The Experts below are selected from a list of 624 Experts worldwide ranked by ideXlab platform
Slawomir Ilnicki - One of the best experts on this subject based on the ideXlab platform.
-
variscan prograde p t evolution and contact metamorphism in metabasites from the sowia dolina karkonosze izera massif sw poland
Mineralogical Magazine, 2011Co-Authors: Slawomir IlnickiAbstract:Several bodies of moderately foliated and porphyroblastic metabasites crop out on the SE side of the metamorphic cover of the Karkonosze granite within metapelites of the Sowia Dolina area (West Sudetes, Saxothuringian zone). Depending on the microstructural setting of the Ca-amphiboles in the rocks, different mineral-chemical trends have been determined for Si, X Mg , Al VI , A [Na+K] which serve as semi-quantitative indicators of temperature and pressure changes. Porphyroblasts and prisms oblique to the main foliation in schistose metabasites show zoning from Mg-hornblende and actinolite to tschermakite, and then to Mg-hornblende (or actinolite). Matrix amphiboles and those in pressure shadows around some porphyroblasts have tschermakitic cores and actinolitic rims. Rarely, Ca-amphibole is accompanied in schists by late- to post-tectonic Cummingtonite. Thermobarometric calculations involving empirically calibrated amphibole equilibria enable a reconstruction of P-T paths for individual rocks and the unravelling of the metamorphic evolution of the metabasites. Peak metamorphic temperatures of 615–640°C and pressures of 7.3–8.2 kbar were preceded by a variably preserved earlier stage ( T = 370–550°C, P = 2.8–6.2 kbar). The final metamorphic episode took place at 450–550°C and 2.5–4.8 kbar and is recorded particularly in rocks close to the Karkonosze pluton. The metabasites shed new light on the history of metamorphism in the Sowia Dolina area. The first two stages of M P -M T metamorphism, coeval with Variscan deformation events (continental collision, burial and subsequent exhumation), took place under epidote-amphibolite then amphibolite facies conditions. The last stage partly concurred with the final stages of Variscan deformation and overlapped the onset of thermal activity associated with the Karkonosze granite. This metamorphic event is documented by metabasites (occasionally Cummingtonite-bearing) outcropping close to the granite. Finally, a prehnite-bearing assemblage reflects retrograde re-equilibration under greenschist/sub-greenschist facies conditions ( T P
-
variscan prograde p t evolution and contact metamorphism in metabasites from the sowia dolina karkonosze izera massif sw poland
Mineralogical Magazine, 2011Co-Authors: Slawomir IlnickiAbstract:Several bodies of moderately foliated and porphyroblastic metabasites crop out on the SE side of the metamorphic cover of the Karkonosze granite within metapelites of the Sowia Dolina area (West Sudetes, Saxothuringian zone). Depending on the microstructural setting of the Ca-amphiboles in the rocks, different mineral-chemical trends have been determined for Si, X Mg, AlVI, A [Na+K] which serve as semi-quantitative indicators of temperature and pressure changes. Porphyroblasts and prisms oblique to the main foliation in schistose metabasites show zoning from Mg-hornblende and actinolite to tschermakite, and then to Mg-hornblende (or actinolite). Matrix amphiboles and those in pressure shadows around some porphyroblasts have tschermakitic cores and actinolitic rims. Rarely, Ca-amphibole is accompanied in schists by late- to post-tectonic Cummingtonite. Thermobarometric calculations involving empirically calibrated amphibole equilibria enable a reconstruction of P-T paths for individual rocks and the unravelling of the metamorphic evolution of the metabasites. Peak metamorphic temperatures of 615–640°C and pressures of 7.3–8.2 kbar were preceded by a variably preserved earlier stage (T = 370–550°C, P = 2.8–6.2 kbar). The final metamorphic episode took place at 450–550°C and 2.5–4.8 kbar and is recorded particularly in rocks close to the Karkonosze pluton. The metabasites shed new light on the history of metamorphism in the Sowia Dolina area. The first two stages of MP-MT metamorphism, coeval with Variscan deformation events (continental collision, burial and subsequent exhumation), took place under epidote-amphibolite then amphibolite facies conditions. The last stage partly concurred with the final stages of Variscan deformation and overlapped the onset of thermal activity associated with the Karkonosze granite. This metamorphic event is documented by metabasites (occasionally Cummingtonite-bearing) outcropping close to the granite. Finally, a prehnitebearing assemblage reflects retrograde re-equilibration under greenschist/sub-greenschist facies conditions (T <300–350°C, P <2.5–3 kbar), which might also be partly due to hydrothermal activity around the pluton.
