General Physical Properties and Structure of Minerals
Structure of Minerals: If by the phrase “structure of minerals” is meant their internal or molecular structure, all minerals may be included in one of two classes: (1) Crystalline; (2) Amorphous. With only a few exceptions, minerals are crystalline in their structure. This does not signify, however, that these minerals necessarily occur in distinct crystals, but only that their internal structure is such that they may under favorable circumstances definitely crystallize.
Commonly, however, the expression “structure of minerals” refers to their outward shape and form. Various descriptive terms are used in this connection that will need short definitions.
- When a mineral consists of distinct crystals the following terms may be used:
- a. Crystallized. In definite crystals (see A, pl. II)
- b. Acicular. In slender needlelike crystals.
- c. Capillary. In hair-like crystals.
- d. Filiform. In threadlike crystals.
- e. Dendritic. Arborescent, in slender divergent branches, somewhat plant-like, made up of more or less distinct crystals.
- f. Reticulated. Lattice-like groups of slender crystals.
- g. Divergent or Radiated. Radiating crystals groups.
- h. Drusy. A surface is drusy when covered with a layer of very small crystals.
- a. Columnar. In stout columnike individuals.
- b. Fibrous. In slender columnar individuals. The fibers may be parallel or radiated.
- c. Stellated. When the radiating individuals form starlike or circular groups.
- d. Globular. When the radiating individuals form spherical or hemispherical groups.
- e. Botryoidal. When the globular forms are in groups. The word is derived from the Greek for a “bunch of grapes”.
- f. Reniform or Mammillary. When a mineral is in broad rounded masses resembling in shape either a kidney or mammac.
- a. Foliated. When a minerals separates easily into plates or leaves.
- b. Micaceous. Similar to foliated but the mineral can be split into exceedingly thin sheets, as in the micas.
- c. Lamellar o tabular. When a mineral consists of flat platelike individuals superimposed upon and adhering to each other.
- d. Plumose. Consisting of fine scales with divergent or feather-like structure.
- Coarse to fine granular. When a mineral consists of an aggreagate of large or small grains.
- a. Compact – Earthy. A uniform aggregate of exceedingly minute particles.
- b. Stalactitic. When a mineral has the shape of cylinders or cones which have been formed by deposition from mineral-bearing waters dripping from the roof of some cavity.
- c. Concentric. Consisting of more or less circular layers super-imposed upon one another about a common center.
- d. Banded. When a mineral occurs in narrow parallel bands of different color or texture.
- e. Geodes. When a cavity has been lined by the deposition of mineral material but not wholly filled, the more or less spherical mineral shell is called a geode. The mineral is often banded owing to successive depositions of the material, and the inner surface is frequently covered with projecting crystals.
- f. Massive. When a mineral is composed of compact material with an irregular form and does not show any peculiar structure like those described above, it is said to be massive.
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