Occurs in crystals, or in cleavable or granular masses; hardness 3.5; specific gravity 2.8; color white to pink or gray; streak white; luster vitreous; transparent on thin edges.
Dolomite crystallizes in the hexagonal system, in rhombohedrons (hemihedral form), which are more or less modified by faces on the corners or edges. The cleavage is parallel to the rhombohedron, and it will effervesce in warm hydrochloric acid. Sometimes the crystal faces are curved, and when this is the case, dolomite is easily determined. Usually however dolomite resembles both calcite and magnesite. From the calcite it is distinguished by the greater hardness, and from magnesite by lesser hardness and not being porcelainous in appearance. Some of the commoner forms are shown on Plate 29, crystals like C being found embedded in anhydrite and gypsum.
Magnesium is a common element and is likely to be present wherever lime is being deposited, so dolomite crystals are common, and much of the limestone is dolomitic.
It may be found in almost any limestone section of the country. Some of the finest crystals of dolomite however come from Roxbury, Vt., Smithfield, R. I., Hoboken, N. J., Lockport, Rochester, and Niagara Falls, N. Y., etc.
Silicon is one of the non-metallic elements, and does not occur as such in Nature. When isolated it is either a dark-brown powder, or steel-gray crystals. However silicon is next to oxygen in its importance in making the crust of the earth. Forty-seven per cent of the surface rocks are composed of oxygen, and 28% of silicon, the latter appearing in a host of minerals. The oxide of silicon is termed silica (SiO₂), its crystal form being quartz, the commonest of all minerals. In non-crystalline form silica is also widely distributed, as chalcedony and opal, even appearing in the tissues of animals and plants, as in the feathers of birds, the shells of certain Protozoa (Radiolaria), the spicules of sponges; and in plants, as the shells of diatoms, and in the stalks of grasses, especially cereals and bamboo. Silica in the form of sand is widely used in making glass, porcelain, china, etc., and in the various cements.
Then there are a considerable number of acids of silicon, which do not occur in Nature, but their salts do, and make a host of minerals, which are known as the silicates, such as mica, feldspar, hornblende, etc. Either as quartz, or as silicates, silicon is represented in most all the igneous and metamorphic rocks and in many of the sedimentary rocks.
Occurs as hexagonal crystals, or in grains or masses; hardness 7; specific gravity 2.65; colorless when pure; luster vitreous; transparent on thin edges.
Quartz is not hard to identify. Its hardness and the crystal-form separate it from most all other minerals. It is the most common mineral, making 12% of the earth’s crust. The usual crystal form is a hexagonal prism with the sides horizontally striated, and a six-sided pyramid on one or both ends. This six-sided pyramid is really two rhombohedrons, a right-handed one and a left-handed one, so that the alternate faces of the pyramid may show peculiarities, for instance three may be large and three small, as in Fig. B, Plate 30, or the alternate ones may be duller or etched in some manner. The crystals are clear and when pure colorless, but there is a tendency for some slight impurity to color them almost any hue.
The most perfect double-ended crystals form only where growth is possible in all directions, as in clay. In cavities and caves there is an opportunity for the crystals to grow in toward the open spaces, and in such places, one finds fine large crystals; the Alps, Brazil, Japan, and Madagascar being especially famous localities. The largest quartz crystal on record is one 25 feet in circumference which came from Madagascar. In this country the caves at Little Rock, Ark., have furnished some very fine large crystals. Smaller, but very clear crystals, come from about Herkimer, N. Y. Some of these have been used as “Rhine-stones” and as cheap imitations of diamonds. Clear quartz is beautiful enough to be a gem, but it is too common to interest people as jewelry, however many objects of art have been carved from it. One of these took the form of crystal balls, which, through the Middle Ages particularly, developed into a form of mysticism. The gazing into the crystal ball was supposed to give some people supernatural vision. It seems to be a form of hypnotism, gazing at the bright reflecting surface tiring the eye, and making possible visions, which are subjective rather than anything external.
Silica is slightly soluble in water, especially when it is alkaline; so that most river-, lake-, and sea-waters have some silica in solution, and are carrying it from one place to another. The waters, which percolate through the rocks, carry even more, and when they come out into open spaces, they give up some of the silica, making crystals lining these openings, whether fissures or cavities. Not infrequently these silica-bearing waters dissolve out some other crystal, and then deposit in its place silica, thus making a crystal which has the form of what was dissolved, rather than that of quartz. Such a form is known as a pseudomorph.
