper powder at shock pressures estimated at 12 to 27 GPa. It is suggested that quartz can invert during shock compression not only to coesite, stishovite and an amorphous or glass phase of silica, but also to Fe₂N-type SiO₂, depending upon the shock history.     

Disequilibrium features in experimentally shocked mixtures of olivine plus silica glass powders

Seventeen shock-recovery experiments were performed on powder mixtures of one part (by weight) olivine (St. John's forsterite) plus two parts silica glass (pure vitreous silica) in order to characterize the physical and chemical interaction of two chemically incompatible components during shock. Powders of <45 m grain size were shocked by impact of projectiles launched from a 20 mm gun which created pressures ranging from 6.2 to 64.2 GPa (1 GPa= 10kbar).Petrographie features observed in thin section attest to mechanical and thermal metamorphism. Samples shocked to pressures from 6.2 to 39.3 GPa form compacted, mosaic, granular aggregates with fractured and strained grains. Samples shocked to pressures from 42.9 to 64.2 GPa form vesicular, mixed melts containing flow schlieren and relict olivine fragments. Petrographic disequilibrium is manifested in cataclastic textures showing deformational anisotropy and in thermal effects showing non-uniform intergranular melting. This disequilibrium is caused by an irregular pressure distribution resulting from the rapid collapse of pore spaces.The chemical composition of the shock melts are similar in each of six samples shocked to pressures of 42.9 to 64.2 GPa. Melt chemistry is bimodal in each sample. Colorless melts are 99.9% SiO₂ and represent pure silica glass melts; pale to dark green melts range in composition from 47% to 64% SiO₂ and represent a progressive mixture of olivine melt (41% SiO₂) with silica glass melt. Surprisingly, the compositions of the colored glasses are intermediate between the composition of pure olivine and the bulk composition of the original starting material (79% SiO₂) and are similar to enstatitic pyroxene compositions (50% to 57% SiO₂; 33% to 37% MgO). Although bulk compositions of shocked samples are unchanged, the creation of melts with pyroxene compositions instead of bulk sample compositions may indicate that an incipient eutectic-type fusion may have occurred in small olivine-normative domains surrounding individual olivine grains. Chemical disequilibrium is evidenced by the creation of these olivine-normative melts from a quartz-normative starting compositions and by the chemical heterogeneity in the melts.     

Studies on the lattice distortion and substructures of shocked lamellae in naturally shocked quartz

One unshocked and 9 naturally shocked single quartz crystal grains with 1–6 sets of shock lamellae from the Ries, West Germany, and the Lake Lappajärvi, Finland, covering a range from unshocked quartz withNo = 1.544 to nearly completely glassy quartz withNo = 1.461 have been used for X-ray precession and Laue investigations. Four of the shocked grains have preliminarily been studied under a transmission electron microscope. It is found that quartz havingNo less than 1.539 shows intensive anisotropic cell expansion and lattice disordering which gradually increase asNo decreases. Shock-induced lattice distortion of quartz is clearly shown on both precession and Laue photographs. For the weakly shocked quartz (p < 200 kb) slight to pronounced spreading of spots is observed. When the pressure reaches 200 kb, both concentric spreading of spots having long ‘tails’ and concentric rings (powder pattern) are revealed on the same photograph, which means that besides a part of single crystal there also exist randomly oriented tiny ‘fragments’ of quartz in this shocked quartz grain. As pressure increases from 230 to 315 kb, more and more crystalline puases in the quartz grains have transformed from solid state into silica glass, and the concentric rings and the long ‘tails’ disappear and the spot spreading becomes slight again, but reflection intensities become much lower in comparison with those of weakly shocked quartz. TEM investigations show three kinds of substructures of shock lamellae. The glass contents of two of the four grains (73% and 84% respectively) were measured on TEM photographs with the help of an image analysis system. On the basis of above investigations a six-terminal-state model for the mechanism of deformation in shock metamorphosed quartz is presented.     

