Feldspar

Feldspar


 

Composition: A group of minerals, alumino silicates of potassium, sodium, and calcium

Physical Properties:
Color: White, pinkish, gray, clear; rarely pale yellow or green
Streak: White
Luster: Vitreous
Hardness: 6 -6.5
Specific Gravity: 2.9-2.9
Cleavage: Perfect in two directions

Crystallography:
System: Monoclinic and triclinic
Twinning:
Habit: Crystals are short to long prismatic, tabular, and acicular. Aggregates are massive, coarse to fine granular, and fibrous.

General Information:
Feldspar is not a single mineral, but is the name applied to a group of closely related and similar-appearing minerals that have almost identical lattice structures. Collectively, they are the most abundant rock- forming minerals in the crust of the earth. Geologists apply separate names to individual minerals in the feldspar group, but they are so similar in physical properties that they are very difficult to distinguish by simple tests. The most abundant feldspar minerals are microcline and orthoclase (both of which have the chemical formula KAISi3O8), albite (NaAlSi3O8), and anorthite (CaAl2Si2O8). The specimen is orthoclase. Characteristics that help to distinguish feldspar from other similar- appearing minerals are its hardness, cleavage, and the nature of its occurrence. None of the minerals of the feldspar group can be scratched by a knife blade, but they can be scratched with a steel file. They have two prominent cleavage directions at, or very nearly at, right angles to each other. As to the nature of occurrence of feldspar minerals, they comprise the bulk of the light-colored minerals in igneous rocks. They are especially distinguishable in such rocks as white or light-colored crystals that are lath shaped or rectangular in outline. Grains of quartz, the other principal light-colored mineral in igneous rocks, tend to have irregular outlines and do not exhibit flat cleavage faces. Feldspar is also an important constituent of many sedimentary and metamorphic rocks. Generally it is considerably more difficult to identify feldspar in these rocks than in igneous rocks because of the lack of the distinctive rectangular outline. The feldspar minerals are commercially important, being used in large quantities in the manufacture of ceramic -lazes and glass. In addition, the alteration of feldspar by weathering is the principal source of clay, an extremely important constituent in most soils.

Information on Specimen Photo:
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