The distribution & significance of deviations from the normal order of crystallization : also the distribution & significance of micropegmatite in granites, as illustrated by the granites of the north of Scotland / by William Mackie.

  • Mackie, William.
Date:
[1908?]
Catalogue details

Licence: In copyright

Credit: The distribution & significance of deviations from the normal order of crystallization : also the distribution & significance of micropegmatite in granites, as illustrated by the granites of the north of Scotland / by William Mackie. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    Orthoclase in Biotite. Orthoclase occurs in biotite much less frequently than quartz, but in the double-mica class of granites it has not been observed in this research in this connection. A reason for this has been already suggested. Where it does occur it usually appears in very small clear grains—not always easily distinguishable from quartz on the one side or plagioclase on the other. Its giving a biaxial reaction in convergent polarised light has been used to distinguish it from quartz, and the absence of lamellar twining to distinguish it from plagioclase. Examples of orthoclase included in biotite are freq uent in many granites; typical examples were seen in a section of Cairngorm granite, where two small elongated orthoclases included in a large biotite were observed to extinguish quite differently from the large bounding ortho- clases outside the biotite. Plagioclase in Biotite. The occurrence of plagioclase in biotite—that is to say, the crystallization of plagioclase before biotite—is not of the same significance as the crystallization of quartz, with or before biotite or hornblende, because in the former case the two minerals crystallize at periods so close together normally that the slight deviation is hardly worth recording. When it does so occur, however, it is always in small crystals, as is usually the case with all these aberrant manifestations. Larger crystals partially embedded in or moulded by the biotite are of frequent occurrence. Inclusions in Sfhene. The natural crystallization period of sphene should come before that of biotite, if it is not actually contemporaneous with or in some instances even prior to the hornblende period ; but there is evidence to show that the crystallizing period of sphene is relatively prolonged, for it has frequently been seen as a fringe of fine crystals on the edges of biotites or occurring in numerous small crystals at a little distance from their margins. The fact that sphene occurs in a proportion of the biotite granites, where there is no hornblende—that is to say, its being of wider dis- tribution in granites than hornblende, is, on the theory of the derivation of biotite granites from originally hornblendic magmas —and of this there is considerable evidence in the area— suggestive that sphene is more soluble at lower temperatures than hornblende, and continues in solution to some extent long after all the hornblende has crystallized out. Where sphene forms