John C. Schumacher - One of the best experts on this subject based on the ideXlab platform.
-
Analytical electron microscopy of nanometer-scale hornblende lamellae; low-temperature exsolution in Cummingtonite
American Mineralogist, 1997Co-Authors: Ulrich Klein, Thomas G. Sharp, John C. SchumacherAbstract:Cummingtonite from central Massachusetts, which has experienced pervasive exsolution of hornblende, contains numerous hornblende precipitates (lamellae and discs) ranging in thickness from 2.5 mm to 4 nm that represent multiple generations of exsolution. The smallest precipitates were investigated using transmission electron microscopy and analytical electron microscopy to characterize the coherent Cummingtonite-hornblende solvus at low temperature. In contrast to older (coarser) generations of exsolution lamellae, which are 0.5‐2.5 mm in width, planar, and extend across entire grains, the youngest generation consists of nanometer-scale discs (4‐80 nm thick and 10‐1000 nm long) that represent exsolution at approximately 300 8C (Klein et al. 1996). Quantitative energy-dispersive X-ray analyses were obtained from discs as small as 20 nm in thickness, providing compositional data for coherent exsolution of hornblende at ;300 8C. Surprisingly, these tiny discs have a lower Ca content and a higher Al content than the coarser, higher temperature lamellae. These compositions appear to lie well within miscibility gaps of the equilibrium and coherent solvi for Cummingtonite-hornblende (in terms of Ca ↔ Fe 21 exchange) at low temperatures, and may represent a more tschermakitic hornblende coexisting with Cummingtonite with excess Al relative to Ca. The composition of the Cummingtonite host between nanometer-scale discs is very low in both Ca and Al relative to bulk Cummingtonite analyses that were obtained from electron microprobe analyses of 1‐2 mm areas. Compositional profiles across host areas were measured to help interpret the unexpected chemistries of the nanometer-scale discs. A profile across the precipitate-free zone between a coarse hornblende lamella and a group of nanometer-scale hornblende discs shows downward concavity for Ca and Al, as expected for incomplete diffusion between the precipitation zone and the pre-existing hornblende lamellae. A profile measured between two nanometer-scale discs shows downward concavity for Al and nearly constant to slight upward concavity for the Ca content, suggesting incomplete Al diffusion before completion of the exsolution process, but a near-equilibrium state for Ca. However, the Ca profile could also represent incomplete diffusion, if the hornblende discs had started coarsening at the expense of other (shrinking) discs by means of the Gibbs-Thomson (capillary) effect. The observed profiles suggest that, relative to Al, Ca was the faster diffusing element when the exsolution and coarsening processes stopped.
