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LAMPROPHYRES (from Gr. Xaurp6s, brigh...
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See also:LAMPROPHYRES (from Gr. Xaurp6s, See also:bright, and the terminal See also:part of the word See also:porphyry, meaning rocks containing bright porphyritic crystals) , a See also:group of rocks containing phenocrysts, usually of See also:biotite and See also:hornblende (with bright cleavage surfaces), often also of See also:olivine and See also:augite, but not of See also:felspar. They are thus distinguished from the porphyries and porphyrites in which the felspar has crystallized in two generations. They are essentially " See also:dike rocks," occurring as dikes and thin sills, and are also found as marginal facies of plutonic intrusions. They furnish a See also:good example of the correlation which often exists between petrographical types and their mode of occurrence, showing the importance of See also:physical conditions in determining the mineralogical and structural characters of rocks. They are usually dark in See also:colour, owing to the abundance of ferro-magnesian silicates, of relatively high specific gravity and liable to decomposition. For these reasons they have been defined as a melanocrate See also:series (See also:rich in the dark minerals); and they are often accompanied by a complementary leucocrate series (rich in the See also: Felspar is restricted to the ground mass; quartz occurs sometimes but is scarce. Although porphyritic structure is almost universal, it is some-times not very marked. The large biotites and hornblendes are not sharply distinct from those of intermediate See also:size, which in turn See also:graduate into the small crystals of the same minerals in the ground mass. As a See also:rule all the ingredients have rather perfect crystalline forms (except quartz), hence these rocks have been called " panidiomorphic." In many lamprophyres the palequartz and felspathic ingredients tend to occur in rounded spots, or ocelli, in which there has been progressive See also:crystallization from the margins towards the centre. These spots may consist of radiate or See also:brush-like felspars (with some See also:mica and hornblende) or of quartz and felspar. A central See also:area of quartz or of See also:analcite probably represents an See also:original miarolitic cavity infilled at a later See also:period. There are two See also:great See also:groups of lamprophyres differing in composition while retaining the See also:general features of the class. One of these accompanies intrusions of See also:granite and See also:diorite and includes the minettes, kersantites, vogesites and spessartites. The other is found in association with See also:nepheline syenites, essexites and teschenites, and is exemplified by camptonites, monchiquites and alnoites. The complementary facies of the first group is the aplites, porphyrites and felsites; that of the second group includes bostonites, tinguaites and other rocks. The granito-dioritic-lamprophyres (the first of these two groups) are found in many districts where granites and diorites occur, e.g. the Scottish See also:Highlands and See also:Southern Uplands, the See also:Lake See also:district, See also:Ireland, the See also:Vosges, Black See also:Forest, Harz, &c. As a rule they do not proceed directly from the granite, but See also:form See also:separate dikes which may be later than, and consequently may cut, the granites and diorites. In other districts where granites are abundant no rocks of this class are known. It is rare to find only one member of the group See also:present, but minettes, vogesites, kersantites, &c., all appear and there are usually transitional forms. For this See also:reason these rock See also:species must not be regarded as sharply distinct from one another. The group as a whole is a well-characterized one and shows few transitions to porphyries, porphyrites and other dike types; its subdivisions, however, tend to See also:merge into one another and especially when they are weathered are hard to differentiate. The presence or See also:absence of the four dominant minerals, See also:orthoclase, See also:plagioclase, biotite and hornblende, determines the species. Minettes contain biotite and orthoclase; kersantites, biotite and plagioclase. Vogesites contain hornblende and orthoclase; spessartites, hornblende and plagioclase. Each variety of lamprophyre may and often does contain all four minerals but is named according to the two which preponderate. These rocks contain also See also:iron oxides (usually titaniferous), See also:apatite, sometimes See also:sphene, augite and olivine. The hornblende and biotite are brown or greenish brown, and as a rule their crystals even when small are very perfect and give the micro-sections an easily recognizable See also:character. Green hornblende occurs in some of these rocks. The augite builds eumorphic crystals of See also:pale green colour, often zonal and readily weathering. Olivine in the fresh See also:state is rare; it forms rounded, corroded grains; in many cases it is decomposed to green or colourless hornblende in radiating nests (pilite). The plagioclase occurs as small rectangular crystals; orthoclase may have similar shapes or may be fibrous and grouped in sheaflike.aggregates which are narrow in the See also:middle and spread out towards