Prograde evolution of Sulu UHP metamorphic rock in Yangzhuang,Junan region,deduced by combined Raman and petrological studies

The prograde metamorphic history of the Sulu ultrahigh‐pressure metamorphic terrane has been revealed using Raman‐based barometry of the SiO₂ phases and other mineral inclusions in garnet porphyroblasts of a coesite eclogite from Yangzhuang, Junan region, eastern China. Garnet porphyroblasts have inner and outer segments with the boundary being marked by discontinuous changes in the grossular content. In the inner segment, the SiO₂ phase inclusions are α‐quartz with no coesite or relict features such as radial cracks. The residual pressures retained by the quartz inclusions systematically increase from the crystal centre to the margin of the inner segment. The metamorphic conditions estimated by calculation from the residual pressure and conventional thermodynamic calculation range from 500 to 630 °C and 1.3 to 2.3 GPa for the stage of the inner segment. Coesite and its pseudomorph occur as inclusions in the outer segment of the garnet and matrix omphacite. This occurrence of coesite is consistent with the pressure and temperature conditions of 660–725 °C and 3.1 GPa estimated by conventional geothermobarometry. Our results suggest that the quartz inclusions in the inner segment were trapped by garnet under α‐quartz‐stable conditions and survived phase transition to coesite at the peak metamorphic stage. The SiO₂ phases and other inclusions in the garnet have retained evidence of the pre‐eclogite prograde stage even during exhumation stage. The combined Raman spectroscopic and petrological approaches used here offers a powerful means for obtaining more robust constraints prograde stages involving garnet growth where different SiO₂ phases are present as inclusions.     

Microstructures of AIPO4 subjected to high shock pressures

Berlinite single crystal specimens were shocked to peak pressures 12 and 24 GPa. Specimens were placed in an Al capsule to minimize shock-wave reflections at interfaces between specimen and capsule. Shock pressures were achieved with a 6.5-m-long two-stage gun. The shock-induced microstructures in recovered specimens were then investigated by Transmission Electron Microscopy. In the sample shocked at 12 GPa, the prominent shock-induced defects are dislocations and basal a glide appears to be the only glide system activated. In contrast, the sample shocked at 24 GPa exhibits no dislocations. The material is partially converted into an amorphous phase occurring under the form of thin amorphous lamellae parallel to the }10 $\bar 1$ n{ planes (n=0, 2, 3, 4). This microstructure is very similar to the one observed in experimentally shocked quartz.     

Blueschist facies metamorphism of peralkaline rhyolites from the Tenda crystalline massif (northern Corsica): evidence for involvement in the Alpine subduction event?

The Tenda crystalline massif (northern Corsica) is a fragment of the western Corsica basement involved in the Alpine orogeny. Rhyolite dykes crosscutting the gabbroic complex of Bocca di Tenda (southern sector of the Tenda crystalline massif) show an unusual metamorphic mineral assemblage, defined by jadeite‐bearing (up to 46 mol percentage) aegirine, riebeckite, celadonite‐rich phengite (Si=3.50–3.65 apfu), quartz, albite and K‐feldspar. Jadeite‐bearing aegirine and riebeckite mostly occur as coronas around jadeite‐free aegirine and arfvedsonite, respectively, which both are relics of igneous origin. This metamorphic assemblage reflects the peralkaline compositions, which are characterised by anomalously high contents of SiO₂ and Na₂O, and negligible CaO and MgO. The evolved rocks of the gabbroic sequence (quartz‐diorites to tonalites) and the surrounding granitoids are characterised by the development of riebeckite/ferroglaucophane, epidote, celadonite‐rich phengite and albite, thus pointing to a metamorphic crystallization in the epidote‐blueschist facies. In all the studied rocks, metamorphic reactions were controlled by fluid‐assisted mass‐transfer through grain boundaries and microfractures. The different mineral assemblages allow the peak P–T metamorphic conditions to be constrained to between 0.8 GPa/300 °C and 1.1 GPa/500 °C. These estimates attest to a geothermal gradient (dT/dP) of 10–13 °C km^?1 and indicate that the Tenda crystalline massif was buried to a minimum depth of 27 km during the Alpine orogeny. The blueschist facies recrystallization in the Tenda crystalline massif has been related to the cessation of an eastward‐dipping subduction event.     

