Temperatures and duration of crystallization within gem-bearing cavities of granitic pegmatites

London, David; Hunt, Lindsey; Duval, Charles L.

doi:10.1016/j.lithos.2020.105417

Cited by 9 publications

(8 citation statements)

References 50 publications

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“…However, following the results obtained with synchrotron and neutron data and temperature-dependent structural modelling, which reveal that PadPar topaz maintain its crystallinity and symmetry up to 1170 K, we can argue that PadPar topaz started its nucleation at this temperature. This value is coherent with the topaz stability field in the system Na 2 O–Al 2 O 3 –SiO 2 –F 2 O 63 , but it is also a rather high temperature for the formation of granitic pegmatites, (1075–625 K 68 – 70 ). However, liquidus temperatures of 1120 K was experimentally obtained for the initial crystallization of the topaz-albite granite assemblage from a supercritical fluid (~ 28 wt% H 2 O and ~ 45 wt% F completely miscible in all proportions at magmatic temperature and pressure) 71 .…”

Section: Discussionsupporting

confidence: 72%

F/OH ratio in a rare fluorine-poor blue topaz from Padre Paraíso (Minas Gerais, Brazil) to unravel topaz’s ambient of formation

Precisvalle

Martucci

Gigli

et al. 2021

Sci Rep

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Topaz [Al2SiO4(F,OH)2] is one of the main fluorine-bearing silicates occurring in environments where variably acidic (F)/aqueous (OH) fluids saturate the silicate system. In this work we fully characterized blue topaz from Padre Paraíso (Minas Gerais, Brazil) by means of in situ synchrotron X-Ray and neutron powder diffraction measurements (temperature range 298–1273 K) combined with EDS microanalyses. Understanding the role of OH/F substitution in topaz is important in order to determine the hydrophilicity and the exchange reactions of fluorine by hydroxyl groups, and ultimately to characterize the environmental redox conditions (H2O/F) required for mineral formation. The fluorine content estimated from neutron diffraction data is ~ 1.03 a.f.u (10.34 wt%), in agreement with the chemical data (on average 10.0 wt%). The XOH [OH/(OH + F)] (0.484) is close to the maximum XOH value (0.5), and represents the OH- richest topaz composition so far analysed in the Minas Gerais district. Topaz crystallinity and fluorine content sharply decrease at 1170 K, while mullite phase starts growing. On the basis of this behaviour, we suggest that this temperature may represent the potential initial topaz’s crystallization temperature from supercritical fluids in a pegmatite system. The log(fH2O/fHF)fluid (1.27 (0.06)) is coherent with the fluorine activity calculated for hydrothermal fluids (pegmatitic stage) in equilibrium with the forming mineral (log(fH2O/fHF)fluid = 1.2–6.5) and clearly different from pure magmatic (granitic) residual melts [log(fH2O/fHF)fluid < 1]. The modelled H2O saturated fluids with the F content not exceeding 1 wt% may represent an anomalous water-dominant / fluorine-poor pegmatite lens of the Padre Paraíso Pegmatite Field.

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Section: Discussionsupporting

confidence: 72%

F/OH ratio in a rare fluorine-poor blue topaz from Padre Paraíso (Minas Gerais, Brazil) to unravel topaz’s ambient of formation

Precisvalle

Martucci

Gigli

et al. 2021

Sci Rep

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“…Miarolitic K-feldspars from two pegmatites in Argentina were at or below a detection threshold that was reported as 0.01 wt% Cs 2 O (Černý et al, 2003). 431 A global survey (London et al, 2020b) found the Cs content of K-feldspars from 12 miarolitic 432 pegmatites to lie below detection by electron microprobe analysis (0.05 wt% Cs 2 O calculated at 433 3σ above mean background). This suite includes samples from the pegmatites of the Little Three 434 mine, Ramona, California (the same locality as the Swamp and Phantom dikes), where quartz 435 and topaz from miarolitic cavities contain daughter minerals including pollucite, ramanite 436 This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.…”

Section: An Assessment Of Case Studiesmentioning

confidence: 99%

“…Subsolidus recrystallization to perthite. London et al ( , 2020b noted that although much of the blocky K-feldspar in the southern California pegmatites is perthitic microcline, that recrystallization mostly occurred without the loss of Cs to an aqueous solution. That is not to say that an aqueous solution was not involved in the transformation to perthitic microcline, but that the recrystallization of the original K-feldspar occurred in a closed system in which the mass fraction of aqueous solution to K-feldspar was negligible.…”

