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Orthoclase is another rock-forming mineral like nepheline and muscovite, but it is responsible for the formation of igneous rocks. It has the formula KAlSi3O8 and belongs to the subclass of the silicates called the tectosilicates. The name was coined from the Greek word for "straight fracture" owing to the fact that its cleavage surfaces are about 90 degrees to each other.

The orthoclase is sometimes called the K-spar or potassium feldspar if it is not easily identified by ordinary methods. These K-spar minerals include the sanidine, anorthoclase and microcline with minor differences. The microcline is more deeply coloured and has a lamellar twinning which is not present in the orthoclase. The crystals of sanidine and anorthoclase are usually flat, and their formation is different from that of the orthoclase.

While the orthoclase is the leading K-spar of the syenites and granites that cool at a moderate speed, the sanidine and anorthoclase are found in extrusive igneous rocks like rhyolites which reduce in temperature quickly. The orthoclase is only fully distinguished from these other K-spar minerals through X-ray methods and optical characteristics.

Characteristics of the Orthoclase

·    Its colour can be yellow, off-white or have shades of orange or brown, with a white streak.

·    It has a vitreous or full lustre if it undergoes weathering.

·    Its crystals are opaque or translucent.

·    It has a monoclinic crystal system.

·    Its crystals are tube-like or blocky, having slanted domes and terminating at a pinnacoid.

·    The adularia variety, located in the alpine veins is either pseudo-trigonal or pseudo-orthohombic with flat and tube-like crystals.

·    Its cleavage forms prisms at right angles and it has an uneven or conchoidal fracture.

·    It is associated with other minerals like the quartz, micas, topaz, tourmalines and garnets.

The orthoclase is usually used as a crude material for making glasses, ceramics and scouring powder. The moonstone, a name given to the attractive orthoclase, is used for making jewellery.

Additional Resources

Onyx Uses   Phanteritic   Plagioclase

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