  Feldspar is the family name of several minerals closely related and indeed grading into each other, but distinguished by mineralogists by separate specific terms. These minerals are all silicates of aluminum, with some alkali or alkali earth, having a hardness of about 6 in the scale in which quartz is 7 and a specific gravity varying from 2.5 to 2.7. They are fusible with difficulty before the blowpipe, crystallize in the monoclinic or triclinic system and cleave in two well-marked directions nearly or quite at right angles to each other. It is this latter property, probably, which led to the grouping of these minerals as spar, since this term is applied in common language to any minerals which break with bright crystalline surfaces. Thus calc spar is a common name for calcite, heavy spar for barite, needle spar for aragonite, and so on. The term field spar, of which Feldspar is probably a corruption, was perhaps given the minerals of this group because of their widespread occurrence. The English spelling of the word is Felspar. The Feldspars form an essential part of nearly all eruptive rocks and by their decomposition produce clays and other soils which may harden into great areas of sedimentary rocks. They are thus of great geological importance and interest. Usually the white crystals to be seen in an eruptive rock in contrast to the dark green or black of the pyroxene or hornblende, or the glassy, nearly colorless quartz, are Feldspar. The Feldspar may, however, contain more or less iron and then take on a flesh color or become even darker. Feldspar crystals can best be recognized by their prominent cleavage, which appears as numerous bright flat surfaces extending in any given crystal in the same direction. The crystals, while they may be of so minute dimensions as to be visible only with the microscope, may on the other hand reach in veins in coarse-grained granites a length of a foot or more. As ornamental stones only certain varieties of Feldspar are valued and their value depends on accidents of color or structure. The first of the Feldspars which may be mentioned as being prized as an ornamental stone is amazonstone or green Feldspar. This in composition is what is called a potash Feldspar, potash being the alkali which in combination with alumina and silica goes to make up the mineral. The percentages of each in a pure amazonstone are silica 64.7, alumina 18.4 and potash 16.9. The mineralogical name of the species is micro-cline, meaning small inclination, and refers to the fact that the angle between the two cleavages of the mineral is not quite a right angle. The common color of microcline is white to pale yellow, but occasionally green and red occur. It is only to the green variety that the name of amazonstone is applied, a name meaning stone from the Amazon river. It first referred probably to jade or some such green stone from that locality and then came to include green Feldspar. No occurrence of green Feldspar in that region is now known. Practically all the amazonstone now used for ornamental purposes comes from three localities. These are the vicinity of Miask in the Ural Mountains, Pike’s Peak, Colorado, and Amelia Court House, Virginia. In all these places the amazonstone occurs in coarse-grained granite and is closely accompanied by quartz and Feldspar. All gradations are found in color from the deep green to white, only the bright green being prized for ornamental purposes. The Feldspar is usually well crystallized and crystals of several pounds weight may be found. A crystal will rarely be of a uniform color, streaks of paler green or white being commonly present. Only the uniformly colored portions are prized for ornamental purposes. The green often takes on a bluish tone and blue sometimes even predominates. The color is doubtless due to some organic matter, as it disappears, leaving the stone white, on heating. The stone is always opaque. Its use is not extensive, its sale being greater to tourists in the vicinity of the regions where it is found than to gem cutters. Several other localities in the United States besides those mentioned afford the mineral, though not in large quantities. It occurs in two or three localities in North Carolina; in Paris, Maine; Mount Desert, Maine; Rockport, Massachusetts; and Delaware county, Pennsylvania. The finest comes from the Pike’s Peak locality. Mr. G. F. Kunz states in regard to these crystals that when they were first exhibited at the Centennial Exposition in Philadelphia in 1876 they were a great surprise to Russian dealers who had brought over some amazonstone from the Urals, expecting to sell it at what would now be considered fabulously high prices. FELDSPAR.
