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Yanjing Chen - One of the best experts on this subject based on the ideXlab platform.

  • the 1 85 ga mo mineralization in the xiong er terrane china implications for metallogeny associated with assembly of the columbia supercontinent
    Precambrian Research, 2011
    Co-Authors: Yanjing Chen, M Santosh, Junming Yao, Yali Sun
    Abstract:

    Abstract The East Qinling region in China, hosting several tens of Mesozoic magmatic-hydrothermal Mo deposits, is one of the largest molybdenum belts in the world. The recently discovered Longmendian Mo deposit, Luoning County, Henan Province, occurs within the Xiong’er Terrane of the East Qinling Mo belt. Here, the Mo mineralization is confined to altered amphibolite which is replaced by Microcline, actinolite, tremolite, epidote and carbonate. Molybdenite is also observed in Microcline-dominated felsic differentiates, but occurs within the altered amphibolite enclaves. Here we report Re–Os isotope analyses of five molybdenite and four pyrite samples from Longmendian which yield Re–Os isochron ages of 1853 ± 36 Ma and 1855 ± 29 Ma, respectively. These ages are in good agreement with the metamorphic age of the Taihua Supergroup (1.87–1.84 Ga). The Re–Os data presented in this study indicate that the period of ∼1.85 Ga was a time of widespread melting generating leucocratic Microcline-dominated granitic melts. The timing also coincides with the collision of the Eastern and Western Blocks of the North China Craton and its final cratonization at ∼1.85 Ga, coinciding with the incorporation of the North China Craton within the Paleoproterozoic supercontinent Columbia.

  • the 1 85ga mo mineralization in the xiong er terrane china implications for metallogeny associated with assembly of the columbia supercontinent
    Precambrian Research, 2011
    Co-Authors: Yanjing Chen, M Santosh, Junming Yao, Yunpeng Sun
    Abstract:

    Abstract The East Qinling region in China, hosting several tens of Mesozoic magmatic-hydrothermal Mo deposits, is one of the largest molybdenum belts in the world. The recently discovered Longmendian Mo deposit, Luoning County, Henan Province, occurs within the Xiong’er Terrane of the East Qinling Mo belt. Here, the Mo mineralization is confined to altered amphibolite which is replaced by Microcline, actinolite, tremolite, epidote and carbonate. Molybdenite is also observed in Microcline-dominated felsic differentiates, but occurs within the altered amphibolite enclaves. Here we report Re–Os isotope analyses of five molybdenite and four pyrite samples from Longmendian which yield Re–Os isochron ages of 1853 ± 36 Ma and 1855 ± 29 Ma, respectively. These ages are in good agreement with the metamorphic age of the Taihua Supergroup (1.87–1.84 Ga). The Re–Os data presented in this study indicate that the period of ∼1.85 Ga was a time of widespread melting generating leucocratic Microcline-dominated granitic melts. The timing also coincides with the collision of the Eastern and Western Blocks of the North China Craton and its final cratonization at ∼1.85 Ga, coinciding with the incorporation of the North China Craton within the Paleoproterozoic supercontinent Columbia.

T. Peter - One of the best experts on this subject based on the ideXlab platform.

  • the impact of bio organic substances on the ice nucleation activity of the k feldspar Microcline in aqueous solutions
    Atmospheric Chemistry and Physics, 2021
    Co-Authors: Kristian Klumpp, C. Marcolli, T. Peter
    Abstract:

    Abstract. Potassium-feldspars (K-feldspars), such as Microcline, are considered key dust minerals inciting ice nucleation in mixed phase clouds. Besides the high ice nucleation activity of Microcline, recent studies also revealed a high sensi-tivity of Microcline towards interaction with solutes on its surface. Here, we investigate the effect of organic and bio-organic substances on the ice nucleation activity of Microcline, with the aim to better understand the underlying sur-face interactions. We performed immersion freezing experiments with Microcline in solutions of three carboxylic acids, five amino acids and two polyols to represent these compound classes. By means of a differential scanning calorimeter we investigated the freezing of emulsified droplets of Microcline suspended in various solutions. Depend-ing on the type of solute, different effects were observed. In the case of carboxylic acids (acetic, oxalic and citric acid), the measured heterogeneous onset temperatures, Thet, showed no significant deviation from the behavior pre-dicted by the water activity criterion, Thet(aw) = Tmelt(aw+Δaw), which relates Thet with the melting point temperature Tmelt via a constant water activity offset Δaw. While this behavior could be interpreted as a lack of interaction of the solute molecules with the surface, the carboxylic acids caused the fraction of heterogeneously frozen water, Fhet(aw), to decrease by up to 40 % with increasing solute concentrations. In combination, unaltered Thet(aw) and reduced Fhet(aw) suggest that active sites were largely deactivated by the acid molecules, but amongst those remaining active are also the best sites with the highest Thet. A deviation from this behavior is citric acid, which showed not only a de-crease in Fhet, but also a decrease in Thet of up to 4 K for water activities below 0.99, pointing to a depletion of the best active sites by interactions with the citrate ions. When neutralized solutions of the acids were used instead, the de-crease in Fhet became even more pronounced. The slope of Thet(aw) was different for each of the neutralized acid solu-tions. In the case of amino acid solutions, we found a decrease in Thet (up to 10 K), significantly below the Δaw-criterion, as well as a reduction in Fhet (up to 60 %). Finally, in case of the investigated polyols, no significant devia-tion of Thet from the Δaw-criterion was observed, and no significant deviation of Fhet in comparison to a pure water suspension was found. Furthermore, we measured the effects of aging on the ice nucleation activity in experiments with Microcline suspended in solutions for up to seven days, and tested the reversibility of the interaction with the solutes after aging for 10 days. For citric acid, an ongoing irreversible degradation of the ice nucleation activity was observed, whereas the amino acids showed completely reversible effects. In summary, our experiments demonstrate a remarkable sensitivity of Microcline ice nucleation activity to surface interactions with various solutes, underscoring the importance of the history of such particles from source to frozen cloud droplet in the atmosphere.

  • ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions part 1 the k feldspar Microcline
    Atmospheric Chemistry and Physics, 2018
    Co-Authors: A. Kumar, C. Marcolli, B. Luo, T. Peter
    Abstract:

    Potassium-containing feldspars (K-feldspars) have been considered as key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar Microcline, which is highly IN active. Freezing of emulsified droplets with Microcline suspended in aqueous solutions of NH 3 , (NH 4 ) 2 SO 4 , NH 4 HSO 4 , NH 4 NO 3 , NH 4 Cl, Na 2 SO 4 , H 2 SO 4 , K 2 SO 4 and KCl, with solute concentrations corresponding to water activities a w   =  0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, T het ( a w ), deviate strongly from T het Δ a w het ( a w ), the values calculated from the water-activity-based approach (where T het Δ a w het ( a w ) =  T melt ( a w  + Δ a w het ) with a constant offset Δ a w het with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH 3 and NH 4 + salts (molalities ≲ 1 mol kg −1 corresponding to a w ≳  0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of Microcline in pure water ( T het ( a w  = 1)) and 5.5 K above T het Δ a w het ( a w ), revealing NH 3 and NH 4 + to significantly enhance the IN of the Microcline surface. Conversely, more concentrated NH 3 and NH 4 + solutions show a depression of the onset temperature below T het Δ a w het ( a w ) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH 4 + as cation exhibit nucleation temperatures T het ( a w )  T het Δ a w het ( a w ) even at very small solute concentrations. In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from Δ a w het   =  const. indicates specific chemical interactions between particular solutes and the Microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K + available on the Microcline surface with externally added cations (e.g., NH 4 + ). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH 3 molecules hydrogen bonded on the Microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen fraction in dilute NH 3 and NH 4 + solutions. Moreover, we show that aging of Microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near-neutral solutions since most of it recovers when aged particles are resuspended in pure water. In contrast, exposure to severe acidity (pH  ≲ 1.2) or alkalinity (pH  ≳ 11.7) damages the Microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for IN in airborne dust containing Microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and acidic conditions to a 5 K enhancement during condensation freezing when Microcline particles experience high humidity ( a w ≳ 0.96) at warm (252–257 K) and NH 3 /NH 4 + -rich conditions.

