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Ch. 2: Minerals: Physical Properties

Ch. 1: Form of Minerals Page of 515 Ch. 2: Minerals: Physical Properties Text size:minus plus Restore normal size   Mail page  Print this page
FORM OF MINERALS                               75
feldspar ,to kaolin, quartz or pearl spar into tale, iron pyrites or iron glance into brown-iron ore, azurite into malachite, augite into green earth. The true nature of such bodies is shown by the internal structure, having no relation to the external form or apparent system of crystallization.
The process of petrifaction of organic bodies is in reality a species of pseudomorphic formation, and has been pro­duced in all the above modes. External and internal casts of organic bodies are not uncommon. In other cases the original substance has been replaced by some mineral which has preserved not merely the external form, but even the "minutest detail of internal structure; so that the different kinds of wood have been distinguished in their silicified trunks. The most common petrifying substances are silica and carbonate of lime. In encrynites, echinites, belemnites, and other fossils, the crystals of calc-spar often occur in very regular positions. In some varieties of petrified wood both the ligneous structure and the cleavage of the calc-spar are observable.
Different from the above are mineralized bodies, in which the original structure is still retained, but their chemical nature partially changed. In these a complete series may be often traded, as from wood or peat, through the varie­ties of brown coal, common coal, anthracite, and graphite, perhaps even to the diamond.
CHAPTER II.
PHYSICAL PROPERTIES OP MINERALS.
The physical characters of minerals comprehend,—1st. Those properties derived from the nature of the substance itself, as coherence, mode of fracture, elasticity, and density
Ch. 1: Form of Minerals Page of 515 Ch. 2: Minerals: Physical Properties
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