Spherulites

Spherulitic aggregate (Spherulites) are radiating arrays of fibrous (needle-like, acicular) crystals that are common in glassy felsic volcanic rocks. Early descriptions of spherulites are by Cross and Iddings in 1891 and by Harker in 1901. Spherulites are typically two-mineral aggregates (mainly quartz and feldspar), formed by initial Spherulitic growth of one mineral and later crystallization of a second mineral from the liquid or glass between the fibres.

Each fibre has the same crystallographic axis parallel to its length, and each has an orientation slightly different from that its neighbors. Thus, in contrast to dendrites, spherulites are aggregate of separate crystals, rather than branched single crystals. A dark "extinction cross" is common in spherulites observed in crossed polars, because many fibres (each with one of the principal optical vibration direction parallel to its length) are parallel ore approximately parallel to the vibration direction of the polarizer and analyzer.

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Fig.1: Extinction cross in spherulite observed in crossed polars. Corsica.



spherulites form during so-called devitrification processes ("de-glassing" processes) in obsidan, a natural glass. During devitrification the glass slowly crystallizes to minerals like cristobalite and feldspars.
In many felsic glassy rocks, flow line and structures pass undisturbed through spherulites, indicating that the spherulites grew after development of the flow structures, probably in solid glass (devetrification).

However, evidence of slight deformation of flow structures around some spherulites has been observed, indicating that spherulites may grow in viscous silicate melt. Spherulites crystallization in melts is also suggested by experiments on the crystallization of plagioclase (Lofgren, 1974).

Spherulitic aggregates typically nucleate on existing crystalline material; Once a radial growth habit is established, growth continues uniformly in all directions, this is probably because the crystals grow in a homogeneous material, such as a liquid or a glass.

If growing spherulites impinge on one another, polyhedral boundaries between them are formed. Incomplete radiation of spherulites results in fan, bow-tie and plumose aggregate; axiolitic spherulites result from fibres radiating or projecting out from a line or plane, probably owing to water penetrating along a crack and promoting crystallization of the adjacent glass. The focus of nucleation for Spherulitic growth may be an existing phenocrysts, forming partly spherical shapes if phenocrysts are small. In other case spherulites are centred on cavities, large cavities are called lithophysea and are common in Rhyolite.

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Fig.2: Spherulites morphologies





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Fig.3: Growth of spherulites (a) cause the progressive and constant filling of the space (b and c). in d you can see the areas that belong to individual spherulites; The boundaries between the spherulites are straight lines, and the spherulites assume a polygonal shape. From Amir Chossrow Akhavan



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Spherulites in a devitrified obsidian. From Amir Chossrow Akhavan





Bibliography



• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• David Shelley (1983): Igneous and metamorphic rocks under the microscope. Campman & Hall editori.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press
• Shelley D (1992): Igneous and Metamorphic Rocks under the Microscope: Classification, textures, microstructures and mineral preferred orientation
• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.

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Photo
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Spherulites (brown) in a Ignimbrite from New Zeland. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Ignimbrite from New Zeland. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Ignimbrite from New Zeland. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Ignimbrite from New Zeland. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Ignimbrite from New Zeland. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Ignimbrite from New Zeland. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) in a Obsidian from Pietre cotte, island of Vulcano (Italy). PPL image, 10x (Field of view = 2mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulites composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 2x (Field of view = 7mm)
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Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). XPL image, 2x (Field of view = 7mm)
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Particular of a Spherulite composed of Feldspar and Acicular torumaline crystals. Corsica (Location Unknown). PPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a pumice. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a pumice. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a pumice. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a pumice. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a pumice. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a pumice. XPL image, 10x (Field of view = 2mm)
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Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
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Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(5).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
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Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
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Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
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Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(9).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(10).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(11).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(12).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(13).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(14).jpg
Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(16).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(17).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(18).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(19).jpg
Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(20).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
sferulitidiquarzoo(21).jpg

Quartz-feldspar Spherulites in a totally devitrified obsidian. XPL image, 2x (Field of view = 7mm)
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Spherulites in a Dacite. XPL image, 2x (Field of view = 7mm)
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Spherulites in a Dacite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a Dacite. XPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 2x (Field of view = 7mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) grown on plagioclase crystals in a Obsidian. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) in a Rhyolite. PPL image, 10x (Field of view = 2mm)
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Spherulites in a Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites (brown) in a Rhyolite. PPL image, 10x (Field of view = 2mm)
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Spherulites (around biotite) in a Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites (brown) in a Rhyolite. PPL image, 10x (Field of view = 2mm)
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Spherulites (brown) in a Rhyolite. PPL image, 10x (Field of view = 2mm)
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Spherulites (around biotite) in a Rhyolite. XPL image, 10x (Field of view = 2mm)
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Dark Spherulites in a pillow lava. PPL image, 2x (Field of view = 7mm)
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Dark Spherulites in a pillow lava. PPL image, 2x (Field of view = 7mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Spherulites in a glassy Rhyolite. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
sferulitipiumose2013(7).jpg

Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)
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Plumose Spherulites in a Obsidian. XPL image, 10x (Field of view = 2mm)