ORPHOCLASE

A silicate which may be colourless, green, red, white or pale yellow. Since pure orphoclase is colourless, any colouration is due to the presence of impurities. For example, yellow specimens owe their colour to presence of iron which is not part of their chemical structure. Its name is derived from the Greek word orphos, which means upright. This refers to its cleavage planes which join at right angles. It is moderately hard (6 on the mohs scale) and is difficult to melt in an open flame. Twinning is often evident.

It is a feldspar mineral which contains less than 67% sodium. Feldspars with a higher concentration of sodium are known as anorphoclase. Both orphoclase and anorphoclase are alkali feldspars which form a solid solution series in which each member contains increasing and decreasing quantities of potassium and sodium. The two end minerals are sanidine (almost pure potassium) and albite (nearly all sodium and no potassium). The internal configuration of aluminium and silicon atoms varies from feldspar to feldspar with microcline being the most ordered and sanidine the most random. Orphoclase lies roughly half way between these two extremes.

It is used mainly in the manufacture of glass and ceramics but the finest specimens are gem quality. Sometimes it can be found in combination with other minerals. For example, moonstone is a made up of alternating layers of orphoclase and albite, which creates a blue colour and a twinkling effects known as schiller.

Orphoclase can be found in granites, pegmatites or lavas (igneous rocks) as well as in some sedimentary rocks. It can be found in St. Agnes, Cornwall (UK) and in Czech Republic, Germany, Italy, Madagascar, Spain and Switzerland.

 

PREHNITE

A silicate which can be found in many colours, shapes and sizes. It is usually colourless but may appear green, grey, white or yellow depending upon the presence of impurities or association with other minerals. Therefore prehnite is said to be allochromatic. Some specimens change colour depending on the angle from which they are viewed (pleochroism), which increases their value. It is moderately hard (6-6.5 on the mohs scale) but is easily workable due to its good cleavage and resistance to heat. It is similar to jadeite and nephrite both chemically and physically and may sometimes be mistaken for chrysoprase or vesuvianite.

It was first identified in 1788 on the Cape of Good Hope (South Africa) and named after Colonel H Von Prehn, the Dutchman who first brought it to Europe. It was originally believed to be a variety of prase quartz until the end of the 18th century, when it was discovered that it had a different chemical and crystal structure.

The finest specimens are gem quality and may be used to imitate precious gemstones. For example, fine green prehnite crystals can be used to imitate jade and some have been marketed under the name of Cape Emerald. Some of the finest yellow prehnite specimens contain fibrous inclusions which produce a beautiful play of light on the surface which resembles the narrowed pupil of a cats eye, This effect is known as chatoyancy. Prehnite is mostly used for jewellery and ornaments.

It is fairly widespread and can be found in the veins and cavities of basaltic lavas (igneous) and in some metamorphic rocks. It can be found in Germany, Cape of Good Hope, Michigan (US) and Scotland.