Metasilicates – Leucite Composition, Crystallization & Structure

Composition. A metasilicate of aluminum and potassium, KAl(SiO₃)₂ = Silica 55.0 alumina 23.5, potash 21.5.

Crystallization. Isometric. Trapezohedral habit (Fig. 302.fter forms rare. Strictly isometric only at temperatures of 500° C. or over. On cooling below this temperature it undergoes an internal molecular rearrangement to that of some other crystal system, but the external form does not change. It is formed in lavas at high temperatures and is then isometric in internal structure as well as outward form.

Structure. Usually in distinct crystals, also in disseminated grains.

Physical Properties. H. = 5.5-6.G. = 2.5. Vitreous to dull luster. Color white to gray. Translucent to opaque.

Tests. Infusible. Decomposed by hydrochloric acid with the separation of silica but without he formation of a jelly. Addition of ammonia to the solution gives precipitate of aluminum hydroxide. When mixed with powdered gypsum and fused gives violet potassium flame (best observed through a blue glass).

Occurrence. A rather rare mineral, occurring only in igneous rocks, and especially in the recent lavas, being rarely observed in deep- seated rocks. Found in rocks in which the amount of potassium in the magna was in excess of the amount necessary to form feldspar. Is not observed, therefore, in rocks that show quartz. Chiefly found un the rocks of central Italy; notably as phenocrysts in the lavas of Vesuvius. Pseudomorphs after leucite are found in syenites of Arkansas, Montana, Brazil, Etc.

Name. From a Greek word meaning white.

Pollucite, H2Cs2Al2 (SiO3)8 is a rare mineral that belongs in the same group as leucite.