|
The Plagioclase Feldspars – Albite, Anorthite Series
The triclinic soda-lime feldspars embrace a series of isomorphous minerals varying gin composition from albite, NaAlSi3O8, to anorthite, CaAl2Si2O8. These two molecules can replace each other in any proportion, and as a consequence a practically complete series may be found from the pure soda feldspar, and then with gradually increasing amounts of the anorthite molecule, to the pure lime feldspar. Definite names have been given to various mixtures of these two molecules, the more important being listed below:
These triclinic feldspars crystallize in forms closely resembling those of the monoclinic orthoclase, and the axial lengths and inclinations are also closely the same. This similarly in the crystal structure between the monoclinic and triclinic feldspars is best shown by a comparison of the cleavage angles of the different species, that of orthoclase being 90° of albite 86° 24’ and of anorthite 85° 50’. The triclinic feldspar are often known as the plagioclase feldspars, because of their oblique cleavage.
The crystals of the plagioclase feldspars are frequently twinned according to the various laws governing the twins of orthoclase, i.e.., the Carlsbad, Baveno and Manebach laws, known as the albite and pericline laws. The twinning plane in the albite law is the brachypinacoid, which corresponds to the clinopinacoid in orthoclase. The angle between the a basal plane and this twinning plane is not 90°, but about 86°; so that if one imagines a triclinic feldspar crystal cut in two along this plane and one-half revolved 180° from its original position upon an axis perpendicular to the plane, there would then be formed a shallow trough along the upper surface of the crystal, because the basal planes of the two adjacent halves would not lie in the same plane, but rather slope at a slight angle toward each other. This sort of twinning is commonly repeated many times in a single crystal, and gives rise to thin lamella, each one in twin position in respect to those on either side. Consequently a basal plane or cleavage surface of such a twinned crystal will be crossed by a number of parallel grooving or striations. Many times these striations are so dine as not to be visible to the unaided eye, but also at time they are coarse and easily see. The presence of these striation lines upon the better cleavage surface of a feldspar is one of the best prods that it belongs to the b crystallographic axis, and when this results in polysynthetic twins the consequent striations are to be seen on the brachypinacoid. |
