Augite

“ The augite exists as a finely granular pale green substance, which might at first be taken for a glass, but it gives the characteristic action of augite with polarized light. It is intimately mixed through the clear glass of the groundmass, which it far exceeds in quantity. ”

“ Whether the latter are a mineral of the spinel-group or belong to perovskite, with which they accord in color, could not be decided.
Augite occurs in only one generation; phenocrysts of augite are wanting. In the rather coarse-grained groundmass, it becomes the most abundant constituent. The mineral shows a grayish-brown color, common to basaltic augite, sometimes with a tint of violet. It generally forms well-shaped crystals, rarely irregular grains, and bears inclusions of magnetite and glass. ”

“ Although this material is very dark greenish brown and not commonly thought of as a gemstone, lapidaries have used it to fashion black faceted stones and cabochons that resemble obsidian. Most of the augite occurs as loose pieces and crystal fragments that have weathered out of nearby igneous rocks; the augite can also be found in situ in the igneous rocks.
Specimens and pieces of cutting quality 1 inch in diameter are common, and fragments over 2 inches in diameter have been found. The augite is associated with black spinel and some dark gray to black pieces of natural glass. ”

“ The species are also exceedingly numerous, but we may note here only a few of the more important. They are composed of silica and various bases, such as lime, potash, magnesia, soda, alumina, &c. Augite or pyroxene is a black or greenish-black mineral, found, either as crystals, which are generally small, or as rounded grains and angular fragments, in basaltic and volcanic rocks. It never occurs in granite rocks. It is brittle, and has a vitreous or resinous lustre. There are a number of varieties or sub-species of augite. Hornblende, like augite, also includes a great many minerals. ”

“ The other mineral, which is colourless with ordinary light, is now seen to be of more than one species. One part exhibits the characteristic cleavage of augite; this occurs in somewhat rounded grains; very thin veins of serpentine frequently traverse the crystal, following the lines of its cleavage-planes. This mineral does not seem the result of change of the augite, but to have formed in the cracks; and the generally open condition of the cleavage-planes rather bears out the idea. The colours of the augite are generally dull ”

“ The augite varies a good deal in character, being green, brown or violet, but aegirine (the dark green pleochroic soda-iron-augite) is seldom present. Among the accessory minerals biotite, brown hornblende, hauyne, iron oxides and apatite 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 of Vesuvius and Somma are familiar examples of this class of rocks. They are black or ashy-grey in colour, often vesicular, and may contain many large grey phenocysts of leucite. ”

“ It is said to occur frequently in those primitive aggregates which contain beds of magnetic iron ore, associated with augite, scapolite, epidote and hornblende, precisely the same rocks in which we find it in this country. There is scarcely any part of Europe where a greater variety of augites are found than in Norway and Sweden ”

“ Description.—Except as regards the prismatic pyroxene these rocks do not differ much from the “granular” augite-andesites. Those before me show phenocrysts of plagioclase and sometimes of augite in a groundmass displaying a mesh-work of felspar-lathes, prismatic pyroxene, and much interstitial glass.... The plagioclase phenocrysts, 1 to 3 mm. in size, show abundant magma-inclusions arranged either zone-wise or parallel to the twinning-planes. They are often eroded.... The phenocrysts of augite, which give extinctions of over 30°, are often rounded and display glass and other inclusions.... ”

“ Augite phenocrysts are usually scanty and small. The felspar-lathes, which are more or less in flow-arrangement, are rather stout, and range in average length in different rocks from ·23 to ·35 mm. They often show a few twin-lamellæ which yield extinctions of medium and basic andesine (15-28°) . The augite granules are large (·03 mm.) in the Ndriti rocks. Magnetites, usually corresponding in size to the augite granules, are abundant. Interstitial glass occurs often in fair quantity and is dark and semi-opaque. ”

“ HORNBLENDE.—A mineral species, placed by Dana in the augite section of the anhydrous silicates. In common use the name is limited, as it was formerly applied only to the dark crystalline minerals which are met with in long, slender prisms, either scattered in quartz, granite, etc., or generally disseminated throughout their mass. The color of the mineral is usually black or dark green, owing to the presence of much iron. It appears to have been produced under conditions of fusion and cooling which cannot be imitated in the laboratory, the crystals obtained artificially being of augite type. ”

“ Other rocks found in these agglomerates contain no rhombic pyroxene, and are referred to genera 13 and 16 of the augite-andesites according to the presence or absence of plagioclase phenocrysts. In the last case we have a dark aphanitic rock (sp. gr. 2·74) , sometimes scoriaceous, where the average length of the felspar-lathes may be as little as ·04 mm. One of the blocks was composed of a highly scoriaceous semi-vitreous rock, the cavities being filled with a zeolite. Another was composed of a black porphyritic augite-andesite, showing large crystals of plagioclase.... ”

“ It is not always easy in the field to distinguish between augite or diallage and hornblende, and one or two rocks which were taken for gabbros turn out on microscopic study to be really basic diorites or hornblende-rocks; while a rock at Um Eleiga, which strongly resembles a rather fine-grained diorite in appearance, turns out to be a gabbro. The limit between gabbros and peridotites [298] is exceptionally difficult to map, the proportions of olivine, augite, bronzite, and felspar changing very frequently in the same rock mass, as for instance at Gebel Gerf. ”

“ The minerals are more or less abundant and may constitute a third of the whole mass. They include plagioclase, augite, rhombic pyroxene, and magnetite, olivine being rare and scanty. Entire crystals of any size are infrequent. However, crystals of augite, 5 or 6 mm. in length, are found in the tuffs at Naivaka and of the coast cliffs near the Salt Lake Passage.... It may be observed that zeolitic minerals which are frequently developed in the tuffs consisting almost entirely of palagonite are not typical of the tuffs of mixed composition. ”