-
Mutual exsolution in hornblende and Cummingtonite; compositions, lamellar orientations, and exsolution temperatures
American Mineralogist, 1996Co-Authors: Ulrich Klein, John C. Schumacher, Michael CzankAbstract:Exsolved pairs of hornblende and Cummingtonite from two localities in southern New England, U.S.A., have been studied using transmission electron microscopy (TEM) to measure the crystallographic orientation of the exsolution lamellae. Both the Cummingtonite and the hornblende show multiple generations or stages of exsolution. Electron microprobe analyses gave compositions of the pre-exsolved (averages) and the coarsest exsolved amphiboles. In amphiboles, two orientations of lamellae are usually present and nearly parallel to {lOa} and {lOT}of the host. The observed lamellar orientations are consistent with orientations predicted by the optimal phase-boundary (OPB) theory and are a function of small differences in a, C,and {3of the host and exsolved amphiboles (~a = ahost - alamella, ~{3 = {3host - {3lamella, and ~c = Chost - Clamella)' Because these lattice parameters vary with T and composition, the precise orientations of the lamellae are controlled by these variables. The widths of determined exsolution lamellae varied from microscopic (micrometer size) to submicroscopic (nanometer size). The TEM images show, in some cases, multiple generations of lamellae with lamellar orientations near {lOT} differing up to 4.1°, whereas orientations near {lOa} differed up to 4°. For two pairs of coexisting amphiboles, the T dependence of the lattice parameters from 25 to 600 °C was measured using a Guinier camera. The OPB calculations indicate that for all the samples the different lamellar generations formed between about 780 and 300 °C (:t80 °C, on the basis of maximum errors of the lattice-parameter determinations). Lamellae of the same generation all showed nearly the same width. Exsolution temperatures could not be derived from the "100" lamellae because relatively small variations in ~c, which controls exact orientation of the "100" lamellae, could not be measured accurately enough.
Ulrich Klein - One of the best experts on this subject based on the ideXlab platform.
-
Analytical electron microscopy of nanometer-scale hornblende lamellae; low-temperature exsolution in Cummingtonite
American Mineralogist, 1997Co-Authors: Ulrich Klein, Thomas G. Sharp, John C. SchumacherAbstract:Cummingtonite from central Massachusetts, which has experienced pervasive exsolution of hornblende, contains numerous hornblende precipitates (lamellae and discs) ranging in thickness from 2.5 mm to 4 nm that represent multiple generations of exsolution. The smallest precipitates were investigated using transmission electron microscopy and analytical electron microscopy to characterize the coherent Cummingtonite-hornblende solvus at low temperature. In contrast to older (coarser) generations of exsolution lamellae, which are 0.5‐2.5 mm in width, planar, and extend across entire grains, the youngest generation consists of nanometer-scale discs (4‐80 nm thick and 10‐1000 nm long) that represent exsolution at approximately 300 8C (Klein et al. 1996). Quantitative energy-dispersive X-ray analyses were obtained from discs as small as 20 nm in thickness, providing compositional data for coherent exsolution of hornblende at ;300 8C. Surprisingly, these tiny discs have a lower Ca content and a higher Al content than the coarser, higher temperature lamellae. These compositions appear to lie well within miscibility gaps of the equilibrium and coherent solvi for Cummingtonite-hornblende (in terms of Ca ↔ Fe 21 exchange) at low temperatures, and may represent a more tschermakitic hornblende coexisting with Cummingtonite with excess Al relative to Ca. The composition of the Cummingtonite host between nanometer-scale discs is very low in both Ca and Al relative to bulk Cummingtonite analyses that were obtained from electron microprobe analyses of 1‐2 mm areas. Compositional profiles across host areas were measured to help interpret the unexpected chemistries of the nanometer-scale discs. A profile across the precipitate-free zone between a coarse hornblende lamella and a group of nanometer-scale hornblende discs shows downward concavity for Ca and Al, as expected for incomplete diffusion between the precipitation zone and the pre-existing hornblende lamellae. A profile measured between two nanometer-scale discs shows downward concavity for Al and nearly constant to slight upward concavity for the Ca content, suggesting incomplete Al diffusion before completion of the exsolution process, but a near-equilibrium state for Ca. However, the Ca profile could also represent incomplete diffusion, if the hornblende discs had started coarsening at the expense of other (shrinking) discs by means of the Gibbs-Thomson (capillary) effect. The observed profiles suggest that, relative to Al, Ca was the faster diffusing element when the exsolution and coarsening processes stopped.