  • enhanced ice nucleation efficiency of Microcline immersed in dilute nh 3 and nh 4 containing solutions
    Atmospheric Chemistry and Physics, 2018
    Co-Authors: A. Kumar, C. Marcolli, B. Luo, T. Peter
    Abstract:

    Potassium containing feldspars (K-feldspars) have been considered key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar Microcline, which is highly IN active. Freezing of emulsified droplets with Microcline suspended in aqueous solutions of NH 3 , (NH 4 ) 2 SO 4 , NH 4 HSO 4 , NH 4 NO 3 , NH 4 Cl, Na 2 SO 4 , H 2 SO 4 , K 2 SO 4 and KCl, with solute concentrations corresponding to water activities a w  = 0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous ice nucleation onset temperatures, T het ( a w ) deviate strongly from T het ∆ a w het ( a w ), the values calculated from the water-activity-based approach (where T het ∆ a w het ( a w ) =  T melt ( a w  + ∆ a w het ) with a constant offset ∆ a w het with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH 3 and NH 4 + -salts (molalities −1 corresponding to a w  > ~ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of Microcline in pure water ( T het ( a w  = 1)) and 5.5 K above T het ∆ a w het ( a w ), revealing NH 3 and NH 4 + to significantly enhance the IN of the Microcline surface. Conversely, more concentrated NH 3 and NH 4 + solutions show a depression of the onset temperature below T het ∆ a w het ( a w ) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH 4 + as cation exhibit nucleation temperatures T het ( a w )  T het ∆ a w het ( a w ) even at very small solute concentrations. In all these cases, the fraction of water volume frozen heterogeneously displays a decrease as solute concentration increases. This deviation from ∆ a w het  = const. indicates specific chemical interactions between particular solutes and the Microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K + available on the Microcline surface with externally added cations (e.g.NH 4 + ). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH 3 molecules hydrogen bonded on the Microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen fraction in dilute NH 3 and NH 4 + solutions. Moreover, we show that aging of Microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near neutral solutions since most of it recovers when aged particles are re-suspended in pure water. In contrast, exposure to severe acidity (pH   ~ 11.7) damages the Microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for ice nucleation on airborne dust containing Microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and NH 3 /NH 4 + -free conditions, to a 5-K enhancement during condensation freezing when Microcline particles experience high humidity ( a w  > ~ 0.96) at warm (252–257 K) and NH 3 /NH 4 + -rich conditions.

  • Ice nucleation activity of silicates and aluminosilicates in pure water and aqueous solutions – Part 1: The K-feldspar Microcline
    Copernicus Publications, 2018
    Co-Authors: A. Kumar, C. Marcolli, B. Luo, T. Peter
    Abstract:

    Potassium-containing feldspars (K-feldspars) have been considered as key mineral dusts for ice nucleation (IN) in mixed-phase clouds. To investigate the effect of solutes on their IN efficiency, we performed immersion freezing experiments with the K-feldspar Microcline, which is highly IN active. Freezing of emulsified droplets with Microcline suspended in aqueous solutions of NH3, (NH4)2SO4, NH4HSO4, NH4NO3, NH4Cl, Na2SO4, H2SO4, K2SO4 and KCl, with solute concentrations corresponding to water activities aw  =  0.9–1.0, were investigated by means of a differential scanning calorimeter (DSC). The measured heterogeneous IN onset temperatures, Thet(aw), deviate strongly from ThetΔawhet(aw), the values calculated from the water-activity-based approach (where ThetΔawhet(aw) = Tmelt(aw + Δawhet) with a constant offset Δawhet with respect to the ice melting point curve). Surprisingly, for very dilute solutions of NH3 and NH4+ salts (molalities ≲1 mol kg−1 corresponding to aw ≳ 0.96), we find IN temperatures raised by up to 4.5 K above the onset freezing temperature of Microcline in pure water (Thet(aw = 1)) and 5.5 K above ThetΔawhet(aw), revealing NH3 and NH4+ to significantly enhance the IN of the Microcline surface. Conversely, more concentrated NH3 and NH4+ solutions show a depression of the onset temperature below ThetΔawhet(aw) by as much as 13.5 K caused by a decline in IN ability accompanied with a reduction in the volume fraction of water frozen heterogeneously. All salt solutions not containing NH4+ as cation exhibit nucleation temperatures Thet(aw) < ThetΔawhet(aw) even at very small solute concentrations. In all these cases, the heterogeneous freezing peak displays a decrease as solute concentration increases. This deviation from Δawhet  =  const. indicates specific chemical interactions between particular solutes and the Microcline surface not captured by the water-activity-based approach. One such interaction is the exchange of K+ available on the Microcline surface with externally added cations (e.g., NH4+). However, the presence of a similar increase in IN efficiency in dilute ammonia solutions indicates that the cation exchange cannot explain the increase in IN temperatures. Instead, we hypothesize that NH3 molecules hydrogen bonded on the Microcline surface form an ice-like overlayer, which provides hydrogen bonding favorable for ice to nucleate on, thus enhancing both the freezing temperatures and the heterogeneously frozen fraction in dilute NH3 and NH4+ solutions. Moreover, we show that aging of Microcline in concentrated solutions over several days does not impair IN efficiency permanently in case of near-neutral solutions since most of it recovers when aged particles are resuspended in pure water. In contrast, exposure to severe acidity (pH ≲1.2) or alkalinity (pH ≳11.7) damages the Microcline surface, hampering or even destroying the IN efficiency irreversibly. Implications for IN in airborne dust containing Microcline might be multifold, ranging from a reduction of immersion freezing when exposed to dry, cold and acidic conditions to a 5 K enhancement during condensation freezing when Microcline particles experience high humidity (aw≳0.96) at warm (252–257 K) and NH3/NH4+-rich conditions

