The only common minerals for which the feldspars are liable to be mistaken are quartz and the carbonates.
Orthoclase crystallizes in the monoclinic system, and all the other feldspars in the triclinic system.
By exposure to the weather, feldspars gradually lose their alkalies and lime, become hydrated, and are changed to kaolin or common clay.
Under the combined influence of carbon dioxide and moisture, feldspars give up their potassium, sodium, and calcium, and take on water, and the result is kaolin.
In other words, the distinction of orthoclase from the basic or triclinicfeldspars is important and comparatively easy, while the distinction of the different basic feldspars from each other is both unimportant and difficult.
In hardness the feldspars range from 5 to 7, being usually near 6, and almost always distinctly softer than quartz.
Like orthoclase, these species may be of almost any color; yet these two great divisions of the feldspars are usually contrasted in this respect.
With the exception of albite, which is a comparatively rare species, the triclinic feldspars all contain less silica than orthoclase; i.
This statement of the relations of the feldspars is, of course, beyond the comprehension of many children, and yet it should be understood by the teacher who would lead the children to any but the most superficial views.
Chemically, the feldspars are silicates of aluminum and potassium, sodium or calcium.
The fact that the feldspars contain little or no iron undoubtedly explains their low specific gravity and light colors, as compared with the other minerals just named.
In these the ophitic arrangement of the coarse feldspars is well defined, and in spite of their subsequent alteration the fragments retain the crystal outlines and polarize together.
There is greater variety in texture, from fine aphanitic traps up to coarse grained dolerites with feldspars one-third of an inch long.
Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic.
The two latter feldspars are rarely if ever found to enter into the composition of rocks containing quartz.
Granite is a compound of feldspar, quartz, and mica, the feldspars being rich in silica, which forms from 60 to 70 per cent of the whole aggregate.
The feldspars which occur in Trachytic rocks are invariably those which contain the largest proportion of silica, or from 60 to 70 per cent of that mineral.
The first of the Feldspars which may be mentioned as being prized as an ornamental stone is amazonstone or green Feldspar.
The Feldspars form an essential part of nearly all eruptive rocks and by their decomposition produce clays and other soils which may harden into great areas of sedimentary rocks.
It is estimated that the feldspars make up 50 per cent of all the igneous rocks and 16 per cent of the sedimentary rocks.
As the igneous rocks are so much more abundant than the sedimentary rocks, the percentage of feldspars in the earth approaches the former rather than the latter figure.
In contrast to the feldspars of granite, those of gabbroes are often dull and colored grayish yellow or greenish.
Like the feldspars a group of complex silicates, but here chiefly of potassium, magnesium, iron, and hydroxyl.
Although with wide variation in chemical composition, the feldspars are yet broadly divided into two classes; the one striated, and the other an unstriated potash or orthoclase variety.
The fine powder of the mineral treated for some time with strong hydrochloric acid forms a perfect jelly of silicic acid, whereas the feldsparsdo not.
The feldspars occur in all three great groups of rocks, but they have most commonly crystallized during the cooling of molten masses of igneous rocks.
The soda-lime feldspars go by the general name plagioclase.
The two potash feldspars are orthoclase and microcline, the former being monoclinic, with cleavages at exactly 90 degrees, and the latter triclinic, with cleavages a little less than 90 degrees.
Where many sedimentary rocks have undergone great change (metamorphism) under conditions of heat, pressure, and moisture, feldspars have very commonly formed.
The feldspars are by far the most abundant of all minerals in the crust of the earth.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "feldspars" in a variety of sentences. We hope that you will now be able to make sentences using this word.
