Electron optical and X-ray powder diffraction study of synthetic and natural potassic cordierites

Daniels, Peter J. L.; Maresch, Walter V.; Schreyer, Werner; Sahl, K.

doi:10.1007/bf00320903

Cited by 7 publications

(1 citation statement)

References 16 publications

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“…As commonly found in high-T low-P pyrometamorphic rocks [81,82], cordierite is characterized by a peculiar high K 2 O content (0.2-1.2 wt.%; [21]) and is mostly concentrated in the higher silica glass of the buchite. As investigated by Del Moro et al [21], all the crystals of cordierites studied show orthorhombic symmetry, although a transformation from precursor hexagonal indialite is considered likely.…”

Section: Mineral Phases and Glass Compositionsmentioning

confidence: 93%

Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism

et al. 2021

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Pyrometamorphism is the highest temperature end-member of the sanidinite facies (high-temperature, low-pressure contact metamorphism) and comprises both subsolidus and partial melting reactions which may locally lead to cryptocrystalline-glassy rocks (i.e., porcellanites and buchites). A wide range of pyrometamorphic ejecta, with different protoliths from Stromboli volcano, have been investigated over the last two decades. Among these, a heterogeneous (composite) glassy sample (B1) containing intimately mingled porcellanite and buchite lithotypes was selected to be studied through new FESEM–EDX and QEMSCAN™ mineral mapping investigations, coupled with the already available bulk rock composition data. This xenolith was chosen because of the unique and intriguing presence of abundant Cu–Fe sulphide globules within the buchite glass in contrast with the well-known general absence of sulphides in Stromboli basalts or their subvolcanic counterparts (dolerites) due to the oxygen fugacity of NNO + 0.5–NNO + 1 (or slightly lower) during magma crystallization. The investigated sample was ejected during the Stromboli paroxysm of 5 April 2003 when low porphyritic (LP) and high porphyritic (HP) basalts were erupted together. Both types of magmas are present as coatings of the porcellanite–buchite sample and were responsible for the last syn-eruptive xenoliths’s rim made of a thin crystalline-glassy selvage. The complex petrogenetic history of the B1 pyrometamorphic xenolith is tentatively explained in the framework of the shallow subvolcanic processes and vent system dynamics occurred shortly before (January–March 2003) the 5 April 2003 paroxysm. A multistep petrogenesis is proposed to account for the unique occurrence of sulphide globules in this composite pyrometamorphic xenolith. The initial stage is the pyrometamorphism of an already hydrothermally leached extrusive/subvolcanic vent system wall rock within the shallow volcano edifice. Successively, fragments of this wall rock were subject to further heating by continuous gas flux and interaction with Stromboli HP basalt at temperatures above 1000 °C to partially melt the xenolith. This is an open system process involving continuous exchange of volatile components between the gas flux and the evolving silicate melt. It is suggested that the reaction of plagioclase and ferromagnesian phenocrysts with SO2 and HCl from the volcanic gas during diffusion into the melt led to the formation of molecular CaCl in the melt, which then was released to the general gas flux. Sulphide formation is the consequence of the reaction of HCl dissolved into the melt from the gas phase, resulting in the release of H2 into the melt and lowering of fO2 driving reduction of the dissolved SO2.

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Section: Mineral Phases and Glass Compositionsmentioning

confidence: 93%

Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism

et al. 2021

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NMR-spectroscopy of synthetic potassium-bearing cordierites

Daniels

Gies

1992

Phys Chem Minerals

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Abstract. Potassic cordierites with the chemical composition KxMgaA14+xSi s_xO18 (x = 0.00, 0.10, 0.20, and 0.25) were synthesized by annealing glasses at 1290 ~ C for different lengths of time. The procedure resulted in cordierites with different states of A1, Si-order for the tetrahedral sites in the structure. The dependence between the potassium-content and the state of order on one side and between annealing time and the state of order on the other side was then studied using 29Si MAS nuclear magnetic resonance (NMR) spectroscopy. The spectra show that the state of order is a continuous function of annealing time for all compositions considered, but the rate of ordering decreases with increasing K-content. Since the substitution K + A1 ~ Si leads to higher A1/Si-ratios; the lower rate of ordering is discussed as a consequence of changed statistics for A1,Si site exchanges. The A1 atoms replacing silicon in the structure to balance the charge of potassium cations are not located close to the potassium ion but at a maximum distance from it. This is shown to be a consequence of an improvement in coordination of all oxygen atoms in the cordierite flamework.hexagonal symmetry (Meagher and Gibbs 1977;Armbruster 1985). Annealing leads to an ordering of aluminium and silicon in the low-temperature polymorph with orthorhombic symmetry (Gibbs 1966). In this phase each six-membered ring contains two aluminium and four silicon atoms with a ratio A1/Si of 1:2 for the T2-tetrahedra. The respective ratio for the ring-connecting Tl-tetrahedra is 2:1.

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On the kinetics of partially conserved order parameters: a possible mechanism for pattern formation

Salje¹

1993

J. Phys.: Condens. Matter

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Local correlations between kinetic events of cation/vacancy ordering in framework structures lead to 'partial conservation' of the order parameter. Mixing non-conserved and conserved kinetic processes leads to a bifurcation behaviour with uniform states for mainly non-conserved order parameter and periodic pattern formation if more than 1 out of 12 steps is conserved. Possible correlations with experimental observations are discussed.

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Electron optical and X-ray powder diffraction study of synthetic and natural potassic cordierites

Cited by 7 publications

References 16 publications

Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism

Sulphide Globules in a Porcellanite–Buchite Composite Xenolith from Stromboli Volcano (Aeolian Islands, Southern Italy): Products of Open-System Igneous Pyrometamorphism

NMR-spectroscopy of synthetic potassium-bearing cordierites

On the kinetics of partially conserved order parameters: a possible mechanism for pattern formation

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