Parampreet Kaur - One of the best experts on this subject based on the ideXlab platform.

  • new evidence for two sharp replacement fronts during albitization of granitoids from northern aravalli orogen northwest india
    International Geology Review, 2015
    Co-Authors: Parampreet Kaur, Naveen Chaudhri, Albrecht W Hofmann
    Abstract:

    We present new evidence of infiltration metasomatism in granitoids that were albitized in a process that produced two sharp replacement fronts, both of which are clearly visible in the field. The two fronts advanced through the original granite simultaneously, but at different rates. Here we focus mainly on the Ajitgarh intrusive in the northern Aravalli orogen of northwest India. This intrusion shows geographically well-defined metasomatic zones on the outcrop scale as well as a large volume of original ferroan granite, both of which were poorly preserved in most of the previously studied Khetri granites. Stage I metasomatism transformed the grey original granite to pink Microcline–albite granite, and stage II converted the Microcline–albite granite to white albite granite. Both these reaction fronts are sharp and are easily recognized in the field by their different colours. The mineralogical and chemical changes during the first stage are expressed by transformation of original oligoclase to albite, bi...

  • Metasomatism of ferroan granites in the northern Aravalli orogen, NW India: geochemical and isotopic constraints, and its metallogenic significance
    International Journal of Earth Sciences, 2014
    Co-Authors: Parampreet Kaur, Susanne Skora, Ingrid Raczek, Martin Okrusch, Naveen Chaudhri, Albrecht W Hofmann, Jürgen Koepke
    Abstract:

    The late Palaeoproterozoic (1.72–1.70 Ga) ferroan granites of the Khetri complex, northern Aravalli orogen, NW India, were extensively metasomatised ~900 Ma after their emplacement, at around 850–830 Ma by low-temperature (ca. 400 °C) meteoric fluids that attained metamorphic character after exchanging oxygen with the surrounding metamorphic rocks. Albitisation is the dominant metasomatic process that was accompanied by Mg and Ca metasomatism. A two-stage metasomatic model is applicable to all the altered ferroan intrusives. The stage I is represented by a metasomatic reaction interface that developed as a result of transformation of the original Microcline–oligoclase (An_12–14) granite to Microcline–albite (An_1–3) granite, and this stage is rarely preserved. In contrast, the stage II metasomatic reaction front, where the Microcline-bearing albite granite has been transformed to Microcline-free albite granite, is readily recognisable in the field and present in most of the intrusives. Some of them lack an obvious reaction interface due to the presence of stage II albite granites only. When studied in isolation, these intrusives were incorrectly classified and their tectonic setting was misinterpreted. Furthermore, our results show that the mafic mineralogy of metasomatised granites has a significant impact on the characterisation of such rocks in the magmatic classification and discrimination diagrams. Nevertheless, the stage I metasomatised granites can be appropriately characterised in these diagrams, whereas the characterisation of the stage II granites will lead to erroneous interpretations. The close spatial association of these high heat producing ferroan granites with iron oxide–copper–gold (IOCG), U and REE mineralisation in the region indicates a genetic link between the metasomatism and the mineralisation. World-class IOCG, U and REE deposits are associated with metasomatised ferroan granites, suggesting that such a relationship may act as a critical first-order exploration target for undiscovered mineral deposits.

