Mineral Species: Albite var. Pericline
Chemical Formula: NaAlSi3O8
Chemical Name: Sodium Aluminium Silicate
Mineral Class: Tectosilicate
Mineral Notes: Albite is a common felspar and is the "pivot" mineral of two different feldspar series. It is most often associated with the Plagioclase series where it is an end member. This series contains feldspars that range in chemical composition from pure NaAlSi3O8 to pure CaAl2Si2O8 . The various plagioclase feldspars are identified from each other by gradations in refractive index and density. Albite is also an end member of the alkali or K-feldspars whose series ranges from pure NaAlSi3O8 to pure KAlSi3O8. This series only exists at high temperatures. At lower temperatures, the K-feldspars seperate from the Albite in a process called exsolution. Albite will then form layers inside the k-feldspar crystals. Some times these layers are visible by the naked eye and the stone is referred to as Perthite. Albite by definition must contain more than 90% Na and less than 10% of either K and/or Cain the cation position in the crystal structure. Albite is the last of the feldspars to crystallize from molten rock. The process of crystallization from a molten rock body serves to isolate rarer elements in the last stages of crystallization and therefore produces rare mineral species. Thus Albite is often found with rare minerals. All plagioclase feldspars show a type of twinning that is named after Albite. Albite Law twinning produces stacks of twin layers that are typically only fractions of millimeters to several millimeters thick. These twinned layers can be seen as striation-like grooves on the surface of the crystal and unlike true striations these also appear on the cleavage surfaces. The Carlsbad Law twin produces what appears to be two intergrown crystals growing in opposite directions. Two different twin laws, the Manebach and Baveno laws, produce crystals with one prominant mirror plane and penetrant angles or notches into the crystal. Although twinned crystals are common, single crystals showing a perfect twin are rare. Pericline is a form of Albite exhibiting elongate prismatic crystal. Pericline twinning is a type of crystal twinning which show fine parallel twin laminae typically also found in the alkali feldspars Sanidine and Microcline. The twinning results from a structural transformation between high temperature and low temperature forms
Description: Many blocky crystals of Pericline on Matrix. They have a very interesting crystal form, appearing as truncated prisms. The mineral has a chalk-like appearance but has a strong pearly luster when viewed from certain angles. A great quality specimen of a rare Albite variety.
Location: Mallnitz, Hohe Tauern, Karnten, Austria.
