Pyroxene

“ Description.—Dark-brown or blackish rocks. Sp. gr. 2·88 to 2·93. Phenocrysts of pyroxene occur in fair quantity in addition to those of the olivine and plagioclase. The groundmass displays a plexus of felspars and augite-granules with much magnetite in grains and irregular patches. The interstitial glass is scanty or almost absent. The olivine phenocrysts, of which the larger are 3 to 4 mm. in size, are as a rule hematised at the borders and in the fissures, and are sometimes partially serpentinised. In some cases small crystals of olivine are enclosed in the pyroxene-phenocrysts. ”

“ As indicated by the extinctions of occasional prism-forms it is composed of both augite and rhombic pyroxene, the former prevailing. The prismatic sub-order is also represented, and here the pyroxene of the groundmass is in great part prismatic, the length of the prisms not often exceeding ·05 mm., whilst both the monoclinic and rhombic kinds occur. Interstitial glass varies in amount, sometimes absent, sometimes scanty and viriditic, at other times abundant and opaque. Magnetite abounds in the groundmass much of it often being of secondary origin. ”

“ The olivine phenocrysts may be fairly fresh with clean outlines, or they may be deeply eroded and stained by iron oxide, or they may be passing into serpentine. The pyroxene phenocrysts may be either eroded or possess regular outlines. They are of pale brown augite and give extinctions over 30°. The pyroxene granules, which average ·02 mm. and are very abundant, are of the same augite. The felspar-lathes are relatively scanty. In the two rocks here included they average in length ·06 mm. and ·08 mm. When lamellæ can be recognised they give extinctions of 30°-40° (labradorite) . ”

“ It is at the base of the limestone that the fossil occurs. This part of the zone is largely composed of great and small irregular masses of white crystalline pyroxene, some of them twenty yards in length by four or five wide. They appear to be confusedly placed one above another, with many ragged interstices, and smoothly-worn, rounded, large and small pits and sub-cylindrical cavities, some of them pretty deep. The pyroxene, though it appears compact, presents a multitude of small spaces consisting of carbonate of lime, and many of these show minute structures similar to that of the fossil. ”

“ Belonging to the same series with diopside and hedenbergite is a manganese pyroxene, known as schefierite, which has the composition (Ca, Mg) (Fe, Mn) (Si03) 2.
Diopside is the characteristic pyroxene of metamorphic rocks, occurring especially in crystalline limestones, and often in association with garnet and epidote. It is also an essential constituent of some pyroxene-granites, diorites and a few other igneous rocks, but the characteristic pyroxene of this class of rocks is augite. ”

“ ACMITE, or AEGIRITE, a mineral of the pyroxene (q.v.) group, which may be described as a soda-pyroxene, being essentially a sodium and ferric metasilicate, NAFe (SiO3) 2. In its crystallographic characters it is close to ordinary pyroxene (augite and diopside) , being monoclinic and having nearly the same angle between the prismatic cleavages. There are, however, important differences in the optical characters: the birefringence of acmite is negative, the pleochroism is strong and the extinction angle on the plane of symmetry measured to the vertical axis is small (3 deg. -5 deg. ) . ”

“ Pyroxene—the type of a large and important group of rock-forming ferromagnesian minerals; varies in composition and embraces a large number of466 varieties; usually a magnesium-iron-calcium silicate; crystals usually thick and stout, but varying greatly; sometimes lamellar and fibrous; H. 5–6; luster vitreous inclining to resinous; green of various shades verging towards light colors, occasionally more often to browns and blacks; among the minerals belonging to the pyroxene group are augite, bronzite, diallage, diopside, enstatite, hypersthene, and others. ”

“ This is the more remarkable as pyroxene is most usually found as an ingredient of igneous rocks; but Dr. Hunt has shown that in the Laurentian limestones and also in veins traversing them, it occurs under conditions which imply its deposition from water, either cold or warm. Gümbel remarks on this:—"Hunt, in a very ingenious « 109 » manner, compares this formation and deposition of serpentine, pyroxene, and loganite, with that of glauconite, whose formation has gone on uninterruptedly from the Silurian to the Tertiary period, and is even now taking place in the depths of the sea ”