  • two stage extreme albitization of a type granites from rajasthan nw india
    Journal of Petrology, 2012
    Co-Authors: Parampreet Kaur, Susanne Skora, Ingrid Raczek, Martin Okrusch, Naveen Chaudhri, Albrecht W Hofmann, Lukas P Baumgartner
    Abstract:

    Albitization is a common process during which hydrothermal fluids convert plagioclase and/or K-feldspar into nearly pure albite; however, its specific mechanism in granitoids is not well understood. The c. 1700 Ma A-type metaluminous ferroan granites in the Khetri complex of Rajasthan, NW India, have been albitized to a large extent by two metasomatic fronts, an initial transformation of oligoclase to nearly pure albite and a subsequent replacement of Microcline by albite, with sharp contacts between the Microcline-bearing and Microcline-free zones. Albitization has bleached the original pinkish grey granite and turned it white. The mineralogical changes include transformation of oligoclase (similar to An(12)) and Microcline (similar to Or(95)) to almost pure albite (similar to An(0 center dot 5-2)), amphibole from potassian ferropargasite (X-Fe 0 center dot 84-0 center dot 86) to potassic hastingsite (X-Fe 0 center dot 88-0 center dot 97) and actinolite (X-Fe 0 center dot 32-0 center dot 67), and biotite from annite (X-Fe 0 center dot 71-0 center dot 74) to annite (X-Fe 0 center dot 90-0 center dot 91). Whole-rock isocon diagrams show that, during albitization, the granites experienced major hydration, slight gain in Si and major gain in Na, whereas K, Mg, Fe and Ca were lost along with Rb, Ba, Sr, Zn, light rare earth elements and U. Whole-rock Sm-Nd isotope data plot on an apparent isochron of 1419 +/- 98 Ma and reveal significant disturbance and at least partial resetting of the intrusion age. Severe scatter in the whole-rock Rb-Sr isochron plot reflects the extreme Rb loss in the completely albitized samples, effectively freezing Sr-87/Sr-86 ratios in the albite granites at very high values (0 center dot 725-0 center dot 735). This indicates either infiltration of highly radiogenic Sr from the country rock or, more likely, radiogenic ingrowth during a considerable time lag (estimated to be at least 300 Myr) between original intrusion and albitization. The albitization took place at similar to 350-400 degrees C. It was caused by the infiltration of an ascending hydrothermal fluid that had acquired high Na/K and Na/Ca ratios during migration through metamorphic rocks at even lower temperatures in the periphery of the plutons. Oxygen isotope ratios increase from delta O-18 = 7 parts per thousand in the original granite to values of 9-10 parts per thousand in completely albitized samples, suggesting that the fluid had equilibrated with surrounding metamorphosed crust. A metasomatic model, using chromatographic theory of fluid infiltration, explains the process for generating the observed zonation in terms of a leading metasomatic front where oligoclase of the original granite is converted to albite, and a second, trailing front where Microcline is also converted to albite. The temperature gradients driving the fluid infiltration may have been produced by the high heat production of the granites themselves. The confinement of the albitized granites along the NE-SW-trending Khetri lineament and the pervasive nature of the albitization suggest that the albitizing fluids possibly originated during reactivation of the lineament. More generally, steady-state temperature gradients induced by the high internal heat production of A-type granites may provide the driving force for similar metasomatic and ore-forming processes in other highly enriched granitoid bodies.

Kim A. Waldron - One of the best experts on this subject based on the ideXlab platform.

