Alkali Feldspar Granite Thin Section

Biotite metaluminous granite showing a close up of one crystal.

Alkali feldspar granite thin section.

The differences can be subtle and confusing. Alkali feldspar also called potassium feldspar or k feldspar has a color range from white to brick red and it s typically opaque. Large number of pages viewed. Orthoclase and sanidine are both optically monoclinic k.

Orthoclase is frequently known as potassium feldspar or k spar and is easily distinguished from the plagioclase feldspars in hand sample by its lack of striations. The abundance of k spar gives the rock a predominant pink to reddish hue. Many rocks have both feldspars like granite. It is a dense rock with a phaneritic texture.

Most alkali feldspars are compositionally closer to or end member which explains why we often refer to them as k feldspars potassium feldspars. Anorthoclase na k alsi 3 o 8 is a crystalline solid solution in the alkali feldspar series in which the sodium aluminum silicate member exists in larger. Cases like that are helpful for learning to tell the feldspars apart. Peppered with minor amounts of black minerals.

You can find the explanation on the alkali feldspar page. The lottah granite is a composite pluton of tin mineralised strongly peraluminous alkali feldspar granite which intrudes the poimena granite a major component of the mid devonian blue tier batholith of northeastern tasmania. Damage produced during thin section grinding causes speckles of light in the biotite where the crystal lattice has been deformed. This means that biotite in standard thin sections rarely goes completely extinct.

As any feldspar cools below the solidus its isopleth eventually intersects the convex upward solvus below which the single feldspar unmixes or exsolves under equilibrium conditions into two stable alkali feldspar solid solutions. At ph 2 o 200 mpa any feldspar precipitated from a melt is a homogeneous alkali feldspar solid solution. Alkali feldspar granite some varieties of which are called red granite is a felsic igneous rock and a type of granite rich in the mineral potassium feldspar k spar. I have the following thin section of alkali feldspar granite.

Microcline is a regular and essential ingredient of sandstone especially feldspar arenaceous rocks and feldspar graywacke sections 5 5 3 2 and 5 5 3 3 section 5 5 3 2 section 5 5 3 3. Microcline is a triclinic k feldspar and has a characteristic cross hatched tartan pattern of twinning seen with a petrographic microscope.