Skeletal olivine

Skeletal crystals are incomplete, polyhedral single crystals with internal cavities that are commonly crystallographically controlled. In skeletal crystals in igneous rocks, the cavities are filled with glass or crystalline groundmass material.

Skeletal and dendritic crystals are forced to develop by constitutional (compositional) supersaturation. In a polycomponent system (such as a silicate melt), if low-temperature components not wanted by the growing crystal cannot diffuse away into the liquid fast enough, they become concentrated in a narrow zone adjacent to the interface, especially if their diffusion rate in the liquid are very low at the temperature concerned (Fig.1). In addition, the liquid right at the interface has the highest temperature of any of the surrounding melt, owing to release of latent heat of crystallization by the growing crystal (Fig.2), although this diffuses away much faster than impurity components, and so has a much smaller effect.

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Fig.1: Sketch of spike projecting from crystal corner, through layer of liquid rich in impurity components into pure liquid, initiating dendritic growth habit. Modified from Vernon, R. H. (2018).



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Fig.2: Temperature field around a growing crystal. Modified from McLachlan & Carlson (1952).



The accumulation of impurity components around the crystal means that the supersaturation of the melt increases with increasing distance from the interface until it reaches a steady value. Further crystal growth is only possible in liquid farther away from the interface, where liquid of normal composition is available. Planar interfaces are unstable under these conditions of strong constitutional supersaturation and change into projections or spikes (Fig.1). The reason is that, if part of the interface advances ahead of the rest of the crystal, it encounters liquid that is poorer in impurity components and richer in nutrient components, and so can continue to grow. Moreover, because of its large interface area, the spike can lose impurity components more easily. However, owing to the tendency for heat and impurity components to accumulate around the spikes themselves, secondary spikes (Fig.3) may be forced to develop on the primary spikes for crystallization to continue. This leads to a dendritic habit (Fig.4). Skeletal, dendritic and spiky crystals of plagioclase, olivine, pyroxene and magnetite occur in quenched volcanic rocks, including submarine basalts and Archaean magnesian basalts or komatiites.

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Fig.3: Sketch showing development of secondary spike on primary spike in response to accumulation of impurity components around the primary spike. Modified from Vernon, R. H. (2018).



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Fig.4: Development of dendritic spike (stages of growth, 1 to 5) in response to accumulation of impurity components around the crystal. . Modified from Kretz, R. (2003).





Bibliography



• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.


Skeletal Olivine
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Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Pillow lava. XPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Pillow lava. XPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Pillow lava. The brown material are spherulites. PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine (altered by iddingsite) in a Basalt from Radicofani volcano (Italy). PPL image, 10x (Field of view = 2mm)
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Skeletal olivine. PPL image, 10x (Field of view = 2mm)
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Skeletal olivine. XPL image, 10x (Field of view = 2mm)
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Skeletal olivine. PPL image, 10x (Field of view = 2mm)