An X-ray diffraction study of shock-wave-densified SiO<Subscript>2</Subscript> glasses

 The densification and structural changes in SiO₂ glass compressed up to 43.4 GPa by shock experiments are investigated quantitatively by the X-ray diffraction technique. Direct structural data (average Si–O and Si–Si distances and Si–O–Si angles, coordination number of the Si atom) of these shock-densified SiO₂ glasses have been obtained by analyzing the radial distribution function curves, RDF(r), calculated with X-ray diffraction data. The coordination number of all densified glasses is about 4 and shows almost no pressure variation. The SiO₂ glass has shown density increase of 11% at a shock compression of 26.3 GPa. This density evolution could not be explained by the coordination change. The reduction of the average Si–O–Si angle (144° at 0 GPa to 136° at 26.3 GPa) obtained from RDF(r) data may account for this density increase. This Si–O–Si angle change may be caused by shrinkage of the network structure and the increase of small rings of SiO₄ tetrahedra. For higher shock pressure, a decrease in the Si–O–Si angle to 140° was observed. This is consistent with the decrease in density at 32.0 and 43.2 GPa. This decrease in the Si–O–Si angle and density could be attributed to an annealing effect due to high after-shock residual temperature. This pressure dependence of average Si–O–Si angles in shock-densified SiO₂ glass agrees with the results of our previous Raman spectroscopic study. On the other hand, the pressure variation for the first sharp diffraction peak (FSDP) was analyzed to estimate the evolution of intermediate range structures. It is suggested that the mean d value (d _m ) obtained from the position of FSDP strongly depends on the shock and residual temperature, as well as shock pressure. Received: 29 June 2001 / Accepted: 14 November 2001     

Raman-spectroscopy and X-ray diffractometer studies of experimentally produced diaplectic feldspar glass

Raman vibrational spectra and X-ray diffractometer scans were obtained from experimentally shocked samples of oligoclase (An₁₉) and andesine (An₄₉). Some 11 oligoclase and 15 andesine targets were shocked between 24 and 40 GPa to address the transition from crystalline to diaplectic states and to explore differences in the structural state of diaplectic feldspar glasses (maskelynite) as a function of peak shock stress. Thy symmetrical VS (T-O-T) (T=Si or Al) stretch bands are the most persistent. They disappear, however, in the noise of an unusually strong luminescent spectrum at > 32 GPa in the oligoclase and at > 30 GPa in the andesine; i.e., at pressures where transition to diaplectic glass is complete. The Raman investigations yield a maskelynite structure that is probably one of a multitude of very small domains with some order, but with a large range of local properties on the scale of small domains, either in heterogeneous size-distribution of domains or in their detailed order, if not both. This results in a very large number of Raman photon-phonon frequencies unlike glasses derived from quenched melts. Our study corroborates conclusions by others, that diaplectic glasses may be the quench products of very dense, disordered phases that exist during shock compression and that subsequently relax to these unusually dense glasses that are only known from shock processes. An origin by relaxation of highly ordered, genuine high pressure polymorphs possessing the structure of hollandite is unlikely, as no evidence for any six-fold Si-coordination was found. Detailed luminescent emission spectra were taken of the oligoclase samples and they show disappearance of the IR band and a strengthening of the green band (the blue band could not be detected with a primary radiation of wavelength 448 nm). This supports previous views that the disappearance of IR emission is most likely caused by shock-induced changes of the crystal field near Fe³⁺ sites, rather than due to quenching by Fe²⁺. The X-ray studies were primarily intended to explore whether differences in structural states of maskelynite occur on sufficiently large scales to be detected by standard diffractometry methods. This is not the case. X-ray diffractometer patterns are grossly similar, if not identical, in samples shocked between 30 and 40 GPa and may not be used to fine-tune the shock histories of naturally produced diaplectic glasses.     

Shock induced planar deformation structures in quartz from the Ries crater,Germany

Crystalline rocks from breccias of the Ries basin, Germany, contain highly deformed quartz. Various planar deformation structures could be observed and classified into five different types: (1) Decorated planar elements, (2) Non-decorated planar elements, (3) Homogeneous lamellae, (4) Filled lamellae, (5) Planar fractures. All these structures are parallel to crystallographic planes: {10¯13}, {10¯12}, {10¯11}, {0001},{11¯21}, {11¯22}, {21¯31}, {51¯61}, {10¯10}. The most typical and most abundant planar structures are decorated and nondecorated planar elements parallel to {10¯13} and {10¯12}. Planar fractures are parallel to {0001} and {10¯11} and form at lower stress levels, probably earlier than the planar elements.Quartz containing planar elements, especially of the non-decorated type, has lower density, index of refraction and birefringence than normal quartz. This quartz is apparently a mixture of an amorphous phase and crystalline quartz, the amount of which can be calculated using average density or refractive index.Comparison of planar quartz structures found in tectonites and those produced artificially under static or dynamic high pressure conditions demonstrates that Ries quartz closely resembles deformed quartz recovered from shock wave experiments. The planar structures found in Ries quartz have been formed by shock wave actions with peak pressures in the 100–400 kbar range.Planar elements are explained to be traces of gliding processes during shock loading visible due to the fact that a high pressure phase (stishovite and/or a stishovite-like glass phase) has been produced along the glide planes. Upon pressure release most of the high pressure phase was transformed into an SiO₂-glass (diaplectic glass).In comparison with experimental data the amount of residual crystalline quartz as well as type and orientation of planar structures in the quartz grains are clues to estimate the peak pressures responsible for these deformations. Shock waves with peak pressures exceeding about 400 kbar completely transform quartz into diaplectic SiO₂-glass.     

