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?]
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.
19/84 (page 259)
![absence of any form of micropegmatite is associated with marked basicity of composition. The ]B form is more characteristic of typical granites than of typical dyke or effusive rocks—indeed, so frequently does this form occur in granites that it may almost be taken as a defining characteristic of a granite. The presence of the C form, on the other hand, is more characteristic of the atypical rocks of the acid class. It will be shown later that the B variety of micropegmatite and the presence of orthoclase or microcline or both go hand in hand, and the two correlatives may be taken as all but defining characters of a granite. Exceptions to the Normal Order op Crystallization in Detail. Quartz in Hornblende. When quartz occurs as an inclusion in hornblende, it frequently occupies a central position in the hornblende aggregates, as if, indeed, it were the very first mineral to crystallize out. It is for the most part of very irregular outline, though occasionally it shows rectilinear outlines. It almost always contains inclusions, which are very frequently small crystals of hornblende itself. At other times, they are magnetite or apatite. I have rarely seen rutile needles in the quartz included in hornblende, though they are of frequent occurrence in the quartz grains of later origin, and in particular in those included in orthoclase. Liquid inclusions with gas bubbles have never been observed in these quartz grains. The occurrence of quartz in hornblende has been attributed to corrosive action set up at a later stage of the crystalhzation in connection with and as a consequence of the separation of biotite. That such an origin is possible in many instances is not denied ; but the cases tabulated are of instances where no biotite occurs in association with the hornblende groups, and in a number of instances even where biotite is all but absent from the rock. Such, for instance, as in Ben Laoghal granite, Dalltullich syenite, etc. The quartz associated with the worm- eaten appearance of hornblende, which is frequently seen in the presence of biotite, is a totally difEerent manifestation from that now under consideration, and calls for separate discussion. Sometimes a condition simulating a micropegmatite of quartz and hornblende has been observed in some hornblende groups, as for example in Lairg granite, Sutherlandshire, and a rock from Dalltullich, Nairnshire. In the instance observed it has been seen to occupy a central position in the hornblende, being surrounded on all sides by solid hornblende, so that corrosion of the kind just indicated would thus be excluded. It should be noted, at-](https://iiif.wellcomecollection.org/image/b2242782x_0021.jp2/full/800%2C/0/default.jpg)