When molten masses of igneous rock were cooling the quartz crystals had their faces interfered with as they grew, and we have resulting crystalline quartz, simply filling in the spaces between the other crystals, such as feldspar and mica, in the granite. Quartz is a large component in many igneous rocks, also in metamorphic rocks, and certain sedimentary rocks like sandstone are almost wholly made up of quartz grains. Quartz is also the gangue mineral in many veins. In this case it seems to have been deposited from hot water or vapors, as they rose from cooling magmas. With it are associated all sorts of metallic ores as has been suggested.
Quartz has been largely used to make imitations of other much rarer minerals, sometimes in its crystalline form to imitate the diamond, at other times ground and made into a “paste,” which is colored to imitate other gems. This paste is a mixture of about 4 parts of quartz, 5 parts of red lead and 1 part of potassium carbonate, melted and cooled slowly. It is clear and has a brilliant luster like the diamond. If some coloring matter is put into it it can be used for rubies, sapphires, etc. When there is any reason to think that this is being used, it is easily detected by being so much softer than any of the true gems, and even than true quartz. Quartz will scratch glass readily, but this imitation has only the hardness of very soft glass, or about 5.
Rock crystal is the term applied to quartz when it is clear and colorless.
Milky quartz is the milky variety, the whiteness being due to imperfections in the crystallization, such as cracks, bubbles, etc.
Smoky quartz is the cloudy brown-colored variety, which results from the presence of small quantities of organic matter (hydrocarbons) in the quartz. If the color is so dark as to be almost black it is termed morion. In the above cases the color will disappear if the stone is heated. Pebbles of smoky quartz from Cairngorm, Scotland, have been so widely used as semiprecious stones that they have come to be known as cairngorms.
Citrine, or false topaz, is a clear yellow variety, the color again due to the presence of organic matter. It is distinguished from true topaz by the lesser hardness, this having the hardness of 7, while true topaz has a hardness of 8.
Amethyst is quartz with a violet color, due to the presence of small quantities of manganese. To be suitable for cutting into gems, the color must be deep or the small pieces will appear almost colorless. It is widely used today as a semiprecious stone in jewelry; and in the fifteenth century it had the traditional virtue of making the wearer sober-minded, whether he had taken too freely of wine, or was over excited by love-passion.
Rose quartz gets its pale-red color from the presence of a small amount of titanium. It is widely distributed, but is more abundant in the Black Hills of South Dakota.
Aventurine is quartz which has inclosed tiny scales of mica or hematite giving it a spangled appearance.
Prase is a green quartz, the color being due to the inclusion of fibrous crystals of green actinolite.
Cat’s Eye is a quartz which has inclosed silky fibers of asbestos. When this is cut parallel to the fibers, the effect is opalescent. The colors are greenish, yellowish-gray, and brown. This form, however, is not to be confused with the true or Oriental Cat’s Eye, which is chrysoberyl and has the hardness of 8.
Non-crystalline, occurring in botryoidal, stalactitic or concretionary masses; hardness, 7; specific gravity, 2.65; color white when pure; luster waxy; translucent to transparent on thin edges.
In addition to the crystalline form, silica is freely deposited in an amorphous or cryptocrystalline form which has the same properties as quartz, except the crystal faces. This is called chalcedony, and it occurs in seams, cavities and free surfaces. When the surface of a chalcedony deposit is free it has a waxy luster. It is generally very brittle and breaks in a peculiar splintery manner. Like quartz it also has a great many varieties, according to the impurities present. Its wide distribution, hardness, and the manner in which it can be chipped have made this a most important stone in the history of the development of civilization. The early men first broke it into rough tools, such as knives, axes, spear points, etc., and used these as cutting tools, of one sort or another, because they held their edge better than most stones. We apply, to the people who used only these chipped stones as tools, the term “Men of the Old Stone Age,” or the period is termed the Palæolithic Age. Later men learned how to grind the edge to a smoother outline, and this much shorter period is termed the Neolithic Age. The use of flints for the first tools is world-wide, and the American Indian when discovered was still using chalcedony in its rough-hewn state.
