In focus: Labradorite

The minerals of the feldspar family are the most abundant in the Earth's crust. In fact, there are more feldspars in the Earth's crust than all other minerals put together. The majority of the common feldspars fall into two groups, the alkali feldspars (the KAlSi₃O₈ through NaAlSi₃O₈ series), and the plagioclase feldspars (the NaAlSi₃O₈ through CaAl₂Si₂O₈ series). Labradorite is a member of the plagioclase series with a chemical composition range just past the midpoint between albite which is a pure sodium aluminosilicate and anorthite which is composed almost exclusively of calcium aluminosilicate. The calcium-rich plagioclase feldspars are the first to begin crystallizing from molten rock at higher temperatures, thereby concentrating sodium in the melt as the temperature decreases. Because Labradorite incorporates two crystals; one of sodium-rich aluminum silicate and the other of calcium-rich aluminum silicate the process often results in twinning. Twinning is not unique to Labradorite - indeed, all members of the plagioclase group display lammerlar twinning called "Albite Twinning". Twinning is a process which occurs during crystal growth when the crystal is subjected to stress or temperature/pressure conditions different from those under which it originally formed. In this way two or more intergrown crystals are formed in a symmetrical fashion. These symmetrical intergrowths of crystals are called twinned crystals.

Another characteristic feature of Labradorite is an optical phenomenon called Iridescence which is a property of the surface in which hue changes according to the angle from which the surface is viewed. Labradorite displays different colours from different angles when exposed to light. Gemstone varieties of Labradorite exhibiting a high degree of iridescence are called Spectrolite and a talented jeweller can produce some stunning results. Spectrolite was discovered in Finland during WWII by troops digging defensive positions. A Finnish professor Aarne Laitakari named the mineral, which has its own classification. Although labradorite is found in Labrador, Norway, Madagascar and Russia in addition to Finland, the Finns have made a speciality of mining and marketing Spectrolite. Labradorites are generally difficult to work with and therefore are regarded as minor gem-stones. Also because Labradorite is very common, the price even of gemstone-quality stone is rather low.

Labradorite occurs in metamorphic or igneous rocks in Labrador (Canada), Finland, Madagascar, Australia, Mexico, Norway, what used to be known as the USSR and the USA. The most spectacular rocks come from Finland and Madagascar. Lesser rocks come from the USA and Mexico.

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