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LEUCITE
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LEUCITE , a See also:rock-forming See also:mineral composed of See also:potassium and See also:aluminium metasilicate KAl(SiO1)2• Crystals have the See also:form of cubic icositetrahedra 12111, but, as first observed by See also:Sir See also:David See also:Brewster in 1821, they are not optically isotropic, and are there-fore pseudo-cubic. Goniometric measurements made by G. vom See also:Rath in 1873 led him to refer the crystals to the tetragonal See also:system, the faces o being distinct from those lettered i in the adjoining figure. See also:Optical investigations have since proved the crystals to be still more complex
in See also:character, and to consist of several orthorhombic or See also:monoclinic individuals, which are optically biaxial and repeatedly twinned, giving rise to twin-lamellae and to striations on the faces. When the crystals are raised to a temperature of about 5oo° C. they become optically isotropic, the twin-lamellae and striations disappearing, reappearing, however, when the crystals are again
cooled. This pseudo-cubic character of leucite rs exactly the same as that of the mineral See also:boracite (q.v.).
The crystals are See also: Hatly's name " amphigene." (L. J. S.) Leucile Rocks.—Although rocks containing leucite are numerically scarce, many countries such as See also:England being entirely without them, yet they are of wide See also:distribution, occurring in every See also:quarter of the globe. Taken collectively, they exhibit a considerable variety of types and are of See also:great See also:Interest petrographically. For the presence of this mineral it is necessary that the See also:silica percentage of the rock should not be high, for leucite never occurs in presence of See also:free See also:quartz. It is most See also:common in lavas of See also:recent and See also:Tertiary See also:age, which have a See also:fair amount of potash, or at any See also:rate have potash equal to or greater than soda; if soda preponderates See also:nepheline occurs rather than leucite. In pre-Tertiary rocks leucite is uncommon, since it readily decomposes and changes to See also:zeolites, See also:analcite and other secondary minerals. Leucite also is rare in plutonic rocks and See also:dike rocks, but leucite-See also:syenite and leucite-tinguaite See also:bear See also:witness to the possibility that it may occur in this manner. The rounded shape of its crystals, their white or grey colour, and rough cleavage, make the presence of leucite easily determinable in many of these rocks by See also:simple inspection, especially when the crystals are large. " Pseudo-leucites " are rounded areas consisting of See also:felspar, nepheline, analcite, 504 &c., which have the shape, See also:composition and sometimes even the crystalline forms of leucite ; they are probably pseudomorphs or paramorphs, which have See also:developed from leucite because this mineral, in its isometric crystals, is not See also:stable at See also:ordinary temperatures and may be expected under favourable conditions to undergo spontaneous See also:change into an aggregate of other minerals. Leucite is very often accompanied by nepheline, See also:sodalite or nosean; other minerals which make their See also:appearance with some frequency are melanite, garnet and mclilite. The plutonic leucite-bearing rocks are leucite-syenite and missourite. Of these the former consists of See also:orthoclase, nepheline, sodalite, See also:diopside and aegirine, See also:biotite and See also:sphene. Two occurrences are known, one in See also:Arkansas, the other in See also:Sutherlandshire, See also:Scotland. The Scottish rock has been called See also:borolanite. Both examples show large rounded spots in the See also:hand specimens; they are pseudo-leucites and under the See also:microscope prove to consist of orthoclase, nepheline, sodalite and decomposition products. These have a radiate arrangement externally, but are of irregular structure at their centres; it is interesting to See also:note that in both rocks melanite is an important See also:accessory. The missourites are more basic and consist of leucite, See also:olivine, See also:augite and biotite; the leucite is partly fresh, partly altered to analcite, and the rock has a spotted character recalling that of the leucite-syenites. It has been found only in the Highwood Mountains of See also:Montana. The leucite-See also:hearing dike-rocks are members of the tinguaite and monchiquite See also:groups. The leucite-tinguaites are usually See also:pale grey or greenish in colour and consist principally of nepheline, See also:alkali-felspar and aegirine. The latter forms See also:bright See also:green See also:moss-like patches