“There the ancient Arrow-maker
Made his arrow heads of sandstone,
Arrow heads of chalcedony,
Arrow heads of flint and jasper,
Smoothed and sharpened at the edges,
Hard and polished, keen and costly.”
Chalcedony is the proper term to use when the color is white to translucent, in which case the surfaces are usually botryoidal and waxy.
Carnelian is chalcedony which is clear red in color and translucent. This is one of the first stones used for ornamental purposes and for engraving. Carnelians with figures engraved on them were used by the Egyptians, Assyrians and The Children of Israel, at least 2000 B.C.; and the Egyptian scarabs of the fifth or sixth century B.C., were often carved from this variety of chalcedony, as well as from jasper and agates.
The brownish varieties are termed sard.
Chrysoprase is an apple-green variety of chalcedony the color being due to the presence of nickel oxide. This is by no means as common as most of the varieties of chalcedony, and was long prized as a gem.
Plasma is chalcedony with a leek- to emerald-green color, and the same stone when it has small red spots of jasper in it is termed blood-stone, or heliotrope. These red spots are said by tradition to be drops of the blood of Christ.
Jasper is a deep red chalcedony, the color being due to hematite, which is so abundant as to make it opaque. A brown variety colored by limonite is also called jasper, and even green jaspers are found. In all cases the opaque character is common.
Flint is an impure brown chalcedony, usually forming concretions. The color is due to organic matter. Flint is mostly found in limestone or chalk, and the concretions are the result of the small particles of silica scattered through the rock being dissolved, and then reprecipitated about some organic center. Generally the silica was obtained by the dissolution of small fossils, like the shells of diatoms or sponge spicules.
Hornstone and Chert are simply impure varieties of flint, brown in color, and with a splintery fracture.
Agate, Plate 32, is a banded or cloudy chalcedony which has formed in a cavity, the layers of different color representing deposition from water, carrying first silica with one impurity, then later, silica with another impurity. Gradually the cavity has been thus filled with silica; and when the mass is freed by the weathering away of the surrounding rock, these banded masses are found. Sometimes the manner of deposition has changed, and while the outer part of the cavity was filled with chalcedony, the central part will contain quartz crystals. On account of the beauty of the colors, and the unusual way in which they may be developed, agates are widely used for semiprecious jewelry and objects of art, and this has been true since ancient times, the name itself coming from the River Achates in Sicily. The center for cutting and polishing agates is at Oberstein, Germany, where this work has been carried on since the middle of the fifteenth century. In spite of the many fine natural colors in agates, they are sometimes artificially colored, in many cases by methods which are kept as “trade secrets.” The color seldom penetrates far; so that even slight chipping reveals whether an inferior agate has been taken and colored up, or whether the stone is natural. Moss agates are chalcedony which has inclosed dendritic masses of some one of the manganese compounds as shown under manganite, p. 73.
Onyx is a variety of agate where the bands are alternately black and white; while sardonyx is agate with red or brown bands alternating with the white. Such agates as these are especially desirable for cameo work, where the figure is carved in the chalcedony of one color, and the other color makes the background.
Silicified or agatized wood is a form of chalcedony, where silica has replaced wood, molecule by molecule; so that in good specimens, all the structure of the wood is still retained, and when thin sections are made it can be studied under the microscope almost as well as modern wood. This takes place under water, usually, if not always, in fresh water. Such fossilized wood is widely distributed in the western United States, the most famous cases being the Fossil Forest of Arizona, now a National Reservation, and the fossil trees in the Yellowstone National Park.
Non-crystalline, massive, stalactitic or nodular; hardness, 6; specific gravity 2; all colors; luster vitreous, resinous, or pearly; transparent on thin edges.
Opal differs from chalcedony in having water, usually about 10%, incorporated in its structure. This is water of crystallization, and not firmly held; so that, if opal is heated in a closed tube to above 100 C., it is given off as a vapor. Opal is distinguished from chalcedony by its lesser hardness, and the resinous to pearly luster. It forms in cavities, in layers often of extreme thinness.