Section: An Assessment Of Case Studiesmentioning

confidence: 99%

A Rayleigh model of cesium fractionation in granite-pegmatite systems

London

2022

American Mineralogist

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The K/Cs ratios of K-feldspars from granitic pegmatites are compared to models derived from the Rayleigh equation. The K/Cs and K/Rb ratios of K-feldspars and micas exhibit decreasing values when plotted against their Cs or Rb contents across cogenetic suites of granites and pegmatites, and from margin to core of individual bodies. The trends in elemental ratios conform to Rayleigh fractionation for the crystallization of feldspars and micas from a silicate melt. Within two individual pegmatite bodies, the K/Cs ratio of K-feldspar initially falls more rapidly than the Rayleigh model predicts. That might reflect a local increase in the concentration of Cs relative to K due to the pile-up of incompatible elements in a boundary layer of melt adjacent to the crystal growth front. The addition of an aqueous solution to the Rayleigh model (i.e., the simultaneous crystallization of K-feldspars from melt and from aqueous solution) predicts high and increasing K/Cs ratios of K-feldspars that are not observed in natural rock suites, except when K-feldspars crystallize in miarolitic cavities or when primary K-feldspar recrystallizes to microcline perthite in an open hydrothermal system. In those cases, the Cs content of K-feldspars falls to nil because of the high solubility of Cs in aqueous solution and low compatibility of Cs in K-feldspar. Otherwise, the observed patterns of K/Rb or K/Cs in K-This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

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“…From that An-rich composition along the margins of charges, feldspars evolve with isothermal cooling to the final composition of solvus pairs (Figure 2 of London, 2014b). The same evolutionary trends of feldspars are typical of zoned pegmatites (Cameron et al, 1949;London et al, 2012bLondon et al, , 2020London et al, , 2021. Oscillatory crystalline assemblages.…”

Section: Crystallinity Undercooling and H 2 O Contentmentioning

confidence: 85%

“…(4) Primary solvus feldspars record temperatures of ~ 400°C along the margins to ~ 450°C at centers of thin dikes (London et al, 2020(London et al, , 2021. The cesium content of primary K-feldspars in pegmatites serves as a measure of the extent to which the feldspars have preserved their original igneous compositions.…”

Section: Geological Contextmentioning

confidence: 99%

An experimental crystallization of the Macusani obsidian in a thermal gradient with applications to lithium-rich granitic pegmatites

London

2023

American Mineralogist

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Drilled cores of Macusani green obsidian pebbles from Peru were hydrated and melted above their liquidus temperatures at 200 MPa to a single column ~ 4 cm in length and then undercooled in a thermal gradient > 150°C along the length of the cores. Despite thousands of hours at subliquidus temperatures, the crystallinity of the products ranged from ~ 30 vol.% to ~ 5 vol.%.Mineral assemblages varied along the length of the cores but not in relation to the thermal gradients in all cases. Oscillations in the abundances of plagioclase, K-feldspar, and quartz were observed across the crystallization fronts and along the lengths of the cores. The originally homogeneous melts became heterogeneous in response to crystallization and to thermal gradients. Chemical gradients in the glasses included sharp boundary layer pile-up of F and B adjacent to the crystallization fronts, as well as linear gradients among several of the elements spanning the entire lengths of the melt columns. Values of molar K/(K+Na) in the glasses, plotted as K*, varied positively with Si, inversely with Al and F, and positively with the domains of K-feldspar that formed at maximum distance from the plagioclase-rich regions. Overall, the results are marked by sharply bounded textural domains, by the spatial segregation of mineral assemblages, by oscillations in mineral assemblages at multiple scales, and by monominerallic crystal aggregates that are hallmarks of pegmatite bodies. Temperatures recorded by feldspars closely approached the actual temperature gradient down to ~ 500°C, and the solvus on the alkali This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

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Temperatures and duration of crystallization within gem-bearing cavities of granitic pegmatites

Cited by 9 publications

References 50 publications

F/OH ratio in a rare fluorine-poor blue topaz from Padre Paraíso (Minas Gerais, Brazil) to unravel topaz’s ambient of formation

F/OH ratio in a rare fluorine-poor blue topaz from Padre Paraíso (Minas Gerais, Brazil) to unravel topaz’s ambient of formation

A Rayleigh model of cesium fractionation in granite-pegmatite systems

An experimental crystallization of the Macusani obsidian in a thermal gradient with applications to lithium-rich granitic pegmatites

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