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The effect of ductile deformation on the kinetics and mechanisms of the aragonite-calcite transformation 总被引：2，自引：0，他引：2
Abstract The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330° C and 360° C) and 900-1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10-6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry-stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry-stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism. 相似文献
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The Ni-S System and Related Minerals 总被引：1，自引：0，他引：1
The system Ni-S has been studied systematically from 200? to1, 030? C by means of evacuated, sealed silica-glass tube experimentsand differential thermal analyses. Compounds in the system areNi3S2 (and a high temperature, non-quenchable Ni3?S2 phase),Ni7S6, Ni1–S4 Ni3S4, and NiS2. The geologic occurrenceof the minerals heazlewoodite (Ni2S2), millerite (ßSNi1-2S),polydymite (Ni3S4), and vaesite (NiS2) can now be describedin terms of the stability ranges of their synthetic equivalents. Hexagonal heazlewoodite, which is stoichiometric within thelimit of error of the experiments, inverts on heating to a tetragonalor pseudotetragonal phase at 556? C. This high-temperature phase(Ni3 has a wide field of stability, from 23.5 to 30.5 wt percent sulfur at 600? C, and melts incongruently at 806??3? C.The ßNi7S6 phase inverts to Ni78 at 397? C6 when inequilibrium with Ni3S2, and at 400? C when in equilibrium withNiS. Crystals of Ni7S6 break down to Ni3-S2+NiS at 573??3?C.The low-temperature form of Ni1-S1 corresponding to the mineralmillerite, is rhombohedral, and the high-temperature form hasthe hexagonal NiAs structure. Stoichiometric NiS inverts at379??3?C, whereas Ni1-S with the maximum nickel deficiency invertsat 282??5OC. The Ni1-alphS-NiS2 solvus was determined to 985??3?C,the eutectic temperature of these phases. Stoichiometric NiSis stable at 600?C but breaks down to Ni2-S2 and Ni1-S below797?C, whereas Ni1-S with 38.2 wt per cent sulfur melts congruentlyat 992??3?C. Vaesite does not vary measurably from stoichiometricNiS2 composition, and melts congruently at 1.007?5?C. Polydymitebreaks down to aNi-S? vaesite at 356??3?C. Differential thermalanalyses showed the existence of a two-liquid field in the sulfur-richportion of the system above 991?C and over a wide compositionalrange. 相似文献
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ABSTRACT There is no significant difference in the diffusion profiles across albite-adularia bicrystals that were simultaneously deformed at a strain rate of 10-6S-1 and those from hydrostatic experiments at the same conditions (1500 MPa and 1000°C for 156 h). This indicates that the bulk alkali diffusion rate, which is the sum of lattice diffusion (D, 1) and dislocation pipe diffusion (Dp), is not significantly enhanced by dislocations at these conditions, and that the maximum value for the ratio of Dp/D1 is about 105. This is equal to the value previously reported for‘oxygen’diffusion in albite. If this ratio is independent of temperature, the contribution of either static (pre-deformed) or moving (syn-deformed) dislocations to the bulk diffusion rate of alkalis is probably minor at all metamorphic conditions. For Al and Si diffusion the ratio of Dp/D1 may be larger if D1 is lower. Thus a significant contribution from dislocations to bulk diffusion cannot be ruled out, especially during simultaneous deformation. 相似文献
4.
The nepheline-kalsilite exsolution reaction was studied isothermallybetween 400 and 700°C. Under nonaqueous conditions the mechanisminvolves nucleation of kalsilite and growth by diffusion ofthe alkalis. As predicted by simple nucleation theory, the nucleationrate and hence the over-all exsolution rate are strongly dependenton the supersaturation of the nepheline. A decrease in temperatureat constant composition increases the supersaturation and therebythe nucleation rate. This increased nucleation rate is opposedby the decrease in the growth rate due to slower volume diffusion.At a supersaturation of more than 8–10 mole per cent thenumber of nuclei is large and the over-all exsolution rate isdetermined primarily by the growth rate. The activation energyfor growth is 28 kcal/mole. An increase of two kilobars in thehydrostatic pressure has little effect on the kinetics of thereaction. Under nonhydrostatic conditions the exsolution rateincreases significantly because the nucleation rate is faster. Under hydrothermal conditions the ‘exsolution’ rateis approximately two orders of magnitude faster due to a modificationin the mechanism. Partial dissolution of the original solidsolution in distilled water creates a condition of nonequilibriumin which the fluid is sodium-rich. Rapid alkali exchange eliminatesthis condition but produces the equilibrium compositions ofthe solids because kalsilite nucleates and grows in contactwith the fluid. The experimental evidence for this mechanismincludes X-ray diffraction data showing a gradual change inthe composition of the initial supersaturated solid, essentiallyidentical activation energies for growth under aqueous and nonaqueousconditions, and a lower percentage of oxygen isotope exchangethan ‘exsolution’ in the same experiment. 相似文献
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Kinetics and Mechanism of Pyrite Exsolution from Pyrrhotite 总被引：4，自引：0，他引：4
