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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.
And Rose paused that they might listen to the following burst of eloquence from Mac's lips: "You know Frenzal has shown that the globular forms of silicate of bismuth at Schneeburg and Johanngeorgenstadt are not isometric, but monoclinic in crystalline form, and consequently he separates them from the old eulytite and gives them the new name Agricolite." "Isn't it awful?
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