  • strain driven disordering of low Microcline to low sanidine during partial phase separation in microperthites
    Contributions to Mineralogy and Petrology, 1997
    Co-Authors: William L Brown, Kim A. Waldron, Martin Lee, Ian Parsons
    Abstract:

    Microperthitic feldspar crystals containing low Microcline in a braid intergrowth often have distinctive microtextures including coarse semi- to in-coherent grain-boundary pleated rims and fine coherent intracrystalline Ab- and Or-rich pleats (Lee et al. 1997). The coarser pleated rims are generally separated from the braid microtexture in the crystal interior by a coherent to semi-coherent transitional zone. Partial phase separation has occurred in the transitional zone in step with that in the Ab- and Or-rich pleats at the grain boundaries, such that Ab-rich lamellar film micro-antiperthite alternates along (010) with more Or-rich lamellar film microperthite; the microtextures and phases are those expected for the respective local bulk compositions. Lamellar microtextures contain tweed orthoclase, whereas low Microcline is the only K-feldspar in the fine coherent pleats and braid microperthite. We propose that the small coherent pleats developed from the braid microtexture by interaction of the spontaneous coherency strains with discontinuities within or at the surface of the crystal, and that their initial spacing is guided by that of the braid microperthite. We infer that the transitional zone formed by straightening of the zig-zag braid microtexture above the pleat heads during coarsening and partial phase separation. We further infer that the resulting coherency shear strains induced a reversal of the K-feldspar phase transformation, involving Si, Al disordering of low Microcline into low sanidine, now tweed orthoclase, although the crystal was at a T within the hydrostatic T-stability of Microcline.

  • exsolution and alteration microtextures in alkali feldspar phenocrysts from the shap granite
    Mineralogical Magazine, 1995
    Co-Authors: M R Lee, Kim A. Waldron, Ian Parsons
    Abstract:

    Alkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ⩽ 590°C by heterogeneous nucleation of thin albite films which coarsened to > 1 µm length. Cryptoperthites developed at < 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular Microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular Microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular Microcline.

  • solution redeposition and the orthoclase Microcline transformation evidence from granulites and relevance to 18o exchange
    Mineralogical Magazine, 1993
    Co-Authors: Kim A. Waldron, Ian Parsons, William L Brown
    Abstract:

    The Or-rich part of optically blebby to lamellar mesoperthite crystals from an Adirondack granulite has been shown by TEM to be a lamellar cryptoperthite, composed dominantly of tweed orthoclase. A fluid-absent, two-stage thermal history is proposed to explain the coarse and fine textures, with the cryptoperthite forming by coherent exsolution below ∼350°C probably during uplift. The mechanism was most probably homogeneous coherent nucleation rather than spinodal decomposition. However, cutting the orthoclase cryptoperthite are thin (<1 μm) seams of tartan Microcline with sharp boundaries, often defined locally by {110} planes, and micropores. The Microcline has replaced orthoclase by solution-redeposition along narrow planes infiltrated by fluid during minor retrogression at T < 350°C Solution-redeposition is a common process in feldspars at T < 500°C potentially accompanied by 18O exchange, because release of elastic strain energy in coherent perthite lamellar boundaries and twin-domain walls, followed by Si, Al ordering, provide driving forces for dissolution and reprecipitation of unstrained, more ordered phases.

  • solution redeposition and the orthoclase Microcline transformation evidence from granulites and relevance to 18o exchange
    Mineralogical Magazine, 1993
    Co-Authors: Kim A. Waldron
    Abstract:

    The Or-rich part of optically blebby to lamellar mesoperthite crystals from an Adirondack granulite has been shown by TEM to be a lamellar cryptoperthite, composed dominantly of tweed orthoclase. A fluid-absent, two-stage thermal history is proposed to explain the coarse and fine textures, with the cryptoperthite forming by coherent exsolution below ∼ 350 °C, probably during uplift. The mechanism was most probably homogeneous coherent nucleation rather than spinodal decomposition. However, cutting the orthoclase cryptoperthite are thin (< 1 μm) seams of tartan Microcline with sharp boundaries, often defined locally by {110} planes, and micropores. The Microcline has replaced orthoclase by solution-redeposition along narrow planes infiltrated by fluid during minor retrogression at T < 350°C

Ian Parsons - One of the best experts on this subject based on the ideXlab platform.

