Editing Pegmatites - St. Kilda: an illustrated account of the geology

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== Chapter 8 Pegmatites ==
 
== Chapter 8 Pegmatites ==
  
'''Keywords: residual fluids, mineralogy, zirkelite'''
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Keywords: residual fluids, mineralogy, zirkelite
  
 
Throughout the Western Gabbro a relatively coarse facies of gabbro occupies irregular fractures parallel to or transgressing the banding and layering. In some places on the eastern slopes of the Cambir, on Dun or below Mullach Bi [[:File:P991513.jpg|(Figure 14A)]] the veins are coarsely pegmatitic with crystals of pyroxene reaching 10 inches (250 mm) in length. The smaller and thinner veins have a mineralogy comparable with that of the gabbro except that olivine is scarcer, but the coarser pegmatites are altered to varying degrees and the primary augite, labradorite–andesine and magnetite–ilmenite grains are partly replaced by chlorite (pycnochlorite), epidote, albite and prehnite. The pale-coloured centres of the wider pegmatite veins consist almost entirely of these secondary minerals [[:File:P991514.jpg|(Figure 14B)]] and here also accessory zircon is found. The composition of the augite is comparable both in major elements (En<sub>42</sub>Wo<sub>43</sub> Fs<sub>15</sub>) and in TiO<sub>2</sub> (0.5%) and A1<sub>2</sub>O<sub>3</sub> (1.6%) contents to augites in the Western Gabbro and it is likely that these pegmatites represent the residual fluids after crystallisation of most of the E<sup>''W''</sup> magma. They indicate a residue rich in Na, Ca and Fe and poor in Si and K, for neither quartz nor alkali feldspar were found with the albite–chlorite–epidote–prehnite association.
 
Throughout the Western Gabbro a relatively coarse facies of gabbro occupies irregular fractures parallel to or transgressing the banding and layering. In some places on the eastern slopes of the Cambir, on Dun or below Mullach Bi [[:File:P991513.jpg|(Figure 14A)]] the veins are coarsely pegmatitic with crystals of pyroxene reaching 10 inches (250 mm) in length. The smaller and thinner veins have a mineralogy comparable with that of the gabbro except that olivine is scarcer, but the coarser pegmatites are altered to varying degrees and the primary augite, labradorite–andesine and magnetite–ilmenite grains are partly replaced by chlorite (pycnochlorite), epidote, albite and prehnite. The pale-coloured centres of the wider pegmatite veins consist almost entirely of these secondary minerals [[:File:P991514.jpg|(Figure 14B)]] and here also accessory zircon is found. The composition of the augite is comparable both in major elements (En<sub>42</sub>Wo<sub>43</sub> Fs<sub>15</sub>) and in TiO<sub>2</sub> (0.5%) and A1<sub>2</sub>O<sub>3</sub> (1.6%) contents to augites in the Western Gabbro and it is likely that these pegmatites represent the residual fluids after crystallisation of most of the E<sup>''W''</sup> magma. They indicate a residue rich in Na, Ca and Fe and poor in Si and K, for neither quartz nor alkali feldspar were found with the albite–chlorite–epidote–prehnite association.
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Although the pegmatitic patches and veins in the gabbros of Glacan Mor, Boreray and Soay vary in composition between the two extremes described above, most contain the albite–chlorite–epidote–prehnite assemblage and there are relatively small quantities of quartz and alkali feldspar.
 
Although the pegmatitic patches and veins in the gabbros of Glacan Mor, Boreray and Soay vary in composition between the two extremes described above, most contain the albite–chlorite–epidote–prehnite assemblage and there are relatively small quantities of quartz and alkali feldspar.
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== [[St. Kilda: an illustrated account of the geology#References|References]] ==
 
== [[St. Kilda: an illustrated account of the geology#References|References]] ==
 
{{EGwalks}}
 
{{EGwalks}}
 
[[Category: 2. Northern Highlands]]
 
[[Category: 2. Northern Highlands]]

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