-
Mutual exsolution in hornblende and Cummingtonite; compositions, lamellar orientations, and exsolution temperatures
American Mineralogist, 1996Co-Authors: Ulrich Klein, John C. Schumacher, Michael CzankAbstract:Exsolved pairs of hornblende and Cummingtonite from two localities in southern New England, U.S.A., have been studied using transmission electron microscopy (TEM) to measure the crystallographic orientation of the exsolution lamellae. Both the Cummingtonite and the hornblende show multiple generations or stages of exsolution. Electron microprobe analyses gave compositions of the pre-exsolved (averages) and the coarsest exsolved amphiboles. In amphiboles, two orientations of lamellae are usually present and nearly parallel to {lOa} and {lOT}of the host. The observed lamellar orientations are consistent with orientations predicted by the optimal phase-boundary (OPB) theory and are a function of small differences in a, C,and {3of the host and exsolved amphiboles (~a = ahost - alamella, ~{3 = {3host - {3lamella, and ~c = Chost - Clamella)' Because these lattice parameters vary with T and composition, the precise orientations of the lamellae are controlled by these variables. The widths of determined exsolution lamellae varied from microscopic (micrometer size) to submicroscopic (nanometer size). The TEM images show, in some cases, multiple generations of lamellae with lamellar orientations near {lOT} differing up to 4.1°, whereas orientations near {lOa} differed up to 4°. For two pairs of coexisting amphiboles, the T dependence of the lattice parameters from 25 to 600 °C was measured using a Guinier camera. The OPB calculations indicate that for all the samples the different lamellar generations formed between about 780 and 300 °C (:t80 °C, on the basis of maximum errors of the lattice-parameter determinations). Lamellae of the same generation all showed nearly the same width. Exsolution temperatures could not be derived from the "100" lamellae because relatively small variations in ~c, which controls exact orientation of the "100" lamellae, could not be measured accurately enough.
David M. Jenkins - One of the best experts on this subject based on the ideXlab platform.
-
Infrared and TEM characterization of amphiboles synthesized near the tremolite-pargasite join in the ternary system tremolite-pargasite-Cummingtonite
American Mineralogist, 2003Co-Authors: David M. Jenkins, Krassimir N. Bozhilov, Kiyotaka IshidaAbstract:High-resolution transmission-electron microscopy (HRTEM) and infrared spectroscopy (FTIR) analyses have been done on amphiboles near the join tremolite (Ca2Mg5Si8O22(OH)2 = TR)-pargasite (NaCa2Mg4Al3Si6O22(OH)2 = PG) in the ternary system tremolite-pargasite-Cummingtonite (Mg7Si8O22(OH)2 = MC) that were synthesized previously by Sharma and Jenkins (1999). Represen- tative samples across the join were examined in detail by HRTEM to document the presence and concentration of chain multiplicity defects (CMFs). There was relatively little change in the defect density with composition, with the tremolitic sample (TREM 23-13) having the highest defect con- centration (6%) and the more PG-rich samples having slightly lower CMF concentrations (4-5%). CMFs with multiplicities of 1, 3, 4, 5, and 6 were observed, usually as isolated chains, with the most common being triple-chain slabs. Correction of the bulk composition of the tremolitic amphibole for the presence of these Mg-rich, wide-chain defects reduces the MC content from an apparent value of 8.5 to 4-7.5 mol% MC, depending on which composition is used for the triple-chain defect. The entire amphibole join was examined by FTIR spectroscopy in the OH-stretching region (3000-3800 cm -1 ) for the purpose of determining the presence of short-range order. A total of 10 component bands were fitted to the spectra across the join. These bands were assigned to specific cation con- figurations on the basis of earlier studies of the FTIR spectra of chemically simplified amphibole joins pertinent to this study. The extent of short-range order was qualitatively determined by com- paring the observed intensities for groups of related bands, corrected for differences in their molar absorptivities, to their calculated intensities based on random-mixing probabilities. From this exer- cise, it is observed that the intensities of sodic amphibole configurations are consistently high, tremolite is lowest near the middle of the join, and aluminous amphibole configurations cross over from being higher (at low Al contents) to being lower (at high Al contents) than expected near the middle of the join. These differences between observed and predicted band intensities may reflect the presence of deviations in the thermodynamic activities of amphibole components from those predicted on the basis of random-mixing models.