The second species of Feldspar which may be mentioned as of use as an ornamental stone is labradorite. This differs in composition from amazonstone in containing soda and lime in place of potash, the percentages in a typical labradorite being, silica 53.7, alumina 29.6, lime 11.8 and soda 4.8. Labradorite has the typical cleavage of Feldspar and cleavage surfaces in the direction of easiest cleavage are usually marked by rows of parallel striae. These show that the mass is made up of a series of crystal twins in parallel position and afford an excellent criterion for determining a triclinic Feldspar. Labradorite is a common rock-forming mineral, especially in the older rocks. It is only, however, when it occurs in large pieces which exhibit a play of colors that it is prized as an ornamental stone. The labradorite exhibiting the latter property in the most remarkable degree and hence most valued is that found on the coast of Labrador near Nain and the adjacent island of St. Paul. It was first found here by a Moravian missionary named Wolfe and brought to Europe in the year 1775. It occurs together with the form of pyroxene known as hypersthene, in a coarse-grained granite, or perhaps a gneiss. From these it is weathered out by wave and atmospheric action and occurs as beach pebbles. It is also mined from veins. Labradorite of pleasing color and opalescence occurs in a few other localities in Canada, and in Essex county, New York, in the United States. Two localities occur in Russia, one near St. Petersburg and the other in the region of Kiew. The labradorite of the latter locality is the better, its occurrence being in a coarse-grained gabbro. The Labrador occurrence exceeds all others, however, in abundance and beauty and by far the larger quantity used in the arts comes from there. The play of colors which gives labradorite its attractiveness is rarely seen to advantage except upon a polished surface, but whether polished or unpolished it only appears when the surface is held at a particular angle with reference to the eye. Emerson thus describes it in his essay on Experience as illustrating the limitations of the individual: “A man is like a bit of Labrador spar, which has no lustre as you turn it in your hand until you come to a particular angle; then it shows deep and beautiful colors.” The play of colors seen in labradorite is not like that of the opal, which presents to the eye fragments of different colors varying in different positions, but appears as broad surfaces of a single color. It is only rarely that these colors change with a change of position. Bauer remarks that the appearance is similar to that seen on the wings of some tropical butterflies. The colors over any given surface are not necessarily alike, but more than two or three tints are rare. Each tint is uniform where it occurs. A surface may be interspersed with many spots exhibiting no sheen. Both colored and uncolored portions have only vague outlines and merge into each other at the edges. Bauer mentions a labradorite from Russia the colored portions of which formed a striking likeness of Louis XVI, the head being a beautiful blue against a gold green background, while above appeared a beautiful garnet red crown. Excellent effects are sometimes produced in labradorite by cutting it in the form of cameos so as to make the base of different color from the figure in relief. Of the different colors shown by labradorite blue and The gems known as moonstone and sunstone owe the play of colors which gives them their respective names to similar causes. These gems are generally some form of Feldspar, although any mineral giving a similar sheen of color might be included under them. The moonstone of commerce comes chiefly from Ceylon, where it occurs in large pieces the size of a fist in a clay resulting from the decomposition of a porphyritic rock. Pieces of these when polished exhibit the beautiful pale blue light coming from within which makes the stone prized as a gem. The cause of this light is undoubtedly minute tabular crystals lying in parallel position through the stone. The stone varies from translucent to opaque, and from colorless to white, the essential feature being the blue opalescent light or chatoyancy exhibited from a polished surface. Good moonstones are worth from three to five dollars a carat. The Ceylon moonstone is sometimes known as Ceylon opal, but it is the variety of Feldspar known as orthoclase, which is a potash Feldspar, differing from the microcline just described in being monoclinic in crystallization and in having two cleavages meeting at right angles. Another species of Feldspar used as moonstone is albite. This is a soda Feldspar and is triclinic, but exhibits the color characteristic of moonstone. One variety is known as peristerite, from the Greek word for pigeon, and is applied on account of the resemblance of the sheen to that of a pigeon’s neck. It is found at Macomb, St. Lawrence county, New York. Albite found at Mineral Hill, Pennsylvania, also exhibits the chatoyancy of moonstone. Amelia Court House, Virginia, is another locality whence come pieces either of orthoclase or oligoclase exhibiting this property. Like most of the more or less opaque gems, moonstone is cut chiefly in the rounded form known as en cabochon. It is of late, however, cut in the form of balls, which are quite popular, the bringing of good luck being attributed to them. The brilliancy of moonstone is considerably increased by mounting it against black. Sunstone is the term by which those kinds of Feldspar are known which reflect a spangled yellow light. The appearance comes from minute crystals of iron oxide, hematite or gothite, which are included in the stone and both reflect the light and give it a reddish color. Like labradorite the sheen is visible only when the stone is held at a certain angle. Some specimens of the mineral carnallite, which is a chloride of potassium and magnesium, exhibit a similar sheen, and being soluble in water the crystals of hematite can be separated out. They are then seen to be perfect little hexagons of a blood-red color. The sheen of sunstone is best visible when the stone is held in the sunlight or strong artificial light. The variety of Feldspar to which the sunstone most in use at the present time belongs is oligoclase, a soda-lime triclinic Feldspar. Like labradorite it usually exhibits on the surface of easiest cleavage parallel striations due to twinning structure. The best sunstone at the present time comes from Tvedestrand, in southern Norway, where it occurs in compact masses together with white quartz, in veins, in gneiss. Some also comes from Hittero, Norway. In Werchne Udinsk, Siberia, another occurrence was discovered in 1831. Previous to this Bauer states that all the sunstone known came from the Island of Sattel in the White Sea, and was very costly, although of a quality which would not now be deemed desirable. At the present time, although stones of fine quality can be obtained, sunstone is little used in jewelry, and its market value is very low. Statesville, North Carolina, and Delaware county, Pennsylvania, are two localities in the United States where good sunstone has been obtained. Both sunstone and moonstone can be accurately imitated in glass and the distinction of the artificial from the real by Gems are occasionally cut from other forms of Feldspar than those here described, which are transparent and colorless and valued for their lustre. The varieties chiefly employed in this manner are adularia, a variety of orthoclase which is often transparent, the best specimens being obtained in Switzerland, and oligoclase, in the transparent form in which it is found near Bakersville, North Carolina. Oliver Cummings Farrington. |
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