Description
The mineral oligoclase, or sunstone, is a sodium-calcium-aluminum tectosilicate with the chemical formula (Na,Ca)(Si,Al)4O8. It belongs to the feldspars. Colorless, greenish, gray, or brown oligoclase has a vitreous luster, a white streak color, perfect cleavage along the [001] crystal plane, and good cleavage along the [010] plane. Its average density is 2.65 and its hardness is 7. The crystal system is triclinic, and the mineral is neither radioactive nor magnetic. The name of the mineral oligoclase is derived from the Greek words oligos and klasein, meaning "little cleavage." Oligoclase is a common feldspar in metamorphic and igneous rocks such as pegmatite. It is part of the plagioclase series (albite-anorthite). The type locality of oligoclase is Twedestrand, Norway. The Vikings likely used oligoclase as a navigational instrument to determine the sun's position at sea in cloudy weather. This mineral can make polarized light visible, which forms rings around the sun through refraction in the atmosphere. The crystal duplicates the rings. When both are equally strong, the crystal is oriented directly toward the sun. The rings are also visible after the sun has set.