both ends. If quartz is present it is the last product of crystallization and the only See also:mineral devoid of idiomorphism; it fills up the spaces between the other ingredients of the rock. As all lamprophyres are prone to alteration by weathering a great abundance of secondary minerals is usually found in them; the See also:principal are See also:calcite and other See also:carbonates, See also:limonite, See also:chlorite, quartz and See also:kaolin. Ocellar structure is See also:common; the ocelli consist mainly of orthoclase and quartz, and may be a See also:quarter of an See also:inch in See also:diameter. Another feature of these rocks is the presence of large See also:foreign crystals or xenocrysts of felspar and of quartz. Their forms are rounded, indicating partial resorption by the solvent See also:action of the lamprophyric magma; and the quartz may be surrounded by corrosion See also:borders of minerals such as augite and hornblende produced where the magma is attacking the crystal. These crystals are of doubtful origin; they are often of considerable size and may be conspicuous in hand-specimens of the rocks. It is supposed that they did not crystallize in the lamprophyre dike but in some way were caught up by it. Other enclosures, more certainly of foreign origin, are often seen, such as See also:quartzite, See also:schists, garnetiferous rocks, granite, &c. These may be baked and altered or in other cases partly dissolved. Cordierite may be formed either in the enclosure or in the lamprophyre, where it takes the shape of hexagonal prisms which in polarized light break up into six sectors, triangular in shape, diverging from the centre of the crystal. The second group of lamprophyric dike rocks (the camptonite, monchiquite, alnoite series) is much less common than those above described. As a rule they occur together, and there are transitions between the different sub-groups as in the granito-dioritic lamprophyres. In See also:Sweden, See also:Brazil, See also:Portugal, See also:Norway, the See also:north of See also:Scotland, Bohemia, See also:Arkansas and other places this assemblage of rock, types has been met with, always presenting nearly identical features. In most cases, though not in all, they have a See also:close association with nepheline or See also:leucite syenites and similar rocks rich in alkalies. This indicates a genetic See also:affinity like that which exists between the granites and the minettes, &c., and further See also:proof of this connexion is furnished by the occasional occurrence in those lamprophyres of leucite, hauyne and other felspathoid minerals. The camptonites (called after Campton, New See also:Hampshire) are dark brown, nearly black rocks often with large hornblende phenocrysts. Their essential minerals in thin See also:section are hornblende of a strong reddish-brown colour; augite purple, pleochroic and rich in See also:titanium, nlivine and plagioclase felspar. They have the porphyritic and panidiomorphic structures described in the rocks of the previous group, and like them also have an ocellar character, often very conspicuous under the See also:microscope. The See also:accessory minerals are biotite, apatite, iron oxides and analcite. They decompose readily and are then filled with carbonates. Many of these rocks prove on See also:analysis to be exceedingly rich in titanium; they may contain 4 or 5% of titanium dioxide. The monchiquites (called after the Serra de See also:Monchique, Portugal) are See also:fine-grained and devoid of felspar. Their essential constituents are olivine and purplish augite. Brown hornblende, like that of the camptonites, occurs in many of them. An interstitial substance is present, which may sometimes be a brown See also:glass, but at other times is colourless and is believed by some petrographers to be See also:primary crystalline analcite. They would define the monchiquites as rocks consisting of olivine, augite and analcite; others regard the analcite as secondary, and consider the See also:base as essentially glassy. Some monchiquites contain hauyne; while in others small leucites are found. Ocellar structure is occasionally present, though less marked than in the camptonites. A See also:special group of monchiquites rich in deep brown biotite has been called fourchites (after the Fourche Mountains, Arkansas). The alnoites (called after the See also:island of AlnB in Norway) are rare rocks found in Norway, See also:Montreal and other parts of North See also:America and in the north of Scotland. They contain olivine, augite, brown biotite and melilite. They are See also:free from felspar, and contain very See also:low percentages of See also:silica. The chemical composition of some of these rocks will be indicated by the analyses of certain well-known examples. SiO2 TiO2 Al203 Fe203 FeO MgO CaO Na20 See also:K20 1.1 52.70 1.71 15.07 8.41 ... 7'23 5'33 3.12 4.81 II. 52.12 1.20 13.52 2.56 4.53 6.36 5.78 2.34 5.36 IV. 54.67 ... 12.68 11.68 2.13 6.11 4.96 3'85 3'65 V. 41'96 4.15 15'36 3'27 9.89 5'01 9.47 5'15 0.19 VI. 43'74 2.80 14.82 2.40 7.52 6.98 1o•81 3.06 2.90 In addition to the oxides given these rocks contain small quantities of See also:water (combined and hygroscopic), See also:CO2, S, MnO, P205, Caz03, &c. (J. S. Additional information and CommentsThere are no comments yet for this article.
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