and growths of indefinite shape, or in other cases scattered acicular prisms, among the felspars and nephelines of the ground See also:mass. Where leucite occurs, it is always eumorphic in small, rounded, many-sided crystals in the ground mass, or in larger masses which have the same characters as the pseudo-leucites. Biotite occurs in some of these rocks, and melanite also is present. Nepheline appears to decrease in amount as leucite increases. Rocks of this See also:group are known from Rio de Janeiro, Arkansas, See also:Kola (in See also:Finland), Montana and a few other places. In See also:Greenland there are leucitetinguaites with much arfvedsonite (See also:hornblende) and euclyalite. Wherever they occur they accompany leucite- and nephelinesyenites. Leucite-monchiquites are See also:fine-grained dark rocks consisting of olivine, titaniferous augite and See also:iron oxides, with a glassy ground mass in which small rounded crystals of leucite are scattered. They have been described from Bohemia. By far the greater number of the rocks which contain leucite are lavas of Tertiary or recent See also:geological age. They are never See also:acid rocks which contain quartz, but felspar is usually present, though there are certain groups of leucite lavas which are non-felspathic. Many of them also contain nepheline, sodalite, hauyne and nosean; the much rarer mineral melilite appears also in some examples. The commonest ferromagnesian mineral is augite (sometimes See also:rich in soda), with olivine in the more basic varieties. Hornblende and biotite occur also, but are less common. Melanite is found in some of the lavas, as in the leucite-syenites. The rocks in which orthoclase (or sanidine) is present in considerable amount are leucite-trachytes, leucite-phonolites and leucitophyres. Of these groups the two former, which are not sharply distinguished from one another by most authors, are common in the neighbourhood of See also:Rome (L. See also:Bracciano, L. See also:Bolsena). They are of trachytic appearance, containing phenocysts of sanidine, leucite, augite and biotite. Sodalite or hauyne may also be present, but nepheline is typically absent. Rocks of this class occur also in the tuffs of the Phlegraean See also:Fields, near See also:Naples. The leucitophyres are rare rocks which have been described from various parts of the volcanic See also:district of the See also:Rhine (Olbrtick, Laacher See, &c.) and from See also:Monte See also:Vulture in See also:Italy. They are rich in leucite, but contain also some sanidine and often much nepheline with hauyne or nosean. Their See also:pyroxene is principally aegirine or aegirine augite; some of them are rich in melanite. Microscopic sections of some of these rocks are of great interest on account of their beauty and the variety of felspathoid minerals which they contain. In See also:Brazil leucitophyres have been found which belong to the Carboniferous See also:period. Those leucite rocks which contain abundant essential See also:plagioclase felspar are known as leucite-tephrites and leucite-basanites. The former consist mainly of plagioclase, leucite and augite, while the latter contain olivine in addition. The Ieucite is often present in two sets of crystals, both porphyritic and as an ingredient of the ground mass. It is always idiomorphic with rounded outlines. The felspar ranges from See also:bytownite to See also:oligoclase, being usually a variety of See also:labradorite; orthoclase is scarce. The augite varies a See also:good See also:deal in character, being green, See also: Leucite lavas from which felspar is absent are divided into the leucitites and leucite basalts. The latter contain olivine, the former do not. Pyroxene is the usual ferromagnesian mineral, and resembles that of the tephrites and basanites. Sanidine, melanite, hauyne and perofskite are frequent accessory minerals in these rocks, and many of them contain melilite in some quantity. The well-known leucitite of the See also:Capo di Bove, near Rome, is rich in this mineral, which forms irregular plates, yellow in the hand specimen, enclosing many small rounded crystals of leucite. Bracciano and Roccamonfina are other See also:Italian localities for leucitite, and in Java, Montana, Celebes and New See also:South See also:Wales similar rocks occur. The leucitebasalts belong to more basic types and are rich in olivine and augite. They occur in great See also:numbers in the Rhenish volcanic district (See also:Eifel, Laacher See) and in Bohemia, and accompany tephrites or leucitites in Java, Montana, Celebes and Sardinia. The " See also:peperino " of the neighbourhood of Rome is a leucitite See also:tuff. U. S. Additional information and CommentsThere are no comments yet for this article.
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