Opal is originally the product of the dissolution of silicate minerals in hot acid waters, the resulting gelatinous silica, when it is deposited and hardened, becoming the opal. There are many varieties, some of them highly prized as gems in spite of the moderate hardness and opacity of the mineral. Gem-quality opal gets its opalescent character from the successive deposition of thin films of opal, the light penetrating and being reflected from different films. This breaks up the white light and causes the play of colors which is the charm of this gem.
Precious opal, in which the play of colors is finest, comes mostly from Hungary, Mexico, and Queensland. The opal was a favorite stone from before Roman times, and in its early history was a charm against the “evil eye.” During the nineteenth century for some reason it came to be considered an unlucky stone.
Fire opal is a hyacinth-red to honey-yellow variety, which has a fire-like play of color, and is found in Mexico and Honduras.
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 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:
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.
Twinning is very common and helps to distinguish orthoclase from the plagioclase feldspars. In orthoclase the twins are simple, that is, only two crystals growing together, and are united on one of the faces, as if one of them had been revolved 180° with the other; or, while related to each other as in the preceding case, they may seem to grow through each other. On plate 34 are three orthoclase crystals showing this simple type of twinning. The first (A) is a simple crystal; the second (B) shows the simplest type of twinning where the left-hand crystal has revolved 180° on the p face, and the end is composed, half of the upper end of one crystal, and half of the lower end of the adjacent crystal. The presence of reëntrant angles calls attention to the twinning. The third figure (C) is a case of intergrowing crystals.
In the plagioclase feldspars twinning is multiple, a large number of crystals, each thin, sometimes as thin as paper, growing side by side, the first one in normal position, the next at 180° with it, the third revolved 180° to the second and thus parallel to the first, and so on. The result is first of all a striated appearance, and second that, as plagioclase crystals have their prism faces intersecting at 86°, there is a series of low roofs and valleys, which are best seen by holding the piece of feldspar so the light reflects from a cleavage face, when it will appear striated; then by tilting it about 8 degrees a second set of reflections, also appearing striated, will appear. The light was first reflected from one side of the roofs, and in the second case from the other side. Figure D, Pl. 34, is a diagram showing the relation of the individual crystals in a multiple twinned piece of plagioclase, in which the crystals are represented as rather large. Plate 35, under labradorite, shows a photograph of a cleavage piece, on which is readily seen the striation which is characteristic of the plagioclase feldspars.
Mixtures of albite and anorthite occur in bewildering numbers, one or the other predominating, and each mixture being uniform throughout the crystal and in the whole mass; so each combination is a mineral, each with its special properties; but the different plagioclase feldspars are so similar in appearance, that by the naked eye it is impossible to separate the closely related ones. This can be done under the microscope by studying the angles at which light is cut off, and also by chemical analyses. For our purposes six types will suffice to illustrate the group, and their composition may be indicated as follows.
Albite is albite with up to 15% of anorthite mixed with it.
Oligoclase is albite with from 15-25% of anorthite mixed with it.
Andesite is albite with from 25-50% of anorthite mixed with it.
Labradorite is anorthite with from 25-50% of albite mixed with it.
Bytownite is anorthite with from 15-25% of albite mixed with it.
Anorthite is anorthite with up to 15% of albite mixed with it.
The best method for distinguishing these feldspars of the plagioclase group is to measure the angle between the two perfect cleavage faces, and even this requires careful measurement. The angles between these faces are as follows:
| Orthoclase | 90° |
| Microcline | 89° 30′ |
| Oligoclase | 86° 32′ |
| Andesite | 86° 14′ |
| Labradorite | 86° 14′ |
| Bytownite | 86° 14′ |
| Anorthite | 86° 50′ |
Occurs in granites, syenites, gneisses and light-colored lavas; hardness, 6; specific gravity, 2.57; color white to gray or pink; cleavage in two directions perfect and at 90°, in the third direction imperfect; luster vitreous; translucent on thin edges.
Orthoclase is monoclinic, and when formed in cavities develops as crystals, but it is usually a constituent of igneous rocks, in which case the crystals have not had the opportunity to develop the crystal faces, and the orthoclase is in grains or irregular masses; and the best way of determining the mineral is the cleavage, the two perfect cleavage planes intersecting at right angles. Twinning is frequent but of the simple type, only two crystals being united, similar to either B or C on plate 34.
It is found in granites, gneisses or lavas, wherever they occur, being especially characteristic of the granites of the Rocky Mountains.
