LEUCITE ROCKS. Igneous rocks bearing leucite, though quantitatively scarce are of wide distribution, and occur in every quarter of the globe, apparently excepting Britain. Taken collectively they exhibit a considerable variety of type and are of great petrographic importance.
Their principal developments are of Tertiary or Recent age and include those of the Roman region and the East Indies (Java, Celebes, Borneo). In pre-Tertiary rocks leucite is uncommon, since it readily undergoes change to other products as analcime, zeolites, etc. The majority of leucite-bearing rocks are surface lavas or thin dykes ; plutonic rocks carrying leucite are known from only two or three localities. In these latter types the insta bility of leucite, cooled slowly under deep-seated conditions, is seen in the frequent occurrence of pseudo-leucite, a paramorph consist ing of aggregates of orthoclase and nepheline, or its alteration products (muscovite, sodalite, etc.). This transformation may be chemically expressed in the equation : 2 [ = Nepheline, sodalite, nosean, hailyne, melanite-garnet and meli lite are frequent associates of leucite, but with rare exceptions it is not found together with free silica. The mineral is readily recog nized by the trapezohedral character of its crystals, its colour and rough cleavage. Leucite rocks belong chemically to a potash-rich suite of rocks which includes potash syenites, monzonites, shonkin ites and latites; in contrast to the sodic Atlantic and calcic Pacific suites, this group is sometimes referred to as the Mediterranean suite, from their relative abundance in the Italian region bordering the Mediterranean.
open crucible, yields leucite as a transformation product. Here may, perhaps, be discovered the reason for the rare occurrence of deep-seated leucite-bearing rocks, the leucite molecule being do silicified to the orthosilicate molecule in the presence of water under cover. The recently demonstrated incongruent melting of orthoclase with separation of leucite is clearly, too, of special petrographic importance, for the occurrence of leucite in compara tively silica-rich glasses or in ground masses containing free silica is thus comprehended. (See PETROLOGY.) Plutonic Rocks Bearing Leucite.—The rare coarse-grained or plutonic rocks carrying leucite are practically limited to occur rences in Arkansas, Montana and Brazil, or occur as cognate xeno liths in leucite lavas, as in those of the Roman region. These rock types include leucite-syenite, leucite-shonkinite, arkite and mis sourite. The first consists of leucite (or pseudo-leucite), ortho clase, nepheline, diopside and aegirine-diopside, melanite and bio tite. Leucite-shonkinite, a melanocratic type known from the Highwood mountains, Mont., and in ejected blocks from Monte Somma, consists of augite, orthoclase and leucite, while arkite and missourite are almost felspar-free rocks in which the chief con stituents are leucite-nepheline, aegirine-diopside and melanite (arkite), and leucite, olivine augite and biotite (missourite). The former occurs at Magnet Cove, Ark., and the latter in the High wood mountains. Hypabyssal rocks carrying leucite belong to the tinguaite and monchiquite groups. The leucite tinguaites are usually pale grey or green and consist principally of nepheline, alkali-felspar and aegirine, together with leucite. Rocks of this group are known from the Laacher See and Kaiserstuhl districts, Serra de Monchique (Portugal), north-west Madagascar, Umptek (Russia), Magnet Cove (Ark.), Bear Paw mountains (Mont.), Alaska, Greenland and Rio de Janeiro. Leucite monchiquites are fine-grained dark rocks consisting of olivine, titaniferous augite and iron oxides, with a glassy ground-mass in which small rounded crystals of leucite are scattered. They are known from Czecho slovakia (Bohemian Mittelgebirge) and the Kaiserstuhl (Baden).