In the dacites felsitic groundmasses are by no means rare, but microcrystalline types consisting of plagioclase and sanidine with quartz are more prevalent.
The hornblende- and mica-andesites have groundmasses composed mainly of acid plagioclase with little orthoclase or glassy base (pilotaxitic groundmass).
Clear brown glass with many small crystals of plagioclase and pale brown augite (hyalopilitic groundmass) is very frequent in pyroxene-andesites.
Plagioclase crystals occur macroscopically in the glass; they are much eroded and contain numerous large inclusions both of the clear isotropic glass and of its palagonitised form.
The plagioclase phenocrysts owe their opacity to the great number of fine and sometimes parallel cracks filled with alteration products, that traverse them.
The plagioclase phenocrysts are for the most part microporphyritic.
The other constituents are fragments of semi-vitreous basic rocks and of palagonite, together with fragments of plagioclase and pyroxene crystals and of an amorphous siliceous mineral which behaves optically like chalcedonic silica.
They belong for the most part to the prismatic sub-order of the group and to the section with plagioclase phenocrysts, and fall naturally into two divisions corresponding to the two genera with glassy and opaque phenocrysts.
Since the exception (which belongs to genus 17 of the synopsis) differs only in the presence of plagioclase phenocrysts, its separate description is not needed.
The plagioclase phenocrysts are as a rule 1 to 2 mm.
They include a large proportion of the acid andesites of the island, and all belong to the prismatic sub-order with prismatic pyroxene in the groundmass, and to the section with plagioclase phenocrysts.
The rock from the gorge below is of the same character, but on account of its opaque plagioclase phenocrysts it is referred to genus 14, and is described on p.
They are usually compact and aphanitic, showing few if any plagioclase phenocrysts and having a semi-conchoidal fracture.
They display in the slide a fair number of plagioclase and pyroxene phenocrysts in a groundmass of felspar-lathes, pyroxene granules, and magnetite, the interstitial glass being scanty or moderate in amount.
It shows no plagioclase phenocrysts, but those of pyroxene are numerous, which, however, do not exceed 2 mm.
Like diorite, this is essentially a plagioclase rock; but there are, nevertheless, important differences.
Olivine is a very common and characteristic constituent, and the plagioclase is often replaced in part by leucite and nephelite.
Plagioclase and hornblende are common, but not abundant, constituents.
On certain faces and cleavage-surfaces of the plagioclase crystals we may often observe a series of straight parallel lines or bands which are often very fine,—fifty to a hundred in a single crystal.
Like norite it consists usually of the more basic varieties of plagioclase with or without augite, diallage, or hypersthene.
As stated, these twinning striæ in plagioclase are often visible to the naked eye; and when they are not, they may usually be revealed by examining a thin section under the microscope with polarized light.
The plagioclase in diorite is mainly the more acidic species, like oligoclase; while in norite the more basic species, such as labradorite and anorthite, predominate.
But the stratified plagioclase rocks seem to have as strong a claim to recognition as gneiss.
Plagioclase decays much more rapidly when exposed to the weather than orthoclase.
Sections of the finer schist in polarized light show many small areas of quartz and plagioclase and numerous crystals of epidote, magnetite, and chlorite, the whole having a marked parallel arrangement.
In many diorites the plagioclase felspar has crystallized before the hornblende, which consequently has less perfect outlines and forms irregular plates which enclose sharply formed individuals of felspar.
These consist of hornblende (dark green) and basic plagioclase felspar, labradorite and bytownite (grey or nearly white).
A crystalline-granular compound of plagioclaseand hornblende with magnetite.
Sections of plagioclase crystals (probably labradorite) from the lava of 1855.
A rock, allied to the felstone family, in which mica is an abundant and essential constituent, thus consisting of plagioclase and mica, with a little magnetite.
Group of leucite crystals of irregular form from the lava of 1855, congregated around a nucleus of crystals of plagioclase and magnetite.
It consists of a light-greyish felsitic paste enclosing grains of smoke-quartz, crystals of sanidine, plagioclase and biotite, with a little magnetite and apatite.
Tschermak to those plagioclase felspars which lie between labradorite and anorthite; and this has been generally adopted by petrologists.
A fine grained compact gray rock, of aggregate structure, consisting chiefly of quartz, plagioclase and biotite, and the alteration products epidote and sericite.
A coarse dark green rock whose principal constituents are pyroxene plagioclase and magnetite.
It has the usual twinning characteristic ofplagioclase and belongs to the andesine-labradorite series, extinction angles proving basic andesine and acid labradorite to be the most common.
The felted or hyalopilitic texture is the most common, and plagioclase is the principal groundmass constituent.
Mohs) Two distinctly different alkali alumino silicates: the Plagioclase and the Alkali Feldspar Series Specific gravity: 2.
The felspar is of two kinds, orthoclase and plagioclase, the former is very turbid and decomposed, while the plagioclase is clear, and retains its usual optical characters.
The felspars are orthoclase and plagioclase; the former is much decomposed, while the plagioclase frequently remains clear, and exhibits well its twin striation.
The best specimen examined is from a quarry near Marston Jabet; it is a fine-grained rock, similar to a basalt in external appearance, but contains numerous crystals of hornblende and plagioclase which are generally quite unaltered.
The plagioclase crystals vary greatly in size, some being mere microliths while others are over the eighth of an inch in length.
On the right hand side part of a plagioclase crystal with inclosures similar to that in the preceding figure.
Their essential minerals in thin section are hornblende of a strong reddish-brown colour; augite purple, pleochroic and rich in titanium, olivine and plagioclase felspar.
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 middle and spread out towards both ends.
They consist essentially of plagioclase felspar, with hornblende or augite, and may contain also biotite, quartz and a limited amount of orthoclase.
Their essential minerals are pyroxene, plagioclase and garnet: chemically they resemble the gabbros.
Of the felspars the plagioclase separates first and forms well-shaped crystals of which the central parts may be more basic than the outer zones.
Under the microscope the felspar is mainly orthoclase with perthite or microcline, while a small amount of plagioclase (ranging from oligoclase to albite) is practically never absent.
The plagioclase felspar, if fresh, is transparent and appears simple in ordinary light, but when polarized breaks up into a series of bars of different colours owing to its complex twinned structure.
Leucite-basalts contain small rounded crystals of leucite in place of plagioclase felspar.
In a very large number of the rocks of this group the plagioclase felspar has crystallized in large measure before the pyroxene, and is enveloped by it in ophitic manner exactly as occurs in the diabases.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "plagioclase" in a variety of sentences. We hope that you will now be able to make sentences using this word.