Occurs in granites and gneisses as crystals or irregular masses; hardness, 6; specific gravity, 2.56; color white to gray, pink, or greenish; luster vitreous; translucent on thin edges.
Microcline has the same composition as orthoclase, but is in the triclinic system, the c axis being inclined a half degree away from a right angle with the b axis. This is best seen in the cleavage pieces, the two perfect cleavage planes meeting at 89° 30′, and this is the only test for determining this mineral by the unaided eye. Pike’s Peak is the best known locality for microcline, and there it occurs in fine large crystals of greenish color, which are known as Amazon stone.
Occurs in small crystals, or more often in lamellar masses in granites or in seams in metamorphic rocks; hardness, 6; specific gravity, 2.62; color white to gray; luster vitreous.
Albite may occur in simple crystals, in which case the two perfect cleavage planes meet at an angle of 86° 24′. However, it is much more frequently found twinned in the multiple manner, the individual crystals often being as thin as paper. This gives rise to a fine striation on the end of a crystal, or on the surface made by the imperfect cleavage plane. Where the crystals are extremely thin, the surface may have a pearly luster. Albite types of granite often inclose secondary minerals, that are prized as gems, such as topaz, tourmaline, and beryl.
It is found at Paris, Me., Chesterfield, Mass., Acworth, N. H., Essex Co., N. Y., Unionville, Penn., and in Virginia, and throughout the Rocky Mountains.
Generally found in cleavable masses in granites and lavas, rarely in crystals; hardness, 6; specific gravity, 2.65; color white, greenish or pink; luster vitreous; translucent on thin edges.
Oligoclase is a plagioclase feldspar and is distinguished by its two perfect cleavage planes meeting at an angle of 86° 32′, but otherwise it is very like albite. Crystals are not common, and it occurs mostly in masses, making one of the components of granite or lava.
It is found in St. Lawrence Co., N. Y., Danbury and Haddam, Conn., Chester, Mass., Unionville, Penn., Bakersville, N. C., etc.
Usually found in cleavable masses in granites and lavas; hardness, 6; specific gravity, 2.71; color gray or white, often with a play of colors; luster vitreous; translucent on thin edges.
Labradorite is distinguished by having the two perfect cleavage planes meet at 86° 14′. The iridescent play of color is also very characteristic and is generally present. It is due to the inclusion of minute impurities. This feldspar is usually associated with granites or lavas in which the dark minerals predominate. It gets its name from being the feldspar of the granites of Labrador, and is also found in the granites of the central part of the Adirondack Mountains and the Wichita Mountains of Arkansas.
The minerals of this group are generally associated with feldspars, and make the dark-colored component of granites, gneisses and lavas. This is especially true of those which have some iron in the crystal. Pyroxenes are salts of metasilicic acid (H₂SiO₃), in which the hydrogen (H) has been replaced by calcium, magnesium, iron, etc. The commoner minerals are orthorhombic or monoclinic, and all agree in their crystal habit, being short stout prisms, with the vertical edges so beveled that a cross section is eight-sided. The cleavage is good in two directions, parallel to the beveling faces (m in figure b, Plate 36), and they intersect at an angle of 87°. This is very characteristic, and if one has a crystal broken across, it is easy to see and measure this angle of intersection. These pyroxenes have the same chemical composition as the corresponding series of amphiboles, but the two are distinguished by several features. Pyroxenes are short and stout crystals, while amphiboles are long and either blade- or needle-like; pyroxenes are eight-sided in cross section, while amphiboles are six-sided; in pyroxenes the cleavage planes intersect at 87°, while in amphiboles they intersect at 55°. The minerals of this group are most frequently one of the components of a lava or granite, and are less frequently associated with metamorphic rocks. Three are common; enstatite, hypersthene, and augite.
Usually occurs in lamellar or fibrous-lamellar masses in dark lavas; hardness, 5.5; specific gravity, 3.3; color gray, bronze or brown; luster vitreous, translucent on thin edges.
