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Journal of Petrology | Volume 4 | Number 2 | Pages 211-222 | 1963
© Oxford University Press 1963

research-article

The Origin and Significance of Snowball Structure in Garnet

ALAN SPRY

Geology Department, University of Tasmania

ABSTRACT

Sigmoidal lines of inclusions (snowball structure) indicate syntectonic crystallization and are due to the rotation of the crystal during growth. The sigmoidal trends form spirals whose mathematical properties may be determined. Structural analysis of garnets from Tasmania and Scotland shows that the spirals are a variant of the Archimedes spiral and the forms of the two examined are given by r2=a{theta} and r3={theta} respectively, where r is the radius of the crystal, a is a constant, and {theta} is the angle of rotation in radians. The amount of movement (slip) along the foliation to cause the measured rotation of the crystal may be calculated but is approximately 30 per cent greater than the length of the visible spiral.


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