Scapolite Group – Wernerite Composition, Crystallization & Structure
Wernerite. Common scapolite
Composition A group of minerals varying in composition by the isomorphous mixture in different amounts of the two molecules, Ca4Al6Si6025(Me) and Na4 al3Si9024CI,(Ma). When the first molecule (Me) alone is present, the subname of meionite is used; when the second molecule (M a) represents the composition, the name marúilite is used. Wernerite, or common scapolite, shows a eombination of the two molecules according to the ratios of Me: M a as 3 : 1 to 1 : 2; while mizzonite corresponds to the ratios of Me: Ma as 1 : 2 to 1 : 3. Mixtures in all proportions may exist.
Crystallization. Tetragonal; tripyramidal. Crystals usually prismatic. Prominent forms are prism of the first and second orders, pyramid of first . Rarely shows the faces of the pyramid of the third order.
Structure. Crystals are usually coarse, with rough faces and often large. Also massive, granular, or with faint fibrous appearance.
Physical Properties. Imperfect prismatic cleavage. H. = 5-6. G. = 2.68. Vitreous luster when fresh and unaltered. Color white, gray or pale green. Transparent to opaque.
Tests. Fusible. Varieties containing sodium give yellow flame on ignition. Imperfectly decomposed by hydrochoric acid, yielding separated silica. but without the formation of a jelly.
Alteration. Easily altered into various other minerals, such as mica, epidote, talc, kaolin, etc.
Occurrence. The scapolites occur in the crystalline schists, gneisses and amphibolites, and in many cases have probably been derived by alteration from plagioclase feldspars. They also characteristically occur in crystalline limestones formed through the contact metamorphic action of an intruded igneous rock. Associated with light colored pyroxene, amphibole, garnet, apatite, titanite, zircon, ete. Crystals of gem quality with a yellow color occur in Madagascar. Found in various places in Massachusetts; Orange, Lewis, and Sto Lawrence counties, New York; at various points in Ontario, Canada.
The other members of the group, meionite, mizzonite and marialite, are much rarer in occurrence. Their crystals are usually smaller and of better quality than those of wernerite. Meionite and missonite are found in limestone blocks on Monte Somma.
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