-
Hydrothermal synthesis of amphiboles along the tremolite-pargasite join and in the ternary system tremolite-pargasite-Cummingtonite
American Mineralogist, 1999Co-Authors: Anurag Sharma, David M. JenkinsAbstract:We report here an investigation of amphibole synthesis within the ternary system Ca 2Mg5Si8O22(OH)2NaCa2Mg4Al3Si6O22(OH)2-Mg7Si8O22(OH)2 (= TR-PG-MC) to define better the limits of solid solution for amphiboles formed along and near the join tremolite-pargasite. Hydrothermal syntheses were conducted in the range of 750‐1000 °C and 1‐6 kbar. Syntheses at 10 mol% compositional intervals along the TR-PG join produced incomplete yields of amphibole, and the resultant amphiboles were found to be associated with varying amounts of accessory clinopyroxene, plagioclase, and gehlenite. Syntheses of end-member pargasite with progressively greater amounts of the MC component produced a very good amphibole yield for a pargasite composition containing 2.5 mol% MC. The persistence of clinopyroxene or gehlenite and the improved yield of pargasitic amphibole with minor addition of the MC component indicate a shift toward MC enrichment for amphiboles made from bulk compositions directly on the TR-PG join. Syntheses within the TR-PG-MC ternary system suggest that the Ca-rich limit of solid solution is a slightly curved field in the TR-PG-MC field. Amphiboles made along this join showed a very systematic change in the (151) and (3 ‐ 31) peak spacing with mol% PG, providing a simple technique for deducing the composition of amphiboles made near the tremolite-pargasite join. Electron microprobe analysis confirms that the amphiboles for the most part conform to the “pargasite” substitution. The one exception being amphiboles formed near 80‐90 mol% PG compositions, which proved both difficult to synthesize (~75 wt% amphibole yield) and displayed a small, but significant, excess of Na in their structure. High yields of amphibole could be formed from this same bulk composition with the use of a dilute NaOH solution instead of pure water, suggesting that there was insufficient Na in the starting mixture to allow complete amphibole formation. Structural refinements using X-ray diffraction (XRD) Rietveld refinement confirmed the high Na content for this sample and also revealed a non-linear trend in the unit-cell volume with composition, with a maximum at the 80 mol% PG bulk composition. The difficulty in synthesizing amphiboles near this bulk composition and its unusually high volume suggest that immiscibility in natural hornblendes may initiate with increasing pressure for pargasite-rich bulk compositions.
C M B Henderson - One of the best experts on this subject based on the ideXlab platform.
-
Mn-Mg disordering in Cummingtonite: a high-temperature neutron powder diffraction study
2020Co-Authors: J J Reece, Simon A T Redfern, Mark D Welch, C M B HendersonAbstract:The crystal structure of a manganoan Cummingtonite, composition [M4] (Na 0.13 Ca 0.41 Mg 0.46 Mn 1.00 ) [M1,2,3] (Mg 4.87 Mn 0.13 ) [Si 8 O 22 ](OH) 2 , (Z = 2), a = 9.5539(2) A Ê , b = 18.0293(3) A Ê , c = 5.2999(1) A Ê , b = 102.614(2)8 from Talcville, New York, has been refined at high temperature using in situ neutron powder diffraction. The P2 1 /m to C2/m phase transition, observed as spontaneous strains +e 1 = e 2 , occurs at~1078C. Long-range disordering between Mg 2+ and Mn 2+ on the M(4) and M(2) sites occurs above 5508C. Mn 2+ occupies the M(4) and M(2) sites preferring M(4) with a site-preference energy of 24.6+1.5 kJ mol 1 . Disordering induces an increase in X M2 Mn and decrease in X M4 Mn at elevated temperatures. Upon cooling, the ordered states of cation occupancy are 'frozen in' and strains in lattice parameters are maintained, suggesting that re-equilibration during cooling has not taken place
-
mn mg disordering in Cummingtonite a high temperature neutron powder diffraction study
Mineralogical Magazine, 2000Co-Authors: J J Reece, Simon A T Redfern, Mark D Welch, C M B HendersonAbstract:The crystal structure of a manganoan Cummingtonite, composition [M4](NaO. 13Ca0.41 Mg0.46Mnl.()O) lMI,2.3] . _ _ . _
