4b in Plagioclase – (series of minerals in the feldspar group), from Greek for oblique and fracture, in reference to the oblique angle between its best cleavages. Bytownite (Ca %70-90 – Na %30-10) – locality at Bytown (now the city of Ottawa), Canada. It is found it all of the three rock types, but is most common in intrusive igneous rocks like granite where the crystals look white or pink. Plagioclase Plagioclase is a member of the feldspar group (like orthoclase) and is a framework silicate.Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals. Dickson, F.W. The names of the minerals in the series are arbitrarily given based upon their relative abundance of albite and anorthite. “Plagioclase” is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite, Na(AlSi 3 O 8), to pure anorthite, Ca(Al 2 Si 2 O 8).Minerals in this series are a homogenous mixture of albite and anorthite. Orthoclase is one of the most common minerals, and occurs in numerous mineral environments. It is polymorphous with the minerals Microcline and Sanidine.These three minerals form the Potassium Feldspar group.They are almost identical in physical properties, and it is sometimes impossible to distinguish one another without x-ray analysis. Location: Ihosy, Betroka district, Toliara Province, Madagascar.Scale: Picture size 3x3 cm. The twinning in plagioclase produces stacks of twin layers that are typically fractions to several millimetres thick. 4a and Fig. Albite twins are bent and fractured, and K-feldspar replaces the plagioclase in irregular veins at top right of plagioclase and on left side. What is Plagioclase? Feldspar is the most common mineral in the Earth’s crust, so you are very likely to find it in the rocks you collect! The gently-inclined, broad, black area is a fracture (hole) in the thin section. deformed plagioclase (tan, center) in the Cape Ann granite. Comments: Dual view of a cleavage fragment of labradorite showing the multi-colored labradorescence (schiller effect) and the same area viewed without the color.The color is due to optical interference on Bøggild exsolution lamellae. (1968) Growth of K-feldspars and plagioclase by replacement processes in rock of the Papoose Flat pluton and in country rocks, Inyo Mountains, California. Plagioclase Labradorite. Anorthite (Ca %90-100 – Na %10-0) – from Greek for not straight, because of its triclinic symmetry. (See Fig. Geological Society of America Special Papers 101, Abstract for 1966: 53-54.
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