Effect of Silicate Matter on Pyrochlore Solubility in Fluoride Solutions at <Emphasis Type="Italic">Т</Emphasis> = 550–850°C, <Emphasis Type="Italic">Р</Emphasis> = 50–100 MPa (Experimental Studies)

The experimental results of natural pyrochlore behavior in KF solutions in the presence of quartz at 550–850°C and 50–100 MPa are presented. It is shown that silicate matter (quartz) exerts a significant effect on pyrochlore solubility in aqueous solutions of fluorides of alkaline metals under hydrothermal conditions. This study of the fluid inclusions has revealed the occurrence of reactions of high-temperature hydrolysis of KF under the experimental conditions: KF + H₂O = KOH + HF; in which case, the interaction with quartz SiO₂ + 2KOH = K₂SiO₃ + H₂O is followed by the formation of a silicate glass phase (an aqueous solution–melt). This phase of alkaline glass is a Nb concentrator (Nb₂O₅ up to 16 wt %). The coefficient of Nb distribution between the glass and the fluid is ≈500 (in favor of the glass). It is determined that the phase of the silicate solution–melt can serve as an effective concentrator of the ore component (Nb) at the last lowtemperature stages of crystallization of rare-metal granites.     

Shock-wave compression of silica gel as a model material for comets

A shock-wave compression experiment using synthesized silica gel was investigated as a model for a comet impact event on the Earth’s surface. The sample shocked at 20.7 GPa showed considerable structural changes, a release of water molecules, and the dehydration of silanol (Si–OH) that led to the formation of a new Si–O–Si network structure containing larger rings (e.g., six-membered ring of SiO₄ tetrahedra). The high aftershock temperature at 20.7 GPa, which could be close to 800 °C, influenced the sample structure. However, some silanols, which were presumed to be the mutually hydrogen-bonded silanol group, remained at pressures >20.7 GPa. This type of silanol along with a small number of water molecules may remain even after shock compression at 30.9 GPa, although the intermediate structure of the sample recovered was similar to that of silica glass.     

Shock metamorphism in sillimanite from the Haughton impact structure,Devon Island,Canada

Clasts of shocked garnet-sillimanite gneisses comprise a minor fraction of the allochthonous breccia at the Haughton impact structure. Refractive indices of the diaplectic and fused components of the gneisses, and reduced specific gravity indicate shock pressures from 35 to 55±5 GPa and effective post-shock temperatures from 500° to 1,000° C in a suite of selected samples.Sillimanites remain birefringent but display several effects of shock metamorphism. Shock-produced planar features and planar fractures are highly developed; optic axial angle (2V _y ) increases from near normal (26°) to over 80° within a sample; there is a reduction in optical relief and a development of a pale brown colouring which generally deepens in shade as shock level increases. There is no unambiguous evidence, optically or from X-ray investigation, of a high-pressure Al₂SiO₅ polymorph or breakdown to mullite and silica. The highly shocked sillimanites have anomalous K₂O contents from 0.11% to 0.92%. Potassium appears to substitute for aluminum and, to a lesser degree, for iron while retaining sillimanite stoichiometry, and the amount of substitution generally reflects increased shock level. The source of the contributed potassium is the coexisting shock-fused feldspar glass. The glass of each sample is derived primarily from melted alkali feldspar with a minor and varied admixture from the breakdown of mafic minerals. The glasses are depleted in K₂O, although Na₂O is unaffected, and the extent of depletion can be correlated with the increased K₂O content of the associated sillimanite. The incorporation of potassium in shocked sillimanites is a function of both degree of shock deformation and availability of potassium from other coexisting shocked phases. It is speculated that the brown colouration is a function of ferrous iron content and may reflect post-crater thermal history rather than shock level.Contribution from the Earth Physics Branch No. 951     

