Labradorite

“ Microscopic examination shows the rock to be a holocrystalline aggregate of plagioclase (labradorite) , augite, hornblende and olivine, with a very little magnetite. The labradorite, which forms about half the rock, is very clear and fresh, in large crystals which frequently show a tendency to idiomorphism. Smaller crystals of labradorite are frequently included in the augite and hornblende. Augite, the next most abundant constituent after the felspar, is present in irregular almost colourless grains with well marked cleavage and numerous irregular cracks, and sometimes slightly clouded. ”

“ The ornamental stone from south Norway, now largely used as a decorative material in architecture, owes its beauty to a felspar with a blue opalescence, often called labradorite, but really a kind of orthoclase which Professor W. C. Brögger has termed cryptoperthite, whilst the rock in which it occurs is an augite-syenite called by him laurvigite, from its chief locality, Laurvik in Norway. Common labradorite, without play of colour, is an important constituent of such rocks as gabbro, diorite, andesite, dolerite and basalt. ”

“ Dolerite, when composed entirely of augite and labradorite, produces rather inferior soils; but when it contains oligoclase and zeolites, and comes under the head of basalt, its disintegration is the source of soils remarkable for their fertility; for these latter substances undergoing rapid decomposition furnish the plants with abundant supplies of alkalies and lime, while the more slowly decomposing hornblende affords the necessary quantity of magnesia. ”

“ The plagioclase phenocrysts owe their opacity in part to their consisting of an aggregate of lesser crystals which are clear and glassy and give lamellar extinctions of 20° to 30° (andesine labradorite) . They do not usually exceed 3 mm. and are sometimes scanty. The pyroxene phenocrysts, which are at times infrequent, may be 5 mm. in size. They are of pale yellowish-brown augite, giving extinctions of 40°. The felspar-lathes are rarely lamellar ”

“ It consists of abundant grains of olivine, imperfect octahedra and shapeless granules of magnetite, little simple or twinned prisms of labradorite, and a pale brown augite. The last-named mineral is always the latest product of consolidation, but it varies in habit, being sometimes in ophitic patches moulded upon or enclosing the other minerals, sometimes in small granules occupying the interstices between the felspars and other crystals. ”

“ The name was afterwards applied by G. Tschermak to those plagioclase felspars which lie between labradorite and anorthite; and this has been generally adopted by petrologists. In chemical composition and in optical and other physical characters it is thus much nearer to the anorthite end of the series than to albite. Like labradorite and anorthite, it is a common constituent of basic igneous rocks, such as gabbro and basalt. Isolated crystals of bytownite bounded by well-defined faces are unknown. ”

“ Other opticians believe the colored reflections to be due to laminæ formed by incipient crystallization, as seen in the equally remarkable mineral known as labradorite. Babinet believes the brilliant colors of the opal to be due to the narrow fissures in the stone, like those produced in the partial fracture of glass or quartz. ”

“ The remarkable sheen of labradorite or blue felspar has its origin in the interference of light at lamellar surfaces in the interior; the uniformity of the colour over comparatively large areas testifies to the regularity of the lamellar arrangement. The finest specimens were brought from the Isle of St. Paul off the coast of Labrador, where they were first discovered in 1770; large masses also occur on the coast itself. Amazon-stone is an opaque green felspar which occurs in the Ilmen256 Mountains, Orenburg, Russia, and at Pike’s Peak, Colorado, United States. ”

“ Greenstone.—This name has usually been extended to all granular mixtures, whether of hornblende and feldspar, or of augite and feldspar. The term diorite has been applied exclusively to compounds of hornblende and triclinic feldspar. Labrador-rock is a term used for a compound of labradorite or labrador-feldspar and hypersthene; when the hypersthene predominates it is sometimes known under the name of Hypersthene-rock. Gabbro and Diabase are rocks mainly composed of triclinic feldspars and diallage. ”

“ Brown hornblende is much less common, and a red biotite is very characteristic of certain nephelinites. Of the felspars, labradorite is probably the most common, with more acid varieties of plagioclase. ”

“ The small plagioclase phenocrysts, which are most evident in the slide, present the two kinds above described under Species A. They give lamellar extinctions varying from those of medium andesine to acid labradorite (15-30°) . The augite phenocrysts, which are small and scanty, occasionally show intergrowths of rhombic pyroxene. The augite granules are generally ·02 to ·03 mm. in size, and here and there a prism form gives extinctions of +25°. The felspar-lathes which average ·11 to ·15 mm. long, are often rather stout, showing a few lamellæ that give extinctions of medium and basic andesine. ”

“ The climate is much too severe for agriculture. Summer is very short, and plagued with mosquitoes. The rivers abound in salmon; the fox, marten, otter, and other animals are trapped for their fur; iron and labradorite are plentiful. The population is largely Eskimo, christianised by the Moravians. The name Labrador specially belongs to the region along the eastern coast, between Capes St. Louis and Chudleigh, presenting a barren front to the sea, precipitous, much indented, and fringed with rocky islands. This region is governed by Newfoundland; its chief industry is cod and herring fishing. ”

“ Orthoclase is rarely entirely absent, and when it is fairly common the rock becomes a tonalite; in this way a transition is furnished between diorites and granites. It is rare to find the pure types of “hornblende-diorite,” “augite-diorite,” &c., but in most cases the rocks contain two or more ferromagnesian silicates, and such combinations as “hornblende-biotite-diorite” are commonest in nature.
The felspar of the diorites ranges in composition from oligoclase to labradorite, and is often remarkably zonal, the external layers being more alkaline than the internal. ”