“ Most strictly, it consists of an intimate mixture of augite, felspar, and iron, to which a mineral of an olive green colour, called olivine, is often superadded, in distinct grains or nodular masses. The iron is usually magnetic, and is often accompanied by another metal, titanium. Augite is the predominant mineral, the felspar being in much smaller proportions. There is no doubt that many of the fine-grained and dark-coloured trap rocks, called basalt, contained hornblende in the place of augite ”

“ Quartz in a rock means an excess of acid or silica, and almost necessarily implies the absence of highly basic minerals. In other words, hornblende is often, and augite very rarely, found in connection with acidic minerals; and it is this difference of association chiefly that makes their distinction essential to the proper recognition of rocks; while at the same time it affords an easy, though of course not absolutely certain, means of determining whether the black constituent of any particular rock is hornblende or augite. ”

“ In some layers the long black prisms of augite are ranged in a general parallelism with the banding.
A specimen selected as typical of the ordinary coarse-grained amorphous rock was sliced and placed in Mr. Harker's for microscopic examination, and he has supplied the following observations regarding it: "The gabbro from Eilean Mhuire [7110] is a crystalline rock showing to the eye lustrous black augites, half an inch long, and (predominating) felspar. The microscope reveals, in addition, irregular grains of black iron-ore and little hexagonal prisms of apatite. No olivine is to be detected. ”

“ Trap-rocks generally may be variously classified according to the aspect under which we view them. Mineralogically they are augitic, when the mineral augite enters largely into their composition; hornblendic, when the augite is replaced by hornblende; and felspathic, where felspar forms the most marked constituent. The first class includes all the dark homogeneous compounds called basalts; the second, the hornblendic greenstones, or diorites; and the third, the felstones, porphyries, and tufas. Geologically, they are beds when they are interstratified with the contiguous rocks ”

“ It consists of two ingredients only, and has at first sight the aspect of those syenitic traps so common in the Cuchullin, which are formed of hornblende and felspar, in large crystals: but instead of hornblende it contains augite. These substances, the felspar and augite, are confusedly crystallized together; and although the concretions vary much in size, they are most generally large, and appear to increase in dimensions on approaching nearer to the summit of the mountain. The crystals of augite are not infrequently an inch or more in length. ”

“ These rocks consist of augite and plagioclase, with little or no olivine, on a brown, vitreous, interstitial matrix. Devitrified forms of tachylyte (sordawilite, &c.) occur at the rapidly chilled margins of dolerite sills and dikes, and fine-grained spotted rocks with large spherulites of grey or greenish felspar, and branching growths of brownish-green augite (variolites) .
To nearly every variety in composition and structure presented by the diabases, a counterpart can be found among the Tertiary dolerites. In the older rocks, however, certain minerals are more common than in the newer. ”

“ The diallage, isolated from the other constituents of the gabbro, may commonly be seen to have broken up into numerous granules, like the augite grains of basalt, while in some cases biotite and hornblende have been developed with the concomitant excretion of magnetite. The acid rock itself has undergone considerable modification owing to the incorporation of basic material into its substance. Professor Sollas distinguishes the following varieties of the rock:—Biotite-granophyre, biotite-amphibole-granophyre, augite-granophyre, diallage-amphibole-augite-granophyre. ”

“ The beautifully complete crystals of augite found in the old tuffs of Vesuvius and on the flanks of Stromboli may be paralleled among Palæozoic tuffs and agglomerates in Britain. Thus the necks belonging to the Arenig and Llandeilo volcanoes of southern Scotland are sometimes crowded with augite, varying from minute seed-like grains up to perfectly formed crystals as large as hazel nuts. The conditions under which such well-shaped idiomorphic minerals were formed were probably different from those that governed the cooling and consolidation of the ordinary lavas. ”

“ 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 mineral can be exactly determined, the rock may be still more definitely named. For instance if it is all augite, then the rock would be called augitite. Like the preceding rocks, pyroxenite is an intrusive rock, usually found in dikes, which are connected with gabbro, and it represents the segregation of one mineral out of the gabbro, and its solidification at one point. Hornblende, magnetite and pyrrhotite may be present as accessory minerals. ”

“ Augite alone rarely constitutes a rock (lherzolite, or augite rock) . The compact rocks, like aphanite, compact basalts, &c., change to amygdaloids, when they include masses of extraneous minerals, which fill, or appear to fill, cavities in the stone like those common in lava; the basis of many amygdaloids is earthy, and is called wacké. The rock called gabbro (euphotide, diallage rock, hypersthene rock) is characterised by its mixed felspar and diallage, or hypersthene ”

“ The specific gravity varies considerably, but is as a rule between 2·55 and 2·75, the more basic rocks containing augite in preponderance and basic andesine, whilst the less basic possess a large proportion of rhombic pyroxene and display oligoclase-andesine. Sometimes, as in the case of a rock composing an agglomerate east of Nanduri, where the porphyritic plagioclase is opaque and there is some degree of alteration, the rock looks very like a porphyrite. ”

“ Whilst fine granular augite prevails, semi-ophitic coarser augites are not uncommon, and prismatic pyroxene, sometimes of the rhombic form, is represented in the groundmass of the rocks composing the agglomerates of Mount Thambeyu and of the Sokena Cliffs. In some localities, as on the south-west slopes of the Korotini Range, rocks of the basic pitchstone kind are predominant. ”