Enstatite rarely occurs in crystals, but when it does they are orthorhombic. Usually it is in irregular masses with the cleavage angles, typical of pyroxene. The color is light, that is gray or brownish, and the streak white or nearly so. In most respects it is similar to hypersthene, which has the same composition, except that a large part of the magnesium is replaced by iron, and there are all sorts of gradations between the two minerals. When some iron takes the place of magnesium, the color darkens to, or towards bronze, until when about a third of the magnesium is so replaced, and the color is fully bronze, this variety is called bronzite. Bronzite is present in some of the dark lavas like gabbro and peridotite. Enstatite is found in the Adirondack Mountains, at Brewster and Edwards, N. Y., etc.
Occurs in cleavable masses in dark lavas; hardness, 5.5; specific gravity, 3.4; color dark-brown or greenish-brown; luster vitreous; translucent on thin edges.
Hypersthene is a pyroxene in which magnesium and iron are present in about equal quantities. It is similar to enstatite, except that the color is darker, and the streak gray or brownish-gray in color. These two minerals grade into each other, so that there are cases where it is simply a matter of preference as to which name should be given to the mineral. This form is associated with dark lavas, of the gabbro or peridotite type, in such places as the Adirondack Mountains, Mount Shasta in California, Buffalo Peaks, Colo., etc.
Usually occurs in short stout monoclinic crystals; hardness, 5.5; specific gravity, 3.3; color dark-green to black; luster vitreous; translucent on thin edges.
Augite is a complex pyroxene having some iron and aluminum always present in it, but the amount not a fixed quantity. It is by far the commonest of the pyroxenes and has a wide distribution, both in the sorts of lavas in which it appears, and in the world. It is commonly the dark component of such lavas, as gabbros and peridotites, and also is common in metamorphic rocks, especially impure crystalline limestones. It is found at Raymond and Mumford, Me., Thetford, Vt., Canaan, Conn., in Westchester, Orange, Lewis and St. Lawrence Counties of N. Y., in Chester Co., Penn., at Ducktown, Tenn., Templeton, Canada, etc.
The amphiboles are a group of minerals made up of the same chemical elements as the pyroxenes, but with the molecular arrangement different, which appears in the forms of the crystals. The commoner ones are all monoclinic but contrast with the pyroxenes as follows. Amphiboles are long and slender crystals, while pyroxenes are short and stout; amphiboles are six-sided, while pyroxenes are eight-sided; amphiboles have the two perfect cleavages intersecting at 55° and 125°, while those of pyroxene intersect at 87° and 93°. With the above in mind it is easy to place the minerals in their proper group, but inside the group it is not always so easy to distinguish one from another. This group is associated rather with metamorphic rocks than with igneous rocks, with which the pyroxenes are mostly associated. The three commoner minerals of the group are tremolite, actinolite, and hornblende.
Occurs in long prismatic crystals or in columnar or fibrous masses; hardness 5.5; specific gravity, 3; color white to gray; luster vitreous; transparent on thin edges.
The long prismatic crystals of tremolite occur especially where dolomitic limestones have been altered by metamorphism. Sometimes these crystals grow side by side, making fibrous masses, where the long slender crystals can be picked apart with the fingers, and yet are flexible, and tough enough so that they can be felted together. This is termed asbestos, which, because it is infusible and a poor conductor of heat, is much used to make insulators, fire-proof shingles, and all sorts of fireproof materials. The varieties in which the crystals are finer and silky in appearance, like the one illustrated on Plate 38 are termed amianthus. There are other minerals, such as actinolite and serpentine, which occur in the same manner, and are also called asbestos, the serpentine variety being just now the most important commercially.
Tremolite is found at Lee, Mass., Canaan, Conn., Byram, N. J., in Georgia, etc.
Occurs in radiating crystals, or in fibrous masses; hardness, 5.5; specific gravity 3; color pale- to dark-green; luster vitreous; translucent on thin edges.
Except for its green color, this mineral is very like tremolite. The difference between the two is due to the small amount of iron in the actinolite. It is usually found in schists, and the radiating character of the crystal groups is enough to determine the mineral, if it is already clear that it is one of the amphiboles. Occasionally it occurs with the crystals parallel to each other, making one of the forms of asbestos.
Actinolite is found at Warwick, Edenville, and Amity in Orange Co., N. Y., at Franklin and Newton, N. J., Mineral Hill and Unionville, Penn., Bare Hills, Md., Willis Mt., Va., etc.
Occurs in well-defined crystals, in grains and in masses; hardness, 5.5; specific gravity 3.2; color black, dark-green, or dark-brown; luster vitreous; translucent on thin edges.
