Plagioclase twin associations from the basic volcanic rocks of the Kamchatka, Russia: growth conditions and formation mechanisms

Punin, Yurii O.; Shtukenberg, Alexander G.; Smetannikova, O. G.; Amelin, Konstantin

doi:10.1127/0935-1221/2009/0021-1979

Cited by 4 publications

(7 citation statements)

References 10 publications

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“…The association between swallowtails and twins is recognized in olivine crystals (Faure et al 2003). In addition, melt inclusions are documented as occurring preferentially along twin boundaries in plagioclase (Punin et al 2009). Some studies attribute linear bands of melt inclusions to synneusis or agglomeration of crystals (Goldstein and Luth 2006;Renner et al 2002) or as the result of new hopper crystal growth on an older preexisting crystal (Kohut and Nielsen 2004).…”

Section: Melt Inclusionsmentioning

confidence: 97%

“…For this process to occur, the crystals must be in an environment that favors their motion and casual interaction so that they may come into random contact (Nespolo and Ferraris 2004) and, when necessary, rotate into structural alignment (Cahn 1954;Spiess et al 2001;Ikeda et al 2002;Ohfuji et al 2005). Some authors consider synneusis a sub-type of growth twinning (e.g., Smith 1974b;Punin et al 2009) although it occurs once the crystals are of observable size (Vance 1969). We consider growth twinning to involve errors in the attachment of molecular-scale feldspar growth units to a crystal face, and thus consider synneusis here as a separate potential mechanism of twin formation.…”

Section: Crystal Growth and Twin Formationmentioning

confidence: 97%

“…It follows that atoms preferentially attach to areas of the growing crystal that do not lie along the twin boundary. This leads to the formation of embayments, swallowtails, hopper crystals, and uneven crystal terminations (Punin et al 2009), thus giving rise to the necessary precursors for the formation of melt inclusions (Roedder 1984;Kohut and Nielsen 2004;Faure and Schiano 2005). aCknowledgments Thanks go to Scott Sitzman for setting up our EBSD system, teaching us how to use it, and answering countless questions. Additional thanks to Kazuhide Nagashima and Gary Huss for assistance with the SEM.…”

Section: Melt Inclusionsmentioning

confidence: 99%

“…This stress can result from regional deformation of the rock or by impingement of growth in a crystal mush (Smith and Brown 1988). For plagioclase crystals with intermediate compositions such as our samples (An 24-33 ), production of mechanical twins requires a high level of stress at elevated temperatures to create high-energy twin boundaries (Borg and Handin 1966;Borg and Heard 1969;Punin et al 2009). However, neither of our samples exceeds 20% crystallinity or displays significant crystal impingement.…”

Section: Causes Of Twinningmentioning

confidence: 99%

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Prevalence of growth twins among anhedral plagioclase microlites

Brugger

Hammer

2015

American Mineralogist

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Crystal textures of volcanic rocks record the processes involved in magma storage and eruptive ascent. Syn-eruptive crystallization, in which groundmass crystals form and grow according to environmental factors such as thermodynamic undercooling, strongly influences the texture of erupted magma. This stage is difficult to isolate for study in natural rocks, but well-suited for laboratory experiments because the chemical compositions and crystallization timescales of eruptive processes can be emulated. This study examines the incipient stages of plagioclase crystallization in hydrous rhyodacite magma undergoing decompression-driven degassing. Experimental samples in which crystal growth at both near-equilibrium and far-from equilibrium conditions were examined using electron backscatter diffraction (EBSD) analysis to ascertain crystallographic lattice orientations of individual crystals. The crystal orientation investigation affirms a common assumption invoked in textural studies of crystal number density: contiguous crystals with parallel faces are crystallographically continuous, whereas contiguous crystals with non-parallel faces have unrelated crystal lattice orientations and as such, represent separate crystals. In the highly undercooled sample, twinning is identified in ~87% of the crystals examined; in the near-equilibrium sample, 38% of the crystals are twinned. We find that the observed twinning is unlikely to be the result of deformation, transformation, or synneusis, but rather a result of growth defects introduced during the incipient stages of crystallization. We suggest internal structural defects (twins) control macroscopic morphological defects (embayments, swallowtails, and melt inclusions) as a result of the high energy of the twin plane boundary. Formation of twins during the incipient stages of plagioclase crystallization is the single most important factor contributing to anhedral morphologies of feldspar microlites growing during magma decompression, and plays a role in the development of some plagioclase-hosted melt inclusions.

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