X-ray investigations related to the shock history of the Shergotty achondrite

X-ray diffraction techniques were employed to evaluate the shock stress suffered by naturally shocked materials from the Shergotty achondrite. Experimentally shocked augite and enstatite, subjected to pressures from 22 to 60 GPa, served as standards. The Shergotty pyroxenes reveal formation of continuous diffraction rings, line broadening, preferred orientation of small scale diffraction domains, and other evidence of substantial lattice disorder; they are hybrids between single crystals and fine-grained, random powders as revealed by Debye-Scherrer techniques. As expected from optical studies related to the degree of mosaicism, which is not sensitively related to peak pressures up to 70 GPa, the pyroxene lattice is very resistant to shock damage on smaller scales also as revealed by Debye-Scherrer and powder difractometer X-ray techniques. While measurable lattice disaggregation and progressive fragmentation occurs up to approximately 25 GPa, little additional damage is suffered from 30 to 60 GPa. Thus, pressure calibration of naturally shocked pyroxenes via optical and x-ray methods continues to be difficult.Powder diffractometer scans on pure maskelynite fractions of Shergotty reveal small amounts of still coherently diffracting plagioclase. Future studies should clarify whether such material, possibly of small size and scattered throughout many maskelynite grains, is responsible for the unusually high refractive indices reported for the diaplectic feldspar glasses of Shergotty.     

Deformation and transformation in experimentally shock-loaded quartz

Single crystals of quartz, shock-loaded along the a axis to pressures of 22 Gpa, 24 GPa, 26 GPa and 30 GPa were examined by high-voltage transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. Asymmetric broadenings of X-ray lines indicate spatial inhomogeneity of shock effects. X-ray streaking angles in the reciprocal lattice planes h0 \(\bar h\) l, 0k \(\bar k\) l and hki0 indicate a slight tilting deformation by rotation about [00.1] in (0001). TEM reveals glass lamellae which are mostly in (01 \(\bar 1\) 2) orientation, and are correlated with optical planar elements and with surface steps seen in SEM. No dislocations are found. There are (0001) lamellar features, probably Brazil twins. The (01 \(\bar 1\) 2) glass lamellae develop directly from bands of quartz in which intense deformation has produced a fine-scale lamellar to blocky structure, possibly also originating by twinning. Relics of crystalline structure are found in almost completely vitrified lamellae. Stishovite occurs in heavily deformed parts of the 22 GPa and 24 GPa specimens, in patches of densified glass distinct from the sharply bounded lamellae. The nucleationless, pervasive transformation of lamellae to glass, with preservation of their sharp boundaries, is attributed to defect coalescence analogous to vitrification by radiation damage (metamictization). Some patchy glass may be due to melting.     

The chemical composition including the Rare Earth Elements of the three major glass types of Europe and the Orient used in late antiquity and the Middle Ages

Sets of 20 soda ash glasses, 16 soda lime glasses and 23 wood ash glasses mainly from excavations in Europe (additional soda ash glasses from Egypt) were analysed on 61 chemical elements. Average SiO₂ is about 62% in soda glasses and 50% in wood ash glasses. The three groups of glasses contain on average 13% Na₂O, 18% Na₂O and 13% K₂O as fluxes to lower the melting temperature of quartz at their production. The starting materials beside quartz were halophytic plant ash for soda ash glass, trona (Na₃H(CO₃)₂·2H₂O) and lime (clamshells) for soda lime glass and beech ash for wood ash glass. Each of the three major glass types contains specific Rare Earth Element (REE) concentrations mainly contained in quartz and its intergrown minerals. 50 Paleozoic and Mesozoic sandstones from Central Europe represent the quartz composition. The REE pattern of these glasses apparently indicates major compositional stages of the Continental Earth's Crust. The boron to lithium and sodium to potassium ratios as in seawater suggest reactions of materials for soda glass with seawater. Negative Ce anomalies in the three glasses are caused by reactions of quartz with seawater.     

