Granite feldspars

Silicon makes up 25.7% of the earth s crust, by weight, and is the second most abundant element, being exceeded only by oxygen. Silicon is not found free in nature, but occurs chiefly as the oxide and as silicates. Sand, quartz, rock crystal, amethyst, agate, flint, jasper, and opal are some of the forms in which the oxide appears. Granite, hornblende, asbestos, feldspar, clay, mica, etc. are but a few of the numerous silicate minerals. 

The hard rock deposits are mined mainly for feldspar with mica and quartz being accessory minerals. These deposits are extensive, often covering hundreds of square meters and are recognized by the light-colored, granite-like appearance with shiny mica flakes being a prominent feature. The mica content of these deposits ranges from approximately 6—10 wt %. 

Two mixtures. On the left is brass, a homogeneous mixture of copper and zinc. On the right is a piece of granite, a heterogeneous mixture that contains discrete regions of different minerals (feldspar, mica, and quartz). 

The feldspars are aluminosilicates in which as much as half the silicon(IV) has been replaced by aluminum(III). They are the most abundant silicate materials on Earth and are a major component of granite, a compressed mixture of... 

FIGURE 14.41 The mineral granite is a compressed mixture of mica, quartz, and feldspar. 

Potassium feldspar (orthoclase) Muscovite mica Quartz Felsic (rhyolite granite)... 

Granite A crystalline intrusive igneous rock consisting of alkaline feldspar and quartz. 

The P-T stability curve of muscovite intersects the incipient melting curve of granite in hydrous conditions at about 2.3 kbar total pressure and T = 650 °C. Thus, muscovite may crystallize as a primary phase from granitic melts above these P and T conditions (interstitial poikihtic crystals) or may form by reaction with pristine solid phases at lower P and T (muscovite as dispersed phase within feldspars, for instance). In this second type of occurrence, the following two equilibria are of particular importance ... 

Based on mineralogical and textural similarities (supported by geochemistry), the rocks of the syenitic dyke swarm can be subdivided into three groups (1) nepheline syenite - nephelinolite group (NS-N), (2) syenite - monzodiorite group (SM), and (3) an alkali feldspar granite dyke. 

The syenitic dyke swarm of the Moose Creek Valley can be subdivided based on mineralogy and geochemistry, into three groups SM, NS-N, and an alkali feldspar granite dyke. 

The granite outcrops are medium- to coarse-grained and have a hypidio-morphic granular texture dominated by rounded or interlocking quartz grains, and laths of pink, white, and perthitic feldspars. 

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