United States or United States Virgin Islands ? Vote for the TOP Country of the Week !
Obsidian is of a black or ash-grey colour, and though opaque in mass is transparent in thin edges. Among the rocks of the trachytic family, or those in which the feldspars are rich in silica, that termed Clinkstone or Phonolite is conspicuous by its fissile structure, and its tendency to lamination, which is such as sometimes to render it useful as roofing-slate.
They consist in great part of feldspars, which vary in composition from anorthite to andesine, or from those kinds in which there is less than one per cent of potash and soda to those in which there is more than seven per cent of these alkalies, the soda preponderating greatly.
The great and wondrous phenomenon then went on without violence the outpouring was enormous, and the seething fused matter, ejected from the bowels of the earth, spread slowly and peacefully in the form of vast level plains, or what are called mamelons or mounds. It was at this epoch that the rocks called feldspars, syenites, and porphyries appeared.
LABRADOR- ROCK is a term used for a compound of labradorite or labrador-feldspar and hypersthene; when the hypersthene predominates it is sometimes known under the name of HYPERSTHENE-ROCK. GABBRO and DIABASE are rocks mainly composed of triclinic feldspars and diallage.
Labradorite, or lime- soda-feldspar, in which the proportions of lime and soda are the reverse to what they are in Oligoclase. 5. Anorthite or lime-feldspar. The two latter feldspars are rarely if ever found to enter into the composition of rocks containing quartz.
Feldspars when arranged according to their system of crystallisation are MONOCLINIC, having one axis obliquely inclined; or TRICLINIC, having the three axes all obliquely inclined to each other. If arranged with reference to their cleavage they are ORTHOCLASTIC, the fracture taking place always at a right angle; or PLAGIOCLASTIC, in which the cleavages are oblique to one another.
Orthoclase is orthoclastic and monoclinic; all the other feldspars are plagioclastic and triclinic. That variety of the Feldspar Group which is called Anorthite has been shown by Rammelsberg to occur in a meteoric stone, and his analysis proves it to be almost identical in its chemical proportions to the same mineral in the lavas of modern volcanoes.
The two principal members of the Plutonic family of rocks are Granite and Syenite, each of which, with their varieties, bear very much the same relation to each other as the trachytes bear to the basalts. 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 terms "glassy" and "compact" feldspars only refer to structure, and not to species or composition; the student should be prepared to meet with any of the above feldspars in either of these conditions: the glassy state being apparently due to quick cooling, and the compact to conditions unfavourable to crystallisation; the so-called "compact feldspar" is also very commonly found to be an admixture of more than one feldspar species, and frequently also contains quartz and other extraneous mineral matter only to be detected by the microscope.
The term was subsequently made to include other rocks, such as clinkstone and obsidian, which have the same mineral composition, but to which, owing to their different texture, the word in its original meaning would not apply. 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.
Word Of The Day