  • strain driven disordering of low Microcline to low sanidine during partial phase separation in microperthites
    Contributions to Mineralogy and Petrology, 1997
    Co-Authors: William L Brown, Kim A. Waldron, Martin Lee, Ian Parsons
    Abstract:

    Microperthitic feldspar crystals containing low Microcline in a braid intergrowth often have distinctive microtextures including coarse semi- to in-coherent grain-boundary pleated rims and fine coherent intracrystalline Ab- and Or-rich pleats (Lee et al. 1997). The coarser pleated rims are generally separated from the braid microtexture in the crystal interior by a coherent to semi-coherent transitional zone. Partial phase separation has occurred in the transitional zone in step with that in the Ab- and Or-rich pleats at the grain boundaries, such that Ab-rich lamellar film micro-antiperthite alternates along (010) with more Or-rich lamellar film microperthite; the microtextures and phases are those expected for the respective local bulk compositions. Lamellar microtextures contain tweed orthoclase, whereas low Microcline is the only K-feldspar in the fine coherent pleats and braid microperthite. We propose that the small coherent pleats developed from the braid microtexture by interaction of the spontaneous coherency strains with discontinuities within or at the surface of the crystal, and that their initial spacing is guided by that of the braid microperthite. We infer that the transitional zone formed by straightening of the zig-zag braid microtexture above the pleat heads during coarsening and partial phase separation. We further infer that the resulting coherency shear strains induced a reversal of the K-feldspar phase transformation, involving Si, Al disordering of low Microcline into low sanidine, now tweed orthoclase, although the crystal was at a T within the hydrostatic T-stability of Microcline.

  • exsolution and alteration microtextures in alkali feldspar phenocrysts from the shap granite
    Mineralogical Magazine, 1995
    Co-Authors: M R Lee, Kim A. Waldron, Ian Parsons
    Abstract:

    Alkali feldspar phenocrysts (bulk composition Or75.0Ab24.6An0.4) in the subsolvus Shap granite comprise a fine-scale mixture of subregular pristine crypto- and micro-perthites with altered, micropore-rich feldspar with irregular microstructures. The regular perthites are strain-controlled intergrowths of Albite and/or Periclinetwinned albite exsolution lamellae within tweed orthoclase. The microperthites formed at ⩽ 590°C by heterogeneous nucleation of thin albite films which coarsened to > 1 µm length. Cryptoperthites developed at < 400°C by homogeneous nucleation of sub-µm long platelets between films. Platelets are coherent, but the coarser microperthite lamellae are semi-coherent, with pairs of misfit dislocations sub-regularly spaced along the albite-orthoclase interface. As much as 30% of any one feldspar crystal is turbid, a result of the formation of numerous µm to sub-µm sized micropores during deuteric alteration. In some areas, deuteric fluids gained access to the interior of feldspar crystals by exploiting semi-coherent film lamellae. Albite was selectively dissolved and micropore-rich irregular Microcline was reprecipitated in its place. In other parts of the feldspars deuteric recrystallization completely cross-cuts the pristine microtextures and patch perthites have formed. These are coarse, incoherent to semi-coherent intergrowths of irregular Microcline (replacing tweed orthoclase) and Albite-twinned albite. The deuteric reactions occurred at < 400°C; the main driving force for dissolution and reprecipitation was decrease in the elastic strain energy at the coherent interfaces of crypto-and micro-perthite lamellae, and the recrystallization of tweed orthoclase to irregular Microcline.

  • solution redeposition and the orthoclase Microcline transformation evidence from granulites and relevance to 18o exchange
    Mineralogical Magazine, 1993
    Co-Authors: Kim A. Waldron, Ian Parsons, William L Brown
    Abstract:

    The Or-rich part of optically blebby to lamellar mesoperthite crystals from an Adirondack granulite has been shown by TEM to be a lamellar cryptoperthite, composed dominantly of tweed orthoclase. A fluid-absent, two-stage thermal history is proposed to explain the coarse and fine textures, with the cryptoperthite forming by coherent exsolution below ∼350°C probably during uplift. The mechanism was most probably homogeneous coherent nucleation rather than spinodal decomposition. However, cutting the orthoclase cryptoperthite are thin (<1 μm) seams of tartan Microcline with sharp boundaries, often defined locally by {110} planes, and micropores. The Microcline has replaced orthoclase by solution-redeposition along narrow planes infiltrated by fluid during minor retrogression at T < 350°C Solution-redeposition is a common process in feldspars at T < 500°C potentially accompanied by 18O exchange, because release of elastic strain energy in coherent perthite lamellar boundaries and twin-domain walls, followed by Si, Al ordering, provide driving forces for dissolution and reprecipitation of unstrained, more ordered phases.