Mallard suggested that all felspars are really asymmetric, and that orthoclase presents only a pseudo-monosymmetric habit, due to twinning.
The melting points offelspars have been investigated by Prof.
All the felspars are silicates containing aluminium with some other metallic base or bases, generally potassium, sodium or calcium, rarely barium, but never magnesium or iron.
In the monoclinic or monosymmetric felspars these, being parallel to the basal pinacoid and clinopinacoid, necessarily make an angle of 90 deg.
Tschermak in 1864 that the other plagioclastic felsparsare isomorphous mixtures in various proportion of albite (Ab) and anorthite (An).
Although the crystals of all felspars present a general resemblance in habit, they are usually regarded as belonging to two systems, some felspars being monoclinic and others anorthic.
The chief constituents of rocks which take part in the production of kaolins appear to be the felspars, but the natural processes by which these felspars are decomposed are by no means perfectly understood.
The last of these minerals occurs in small irregular patches between the idiomorphic felspars which Dr J.
In microscopic characters the andesites present considerable variety; their porphyritic felspars are usually of tabular shape with good crystalline outlines, but often filled with glass enclosures.
The andesites have mostly a porphyritic structure, and the larger felspars and ferro-magnesian minerals are often visible to the naked eye, lying in a finer groundmass, usually crystalline, but sometimes to a large extent vitreous.
Aventurine and moonstone varieties occur, though these special appearances are more usually displayed by the oligoclase and orthoclase felspars respectively.
Like all the felspars it possesses two cleavages, one perfect and the other less so, which are here inclined at an angle of 86 deg.
According as these various felspars are present in a soil, so will the quality of the soil be.
The felspars are composed of silica and alumina, along with potash, soda, and lime, with traces of iron and magnesia.
Illustration: Sections of Etna Lavas seen under the Microscope] Plagioclastic felspars are unquestionably the dominant constituents of these lavas.
The felspar crystals are lighter and more translucent than the matrix, but are of much the same character, having a granulated or flocculent appearance, somewhat like that of the decomposedfelspars in diabase.
Equally characteristic of the gabbros is the alteration of the felspars to cloudy, semi-opaque masses of saussurite.
In these the minerals have a parallel arrangement, the felspars are often broken down by pressure into a mosaic of irregular grains, while greenish fibrous or bladed amphibole takes the place of pyroxene and olivine.
They contain a small amount of quartz, and often micropegmatite, as the last element to consolidate, filling up little angular interspaces between the felspars and pyroxenes, which had previously crystallized.
The commonest type of structure in dolerite is the ophitic, which results from the felspar of the rock having crystallized before the augite; the latter mineral forms shapeless masses in which the idiomorphic felspars lie.
Epidote is another secondary lime-bearing mineral which results from the decomposition of the soda lime felspars and the pyroxenes.
Their olivine tends to become serpentinized; their augite changes to chlorite and uralite; their felspars are clouded by formation of zeolites, calcite, sericite and epidote.
From the varying size of the felspars of the groundmass it is apparent that the flows are not all of the same character.
The broad felspars of the groundmass are sometimes rectangular and give lamellar extinctions of medium and acid andesine (12°-17°).
Felspars (lathes and prisms) of the groundmass not in flow-arrangement.
The felsparsof the groundmass, which average ·1 mm.
The felspars of the groundmass, which for the most part form a plexus, are small and stout, their average length varying from ·08 to ·13 mm.
The basic rocks prevailing between Vatu-levoni and Vandrani belong to genus 13 of the augite-hypersthene-andesites and have a groundmass of much finer texture, the felspars only measuring ·05 mm.
The small size of the felspar-lathes of both rocks distinguishes them from the dyke rocks of the basin, where the felspars are twice as long.
Brownish-black rocks which cannot be distinguished, except in the flow-arrangement of the felspars of the groundmass, from those described under genus 25.
The felspars of the groundmass give the small extinctions of oligoclase; and in this respect it differs from the other augite-andesites.
Of thefelspars the plagioclase separates first and forms well-shaped crystals of which the central parts may be more basic than the outer zones.
These larger felspars have no crystalline outlines and consist of orthoclase or microcline surrounded by borders of white oligoclase.
Exceptions to this sequence are unusual; sometimes the first of the felspars have preceded the hornblende or biotite which may envelop them in ophitic manner.
These spots may consist of radiate or brush-like felspars (with some mica and hornblende) or of quartz and felspar.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "felspars" in a variety of sentences. We hope that you will now be able to make sentences using this word.