A Raman spectroscopic study of shock-wave densification of vitreous silica

The densification processes in SiO₂ glass induced by shock-wave compression up to 43.4 GPa are investigated by Raman spectroscopy. At first, densification increases with increasing shock pressure. A maximum densification of 11% is obtained for a shock pressure of 26.3 GPa. This densification is attributed to the reduction of the average Si−O−Si angle, which occurs first by the collapse of the largest ring cavities, then by further reduction of the average ring size. For higher shock pressures, a different structural modification is observed, resulting in decreasing densification with increasing shock pressure. Indeed, the recovered densification becomes very small, with values of 1.8 and 0.5% at 32 and 43.4 GPa, respectively. This is attributed to partial annealing of the samples due to high after shock residual temperatures. The study of the annealing process of the most densified glass by in situ high temperature Raman spectroscopy confirms that relaxation of the Si−O−Si angle starts at a lower temperature (about 800 K) than that of the siloxane rings (about 1000 K), thus explaining the high intensity of the siloxane defect bands in the samples schocked at compressions of 32 and 43.4 GPa. The large intensity of the siloxane bands in the nearly undensified samples shocked by compressions above 30 GPa may be explained by the relaxation during decompression of five- and six-fold coordinated silicon species formed at high pressure and high temperature during the shock event. Received: March 30, 1998 / Revised, accepted: August 21, 1998     

Dissolution of orthopyroxene in basanitic magma between 0.4 and 2 GPa: further implications for the origin of Si-rich alkaline glass inclusions in mantle xenoliths

A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO₂-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures of 1 GPa and below under anhydrous and CO₂-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and CO₂-bearing experiments is only slightly enriched in SiO₂ and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass of similar composition whereas the glass from CO₂-bearing experiments is less SiO₂ rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene, combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K also causes minor SiO₂ enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor. Received: 24 August 1998 / Accepted: 2 December 1998     

Shock effects in MgAl2O4-spinel

Shock recovery experiments on synthetic MgAl₂O₄-spinel samples in the pressure range 25.5 to 50.5 GPa have been performed in order to examine the effects of shock waves on this material. The shocked samples were subsequently studied in the transmission electron microscope. All samples showed shock-induced dislocations with the Burgers vector 1/2 〈110〉 and twin lamellae of the twin-law {111}. In addition, samples, which had experienced the higher pressures, showed lamellar areas of a crystalline phase that we have not yet been able fully to characterize. It is probably not ε-MgAl₂O₄.     

Diffusive dissolution in ternary systems: analysis with applications to quartz and quartzite dissolution in molten silicates

Exact solutions to equations governing isothermal diffusive dissolution of a crystalline slab in a ternary liquid were obtained to include the effect of coupled chemical diffusion in the liquid. These analytical results, supplemented by approximate solutions valid for slow dissolving, provide new insights into the characteristics of diffusive dissolution in ternary systems. Dissolution rate is proportional to square root of time in diffusive dissolution. The coefficient of proportionality is a function of diffusion coefficients, liquidus relation, melt composition at the crystal–melt interface, and compositions of the dissolving crystal and starting melt. In the limit of slow dissolving, the dissolution rate can be written in terms of three dimensionless parameters that are functions of the aforementioned parameters. Dissolution rate is proportional to the diffusion rate of the slow eigen component in the melt when the diffusion rate of the minor eigen component is much slower than the diffusion rate of the major eigen component.Laboratory experiments of diffusive dissolution of single crystals and polycrystalline aggregates of quartz in a haplodacitic melt (25 wt.% CaO, 15 wt.% Al₂O₃, and 60 wt.% SiO₂) were conducted at 1500°C and 1 GPa. Measured dissolution distances (X_b, in microns) are proportional to the square root of experimental run time (t, in seconds), X_b = −0.620 (±0.019) √t. Chemical concentration profiles measured from quenched melts are invariant with time when displayed against the distance (measured from the crystal–melt interface) normalized by the square root of time. The melt compositions at the crystal–melt interface, extrapolated from the measured diffusion profiles in the quenched melts, are within 0.2 wt.% of the independently measured quartz liquidus in the ternary CaO–Al₂O₃–SiO₂ at 1500°C and 1 GPa. These results suggest that crystal and melt are in chemical equilibrium at their interface shortly after the onset of dissolution. Diffusive dissolution of quartz and quartzite is characterized by slow dissolving. Using quartz liquidus as one of the boundary conditions, it has been shown that the calculated dissolution distances and concentration profiles are in good agreement with the experimentally measured ones. Coupled diffusion played an essential role in quartz and quartzite dissolution in haplodacitic to haplobasaltic melts, and is likely to play an important role in diffusion-limited kinetic processes such as crystal growth and dissolution in natural melts of basaltic–rhyolitic compositions.