Feldspar


Plagioclase

Orthoclase

Feldspar is a name given to a class of aluminum-containing silicates which form with no cleavage planes. The feldspars include orthoclase (KAlSi3O8) and plagioclase ((Ca,Na)AlSi3O8). Note that the plagioclase differs from the orthoclase in terms of the additional metals, plagioclase having calcium or sodium and orthoclase having potassium.

The feldspars are the most abundant minerals on the earth.

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Plagioclase


Plagioclase

Plagioclase ((Ca,Na)AlSi3O8) is one of the forms of feldspar. It is an aluminum-containing silicate which forms with no cleavage planes. This feldspar is distinguished from the orthoclase form by its content of calcium and sodium.

The feldspars are the most abundant minerals on the earth. In the Bowen reaction series, plagioclase can be seen to form throughout the process of magma crystallization in what is called the continuous reaction series. As the cooling continues, the calcium content of plagioclase diminshes while the sodium content increases.

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Orthoclase


Orthoclase

Orthoclase (KAlSi3O8) is one of the forms of feldspar. It is an aluminum-containing silicate which forms with no cleavage planes. This feldspar is distinguished from the plagioclase form by its content of potassium .

The feldspars are the most abundant minerals on the earth.

Orthoclase can form in single crystals like that shown at right, although the polycrystalline form above is the common occurrence. This crystal sample is about 5x3 cm and the gem is 61 carats. The sample is from Madagascar.

The orthoclase sample at left is about 3x8 cm and is from Rhone Glacier, Aar Massif, Switzerland. The sample at right is about 5x10 cm and is from St. Gotthare, Ticino, Switzerland. That sample is labeled as variety adulara with quartz and chlorite.

The orthoclase sample above left is a single crystal about 2x2 cm from Binnatal, Valais, Switzerland. The sample above right is about 5x3 cm and is from Baveno, Italy. At left is a sample about 7x5 cm from Rhone Glacier, Aar Massif, Switzerland.

These orthoclase gems are 249.5 and 126.4 carats and are from Madagascar.
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