Pyrite exsolution from pyrrhotite at 325 °C occurs by heterogeneousnucleation of pyrite and subsequent growth by volume diffusionof iron away from the nuclei. Sulfur atoms are required to moveonly short distances and although their diffusivity is muchlower than iron, their movement is not the rate determiningfactor. The exsolution rate is primarily dependent on the nucleationrate, which increases with the degree of supersaturation. Inaddition, the impurities in National Bureau of Standards 55diron or 500 ppm of As, Sb, or Bi retard the exsolution rateby two or more orders of magnitude at 325 °C. This reductionis primarily the result of a lower nucleation rate and is believedto be due to a decreased vacancy mobility caused by a high bindingenergy of vacancies with the slow diffusing impurities. Thusthe strain associated with the nucleation cannot be as easilyrelieved. The impurities may also reduce the growth rate bycausing a decrease in mobility of the pyrite interface. The most important general aspect of this study is the effectof a few hundred parts per million of an impurity on the exsolutionrate of this reaction. This suggests that the rate of othermineral reactions may also be dependent on impurities; however,the importance of this factor in any particular mineral reactionrequires experimental verification. 相似文献
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Thermal Stability of Assemblages in the Cu--Fe--S System 总被引：1，自引：0，他引：1
The phase relations in the Cu-Fe-S system were determined from700 C to approximately 200 C in most portions of the systemand below 100 C in restricted areas. Approximate solid solutionlimits for bornite, chalcopyrite, and pyrrhotite were determinedat elevated temperatures. At low temperatures emphasis was placedon establishing the stable assemblages and less on determiningthe compositions of coexisting phases. At 700 C two extensiveternary solid solutions dominate the phase relations in thissystem. One of these solid solutions (bornite) includes thecompositions Cu2S, Cu18S, and Cu5FeS4and the other (chalcopyrite)lies with in the area bounded by the compositions CuFeS2 CuFe2S3,and CU3Fe4S4. The two fields are separated by approximately10 weight per cent copper at 700 C. The chalcopyrite volume,as seen in a trigonal prism representing temperature and composition,is intersected by a miscibility gap below approximately 600C.Below this temperature the two one-phase volumes are referredto as chalcopyrite and cubanite. Chalcopyrite is tetragonalat low temperature but isometric above approximately 550C.The temperature of the transformation is a function of composition.Cubanite is isometric above 252C, tetragonal from 252 to atleast 213C, and orthorhombic at lower temperature. The temperatureof the second transformation is unknown because the tetragonal-to-orthorhombictransformation has not been achieved in the laboratory. Borniteand pyrite become stable together at 568C and coexist downto 228C. Covellite appears with lowering temperature at 507C,and idaite at 501C. Idaite—pyrite and idaite—borniteare stable assemblages below 501 C. The composition of bornitecoexisting with idaite changes gradually towards digenite withdecreasing temperature, thus permitting the change from thebornite—pyrite tie-line to the digenite—chalcopyritetie-line at 228C. Other major tie-line changes are bornite—ironto pyrrhotite—copper below 475C and cubanite—pyriteto chalcopyrite—pyrrhotite below 334C. A new syntheticphase, x-bornite, which has a composition close to bornite (Cu5FeS4)but contains about 04 weight per cent more sulfur, forms whensulfur-rich bornite synthesized at high temperature is annealedbetween 62 and 140C. Optically this new phase is very similarto bornite, and their X-ray powder diffraction patterns aregiven for comparison. o The determined phase relations are applicable to numerous deposits.The tie-line changes involving bornitepyrite reacting to producedigenitechalcopyrite below 228 C and cubanite (isometric)pyritegoing to chalcopyritepyrrhotite below 334 C are of considerablegeological interest. The rates of these reactions are sufficientlyslow to allow the higher temperature assemblages to be observedin some ores. The cubic—tetragonal inversion in chalcopyriteis often deduced in ores by inversion twins. However, twinningis also commonly produced through deformation. Geological applicationof the inversion therefore depends on correct interpretationof the twinning. Because of the considerable solubility of copperin pyrrhotite the pyrrhotite—pyrite solvus of the pureFe—S system cannot be applied indiscriminately to oresthat also contain chalcopyrite or cubanite, or both. The newx-bornite phase was identified with the natural ‘anomalousbornites’, which when heated exsolve chalcopyrite and,depending on their composition, also digenite. The experimental results indicate that the mineral commonlyidentified as chalcopyrrhotite is in reality tetragonal or evenisometric cubanite. Experimental evidence could not be obtainedfor the existence of a phase of Cu2Fe4S7 or Cu2Fe4S7 composition,the older formulae given foor valleriite. The thermal breakdownof natural material supports the idea that valleriite is a low-temperaturepolymorph of chalcopyrite. The relatively uncommon occurrenceof idaite in comparison to covellite is attributed to the greaterdifficulty in nucleating idaite. The possibility of stable coexistenceof chalcocite and pyrite was investigated but was found to beprohibited by tie-lines between bornite and digenite even aslow as 100 C. 相似文献
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