“ What has been called "pyroxene-granulite" is a dark granular eclogite, including rhombic pyroxene side by side with garnet, and associated, in Saxony - 149 - and Skye, with igneous intrusions. In both localities it has been shown to result from the inclusion of basic rocks, such as dolerites and gabbros, in a bath of some invading magma. The lens-like form of the Saxon masses, and the occurrence also of sheets of pyroxene-granulite interlaminated with fine-grained granite, were till lately attributed to the rolling-out action of pressure-metamorphism. ”

“ Proceeding along the coast from Ngara-vutu to Navetau, the Buli’s town of the Tunuloa district, one observes blocks of basic rocks. Farther to the eastward for a distance of two or three miles basic agglomerates and basic tuff-agglomerates prevail along the sea-border, being often extensively exposed in the cliff-faces of the hills. The rock composing the blocks is a hemi-crystalline pyroxene-andesite remarkable for the prismatic pyroxene in the groundmass, and referred to genus 17 of the hypersthene-augite sub-class. ”

“ Olivine, however, is not invariably present. [330] The pyroxene also does not always show the peculiar fibrous structure of diallage. Professor Judd, indeed, maintains that the diallagic form is due to a deep-seated process of alteration (schillerization) , and that the same crystal may consist partly of ordinary augite and partly of diallage. [331] Ilmenite (with leucoxene) , magnetite, apatite, biotite, and epidote are not infrequent constituents. ”

“ As a matter of fact, the pyroxenes are, next to the felspars, the commonest constituents of igneous rocks of almost all kinds, being especially characteristic of those of basic composition. An igneous rock composed almost wholly of pyroxene is known as a pyroxenite. Besides being minerals of primary origin in igneous rocks. the pyroxenes are also of frequent occurrence in metamorphic rocks, for example, in crystalline limestones, being then of secondary origin. ”

“ This hornblende is not a fibrous variety but forms compact individuals, hence in the specimens studied it could not be certain that some of it was not primary. In some cases it is further altered to chlorite. Reaction between 35plagioclase and pyroxene seems to have taken place, for chlorite occurs among the plagioclase decomposition products as well as those of the pyroxene. Next to these, ilmenite is the most abundant constituent, occurring as irregular masses largely altered to leucoxene in which the original ‘gridiron’ structure is distinct. ”

“ The mineral is always a little pleochroic and is never colourless, and it is only in very rare cases that the term “enstatite” could be used. As a matter of fact there is practically only one form of rhombic pyroxene represented in my collections whether in acid or basic andesites or in hemi-crystalline and plutonic rocks. In the acid andesites it occurs not only as phenocrysts but also as minute prisms forming a constituent of the groundmass. ”

“ The principal minerals are plagioclase (normally labradorite) and pyroxene (normally diallage) with magnetite or ilmenite. They are usually dark, heavy rocks. The pearly luster of the cleavage faces of the diallage, when present, gives a peculiar sheen to a fresh surface of the rock. In the recently proposed field names, gabbro is made to include all phanerocrystalline rocks in which pyroxene predominates, attended by feldspar of any kind in subordinate quantity, with or without hornblende or mica. ”

“ It occurs in various parts of the Kola peninsula in north Finland on the shores of the White Sea. The pyroxene is morphic, yellow or green, and is surrounded by formless areas of nepheline. The accessory minerals are apatite, cancrinite, calcite, titanite and jiwaarite, a dark-brown titaniferous variety of melanite-garnet. This rock is the plutonic and holocrystalline analogue of the nephelenites and nepheline-dolerites; it bears the same relation to them as the nepheline-syenites have to the phonolites. ”

“ The most pronounced lamination is that parallel to the orthopinacoid; another, less distinct, is parallel to the basal plane, and a third parallel to the plane of symmetry; these planes of secondary parting are in addition to the ordinary prismatic cleavage of all pyroxenes. Frequently the material is interlaminated with a rhombic pyroxene (bronzite) or with an amphibole (smaragdite or uralite) , the latter being an alteration product of the diallage. ”

