Common opal does not have the play of color, but comes in a variety of colors; is waxy or greasy in luster; and occurs mostly as fillings of seams or cavities, especially those in igneous rocks, like the steam holes in lavas, etc. It is found in Cornwall, Penn., in Colorado, California, etc.
Opal-agate is a variety in which there are color bands, and it is widely distributed.
Opalized wood is formed in exactly the same manner as agatized wood, much of the fossil wood called silicified being really opalized.
Siliceous sinter is the porous mass of opal which is so frequently deposited about hot springs and geysers. It is readily recognized by its porous character.
The shells of the diatoms, which are microscopic plants, are made of opal; and while they are so small, there is certainly no other plant so abundant or omnipresent, living as it does in every pool, lake, or sea by the millions. These shells are very indestructible so that they accumulate at the bottom of ponds, bogs, and sea-bottoms, making at times extensive deposits. This material in quantities is termed diatomaceous earth, or tripolite (from Tripoli where it was first used commercially). It is used as a polishing powder for metals, marble, glasses, etc.
The Feldspars
The term feldspar is a family name for a large variety of very common minerals, which altogether make up nearly 60% of the crust of the earth, being the predominant part of granites, gneisses, and lavas. In composition they are silicates of aluminum, together with potassium, sodium and calcium, and their mixtures. They may be tabulated as follows:
1. KAlSi₃O₈, orthoclase, the silicate of aluminum and potassium. 2. NaAlSi₃O₈, albite, the silicate of aluminum and sodium. 3. CaAlSi₂O₈, anorthite, the silicate of aluminum and calcium. 4. Mixtures of 1 and 2 are alkalic feldspar. 5. Mixtures of 2 and 3 are plagioclase feldspar.
Orthoclase is monoclinic, but the rest of the feldspars are triclinic. If crystals are available they may be short and stout, or tabular and thin, but as the feldspars are mostly components of the igneous rocks, where perfect crystals have not had a chance to grow, they are mostly determined by their hardness and cleavage. The hardness of all the feldspars is 6 or very close to it.
They all have three planes of cleavage, two of which are good and intersect either at 90° as in orthoclase, or at about 86° as in the plagioclase series; while the third cleavage plane is imperfect. In figure 1, [Plate 34], a and b are the two perfect cleavages, while c is the imperfect one. Breaking into such cleavage masses as the one illustrated is characteristic of feldspar. The specific gravity ranges from 2.55 to 2.75. The luster is vitreous, and the color white, ranging to various shades of gray and pink, and, sometimes in recent lavas, colorless.