Leucitite orleucite rock is anigneous rock containingleucite. It is scarce, many countries such asEngland being entirely without them. However, they are of wide distribution, occurring in every quarter of the globe. Taken collectively, they exhibit a considerable variety of types and are of great interest petrographically. For the presence of this mineral it is necessary that the silica percentage of the rock should be low, since leucite is incompatible with freequartz and reacts with it to form potassiumfeldspar. Because it weathers rapidly, leucite is most common inlavas of recent andTertiary age, which have a fair amount of potassium, or at any rate have potassium equal to or greater than sodium; if sodium is abundantnepheline occurs rather than leucite.
In pre-Tertiary rocks leucite readily decomposes and changes tozeolites,analcite and other secondary minerals. Leucite also is rare inplutonic rocks anddike rocks, but leucitesyenite and leucite tinguaite bear witness to the possibility that it may occur in this manner. The rounded shape of its crystals, their white or grey color, and absence of planar cleavage make the presence of leucite easily determinable in many of these rocks by inspection, especially when the crystals are large.
"Pseudoleucites" are rounded areas consisting offeldspar,nepheline,analcite, &c., which have the shape, composition and sometimes even the outward crystalline shape of leucite; they are probablypseudomorphs or paramorphs, which have developed from leucite because this mineral is not stable at ordinary temperatures and can be expected under favorable conditions to undergo spontaneous change into anaggregate of other minerals. Leucite is very often accompanied by nepheline,sodalite ornosean; other minerals which make their appearance with some frequency aremelanite,garnet andmelilite.
The plutonic leucite-bearing rocks are leucite syenite andmissourite. Of these the former consists oforthoclase, nepheline, sodalite,diopside andaegirine,biotite andsphene. Two occurrences are known, one inArkansas, the other in Sutherland,Scotland. The Scottish rock has been calledborolanite. Both examples show large rounded spots in the hand specimens; they are pseudoleucites, and under the 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; in both rocks melanite is an important accessory. The missourites are moremafic and consist of leucite,olivine,augite andbiotite; the leucite is partly fresh partly altered toanalcite, and the rock has a spotted character recalling that of the leucite-syenites. It has been found only in theHighwood Mountains ofMontana.
The leucite-hearing dike-rocks are members of thetinguaite andmonchiquite groups. The leucite tinguaites are usually pale grey or greenish in color and consist principally of nepheline, alkali feldspar and aegirine. The latter forms bright green moss-like patches and growths of indefinite shape, or in other cases scattered acicular prisms, among the feldspars and nephelines of the ground mass. Where leucite occurs, it is always euhedral in small, equant, many-sided crystals in the ground mass, or in larger masses which have the same characters as the pseudoleucites. Biotite occurs in some of these rocks, and melanite also is present. Nepheline decreases in amount as leucite increases since the abundances of the two reflect the Na:K ratio of the rock. Rocks of this group are known fromRio de Janeiro, Arkansas,Kola Peninsula (inRussia),Montana and a few other places., InGreenland there are leucite tinguaites with mucharfvedsonite, (hornblende) and eudialyte. Wherever they occur they accompany leucite- and nepheline syenites. 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 have been described fromCzechoslovakia.
By far the greater number of the rocks which contain leucite arelavas ofTertiary or recent geological age. Although these never contain quartz, butfeldspar is usually present, though there are certain groups of leucite lavas which are non-feldspathic. Many of them also contain nepheline, sodalite,hauyne and nosean; the much rarer mineralmelilite appears also in some examples. The commonest ferromagnesian mineral isaugite (sometimes rich insodium), with olivine in the more basic varieties.Hornblende andbiotite occur also, but are less common. Melanite is found in some of the lavas, as in the leucite syenites. An extrusive leucitite rich in olivine andclinopyroxene, with a glassygroundmass rich in sodium, is sometimes called augandite, though this terminology is not recommended and such rocks are better described as olivine leucitites.[1]
The rocks in whichorthoclase (or sanidine) is present in considerable amount are leucite-trachytes, leucite-phonolites and leucitophvres. Of these groups the two former, which are not sharply distinguished from one another by most authors, are common in the neighborhood ofRome. They are of trachytic appearance, containing phenocrysts ofsanidine, leucite, augite and biotite. Sodalite or hauyne may also be present, but nepheline is typically absent. Rocks of this class occur also in thetuffs of thePhlegraean Fields, nearNaples. The leucitophyres are rare rocks which have been described from various parts of the volcanic district of the Rhine (Olbrck. Laacher See, etc.) and from Monte Vulture in Italy. They are rich in leucite, but contain also some sanidine and often much nepheline with hauyne or nosean. Their 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 offeldspathoid minerals which they contain. InBrazil leucitophyres have been found which belong to theCarboniferous period.
Those leucite rocks which contain abundant essential plagioclase feldspar 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 leucite 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 feldspar ranges from bytownite tooligoclase, being usually a variety oflabradorite; orthoclase is scarce. The augite varies a good deal in chemistry and optical character, being green, brown or violet (suggesting high Na and Ti content), but it is rarely high enough in Na and Fe to qualify as aegirine-augite oraegirine. Among the accessory minerals biotite, brown hornblende, hauyne, iron oxides andapatite are the commonest; melanite and nepheline may also occur. The ground mass of these rocks is only occasionally rich in glass. The leucite-tephrites and leucite-basanites ofVesuvius and Somma are familiar examples of this class of rocks. They are black or ashy-grey in color, often vesicular, and may contain many large grey phenocrysts of leucite. Their black augite and yellow green olivine are also easily observed in hand specimens. From Volcan Ello, Sardinia and Roccamonfina similar rocks are obtained; they occur also inBohemia, inJava,Celebes,Kilimanjaro (Africa) and nearTrebizond inAsia Minor.
Leucite lavas from which feldspar is absent are divided into the leucitites and leucitebasalts. 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 andperovskite are frequent accessory minerals in these rocks, and many of them contain melilite in some quantity, The well-known leucitite of the 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 Italian localities for leucitite, and in Java, Montana, Celebes andNew South Wales similar rocks occur, The leucite basalts belong to more basic types and are rich in olivine and augite. They occur in great numbers in the Rhenish volcanic district (Eifel, Laacher See) and in Bohemia, and accompany tephrites or leucitites in Java, Montana, Celebes and Sardinia. Thepeperino of the neighborhood of Rome is a leucititetuff.
This article incorporates text from a publication now in thepublic domain: Spencer, Leonard James (1911). "Leucite". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 16 (11th ed.). Cambridge University Press. pp. 503–504.| Types |  | |
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