“ Tracing the variations of an intrusive dolerite outwards in the direction of the rocks which it has invaded, we perceive change first in the augite. The large crystals and kernels of that mineral grow smaller until they pass into a granulated form like that characteristic of basalts. The large plates and - 453 - amorphous patches of titaniferous iron or magnetite give place to minute particles, which tend to group themselves into long club-shaped bodies. The labradorite continues but little affected, except that its prisms, though as defined, may not be quite so large. ”

“ The structure and mineral constitution render the term quartz-diabase appropriate. Finer grained specimens secured from dikes were found to be much fresher than the coarser types and yielded more satisfactory thin sections; the ophitic structure is more pronounced, but the mineral composition is the same. The plagioclase laths of one section were determined optically to be an intermediate labradorite. Small flakes of biotite partially altered to chlorite occur accessorily.
The aplite is also so much weathered that doubt sometimes exists as to its original composition. ”

“ It has a specific gravity of 2·77 and displays phenocrysts of plagioclase, olivine, and augite. The first-named, which give the lamellar extinction of acid labradorite, (22°-28°) , are fresh-looking and only affected to a small extent by the magma. Those of olivine and augite are in much the same condition. The glass of the groundmass is rather turbid and displays numerous dark patches of incipient crystallisation, which in some cases prove to be composed of brush-like crystallites around a clear H-shaped nucleus, and in other cases have a more prismatic form. ”

“ If the zÂrolite of Juvenas contain separable crystals of augite and labradorite, the numerical relation of the constituents p 132 render it at least probable that the meteoric masses of Chateau-Renard may be a compound of diorite, consisting of hornblende and albite, and those of Blansko and Chantonnay compounds of hornblende and labradorite. ”

“ The more abundant minerals are plagioclase feldspar (labradorite or anorthite) , with one or more of the ferromagnesian minerals (augite, olivine, or biotite) , and magnetite or ilmenite. In the growth of the minerals one crystal frequently embraces others, giving an ophitic structure. The dolerites have many varieties, due either to accessory minerals or to the development of some of the constituents more amply than the rest. The type may be said to consist of plagioclase and augite, the418 other minerals being regarded as accessories. Magnetite or ilmenite is almost universally present. ”

“ An instance of this rock occurs in Kincardineshire, from which the large flat twins of labradorite have been analyzed by Dr. Heddle (Trans. Roy. Soc. Edin. vol. xxviii. (1879) , p. 257) .
[330] C. T. Clough, "The Cheviot Hills," Mem. Geol. Survey (1888) , p. 12.
The texture of the andesites occasionally becomes faintly resinous, where a considerable proportion of glass still remains undevitrified, as in the well-known varieties from the Cheviot Hills, and in another pitchstone-like rock from above Airthrey Castle in the Ochil Hills, near Bridge of Allan. ”

“ The medical profession has long appreciated the dangers involved in the administration of various mineral remedies now so commonly employed, and the value of a safe, effective and reliable vegetable antipyretic is universally recognized. Such a remedy is Labordine. It is purely vegetable in its composition and produces none of the evil after-effects of the coal-tar derivatives.... Labordine ... is a purely vegetable cardiac stimulant.... There is nothing mysterious about Labordine or its constituents.... ”

“ The common termination ite (originally itis or ites) was adopted by the Greeks and Romans for the names of stones, the names themselves indicating some character, constituent, or use of the stone, or the locality at which it was found. For example, haematite, because of the blood-red colour; siderite, containing iron; alabaster (originally alabastritis) , a stone from which a vessel called an alabastron was cut; magnetite, from the locality Magnesia. The custom of naming minerals after persons is of modern origin; e.g. prehnite, biotite, haüyne, zoisite. ”

“ 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 iron oxides (usually titaniferous) , apatite, sometimes 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 character. Green hornblende occurs in some of these rocks. The augite builds eumorphic crystals of pale green colour, often zonal and readily weathering. Olivine in the fresh state is rare ”

“ This is in parts very like an ordinary basalt or anamesite, with a glistening surface and weathering brown. Parts of it are rather porphyritic, having very white felspar crystals up to about 1/4 inch long. I have had slides cut from this; and on examination it proves to be a basalt, the plagioclase being fairly well preserved, the augite sometimes very characteristic, and a good deal of olivine with a granulated dusky aspect like (though on a smaller scale) that described above; there is also a fair quantity of magnetite. ”

“ In addition to the blue cubic minerals in lapis lazuli, the following minerals have also been found: a non-ferriferous diopside, an amphibole called, from the Russian mineralogist, koksharovite, orthoclase, plagioclase, a muscovite-like mica, apatite, titanite, zircon, calcite and pyrite. The calcite seems to form in some cases a great part of the lapis; and the pyrite, which may occur in patches, is often altered to limonite. ”

“ Labrador is a miracle of youth and age combined. It is still growing out of the depths with the irresistible vigour of youth. But its titanic tablelands consist of those azoic rocks which form the very roots of all the other mountains in the world, and which are so old, so immeasurably older than any others now standing on the surface of the globe, that their Laurentians alone have the real right to bear the title of "The Everlasting Hills". ”

“ This may be true in some cases, for alabandite has been found in a few metalliferous deposits in Colorado, Mexico, Germany, Peru and elsewhere, but in most cases, at least in the Rocky Mountains, when the level is reached at which the oxidized forms of lead, zinc, iron and other metals pass into sulphides, the manganese passes into carbonate or silicate, and remains in one or both of those forms to all depths that have been reached. ”

“ Among the numerous species of the trap family here existing, basalt is the most conspicuous, and it occurs almost every where, alternating in an irregular manner with all the other species or varieties. It is most frequently amorphous, displaying at the same time so great a variety both in its natural mode of breaking, in its external appearance on weathering, and in its texture and colour, as to form a great number of subordinate varieties much more remarkable in their natural situations than when broken into hand specimens. ”