“ It is a common and very widely disseminated mineral in rocks of all kinds and ages, but especially in metamorphic rocks as veins, and banded or lenslike deposits. Most igneous rocks contain small scattering grains of pyrite. Many deposits of commercial value are known. Great quantities are burned for the manufacture of sulphuric acid (“oil of vitriol”) which is one of the most important of all chemicals.
Pyroxene Group. Along with quartz and feldspars, the pyroxenes rank among the most common of all minerals. (See Figure 74k.) Composition, very similar to amphibole (see above) . ”

“ Further, how does it happen that in some specimens serpentine and pyroxene seem to have encroached upon the structure, as if they and not calcite were the eroding minerals? How any one who has looked at the structures can for a moment imagine such a possibility, it is difficult to understand. ”

“ Pyrotic, pī-rot′ik, adj. burning: caustic.—n. a caustic medicine. [Gr. pyrōtikos—pyr, pyros, fire.]
Pyroxene, pī′rok-sēn, n. an important mineral species, occurring in monoclinic crystals.—adj. Pyroxen′ic. [Gr. pyr, fire, xenos, a guest.]
Pyroxylic, pī-rok-sil′ik, adj. obtained by distilling wood.—ns. Pyrox′yle, Pyrox′ylin, -e, gun-cotton.—Pyroxylic spirit, a mixture of acetone, methyl-alcohol, acetate of methyl, &c., obtained by the destructive distillation of wood in the manufacture of pyroligneous acid. [Gr. pyr, fire, xylon, wood.] ”

“ Quartz occurs both in irregular simple grains and as micropegmatite. Occasionally pale green pyroxene is visible in the centre of crystals of dark green hornblende. The accessory minerals apatite, magnetite and zircon are always present, and very common also are orthite in coffee-coloured zonal prisms practically always encircled by yellow epidote, and reddish-brown crystals of sphene, simple or twinned. ”

“ Labradorite, showing multiple twinning (the striation) and the iridescent play of colors (in color) 299 Plate 36.—Crystal form of a pyroxene. Cross sections of a pyroxene crystal showing the lines of intersection of two cleavage planes. ”

“ In contrast with these light-coloured minerals, iron, magnesium, and part of the calcium, appear in another series of silicates, usually dark in colour, and this series may be broadly styled "ferromagnesian." The pyroxenes, of which augite is the type, the amphiboles, of which hornblende is the type, dark mica (mostly biotite) , and olivine, are the ordinary ferromagnesian minerals. ”

“ Ether when shaken with the concentrated aqueous solution and left to evaporate, leaves colourless crystals of pyrocatechin which after purification are devoid of acid reaction. They have a peculiar burning persistent taste, and are very pungent and irritating when allowed to evaporate. A solution of pyrocatechin yields with perchloride of iron a dark green coloration changing to black after a few moments, and becoming red on the addition of potash. ”

“ No alteration is produced in a solution of pyrocatechin by protosalts of iron.
Among the few medicinal preparations of tar, is Tar Water, called Aqua vel Liquor Picis, made by agitating wood-tar with water. The presence in it of pyrocatechin is easily proved by the above-mentioned reactions, or by a few drops of red chromate of potassium, which produces a brownish black colouration. It may hence be inferred that pyrocatechin is perhaps the active ingredient in tar-water, and that for making this liquid the granular, crystalline sorts of tar should be preferred. ”

“ It has long been known in photography, that a developer must be of the nature of a reducing agent, either inorganic or organic, and many hydroxylic and amidic derivatives of hydrocarbons come under this category. Thus, pyrogallol, which has already been referred to as a trihydroxybenzene (p. 146) , when dissolved in alkali rapidly absorbs oxygen—it is a strong reducing agent, and is thus of value as a developer. But although pyrogallol is a benzene derivative, and could if necessary be prepared synthetically, it can hardly be claimed as a tar product, as it is generally made from gallic acid. ”

“ It is resinified by the action of concentrated mineral acids. On warming solutions of pyrrol in dilute acid, ammonia is evolved, and an amorphous powder of variable composition, known as pyrrol-red, separates out. The pyrrol ring is easily broken, e.g. hydroxylamine gives the dioxime of succinic aldehyde. Pyrrol is readily converted into pyridine derivatives by acting with bromoform, chloroform, or methylene iodide on its potassium salt, β-brom-and β-chlorpyridine being obtained with the first two compounds, and pyridine itself with the last. ”