In composition hornblende corresponds to augite, but occurs in long slender, six-sided crystals with cleavage planes intersecting at 55°, so that it is a typical amphibole. It occurs in a very wide range of rocks, such as granite, syenite, diabase, and gabbro; and in such metamorphic rocks as schists and gneisses; and sometimes igneous rocks are made up almost entirely of hornblende, when they are known as amphibolites or hornblendite. It is found all through the New England States, down along the Piedmont Plateau, through the Blue Ridge Mountains, and in many of the western mountainous areas.
The garnets are a series of double silicates, which occur with surprisingly uniform characters. They are all isometric, and occur either as dodecahedrons, or as the 24-sided figure (the trapezohedron), which is formed by the beveling of the edges of the dodecahedron, and developing these new faces to the exclusion of the dodecahedron faces. Combinations of the dodecahedron and trapezohedron (36 faces) may occur. All the garnets have a hardness of 7 to 7.5, and the specific gravity runs from 3.2 to 4.3, according to the composition. In size they run from as small as a grain of sand up to as large as a boy’s marble, and occasionally even to four inches in diameter. The color varies with the composition, from colorless to yellow, red, violet, or green. There is no cleavage, and the luster is always vitreous.
Garnets are usually accessory minerals, found in metamorphic rocks, though they are sometimes also present in granites and lavas. They are always segregations which have taken place in the presence of high temperatures. When clear and perfect several of the garnets are used as gems. On the other hand some of the common garnets occur in such quantities that they are crushed and used as abrasives, for such work as dental polishes, or for leather and wood polishing.
The following is the composition of some of the commoner garnets.
| Ca₃Al₂(SiO₄)₃ | = grossularite |
| Mg₃Al₂(SiO₄)₃ | = pyrope |
| Fe₃Al₂(SiO₄)₃ | = almandite |
| Mn₃Al₂(SiO₄)₃ | = spessartite |
| Ca₃Fe₂(SiO₄)₃ | = andradite |
| Ca₃Cr₂(SiO₄)₃ | = uvarovite |
Grossularite is chiefly found in crystalline limestones, which have resulted either from contact with lavas, or from general metamorphism of impure limestones. These garnets are colorless to white, or more often shades of yellow, orange, pink, green or brown, according to traces of impurity which they may contain. The cinnamon-colored variety from Ceylon is termed cinnamon stone, and is a fairly popular gem.
Pyrope is a deep-red color and when perfect is highly prized as a gem. It is found in dark-colored igneous rocks, like lavas, or serpentines. Some of the finest come from South Africa, where they are found in company with the diamond.
Almandite is dark-red to brown in color, the brownish-cast distinguishing it from pyrope. It is one of the garnets known as “common garnet.” In some cases it is clear and deep colored enough to be used as a gem, but mostly it is muddy in appearance. The name almandite comes from Alabanda, a city of the ancient district of Caria, Asia Minor, whence garnets were traded to ancient Rome. The finest garnets “Sirian garnets” came from the city of “Sirian” in Lower Burma, and were supposed to have been found near there, but careful investigation shows that no garnets occurred near there, and this town was therefore, even at that early time, a distributing point for garnets, found probably further to the east. The “Sirian” garnet had a violet cast and now the term is used to indicate a type of garnet, rather than a locality.
Spessartite is dark-hyacinth-red, or red with a violet-tinge, and is one of the less-common garnets. It is usually found in granites. The finest garnets of the type come from Amelia Court House, Va., which has yielded some ranging from one up to a hundred carats.
Andradite is another garnet which is termed “common garnet.” It is red in color, but with a yellowish-cast which distinguishes it from almandite, but these two are not easy to separate. It is found mostly in metamorphosed limestones. One variety is black in color and called malanite. It is found in lavas. The common yellowish-red garnets are found through New England and the Piedmont Plateau.
Uvarovite is a rare garnet of emerald-green color, found in association with chromium ores.
The number of localities for garnets is so great that a list would suggest most of the regions where metamorphic rocks occur, as all over New England, throughout the Piedmont Plateau, the Rocky Mountains, etc. Certain fine clear garnets, found in Montana, northeastern Arizona, and northwestern New Mexico are sold under the trade name of “Montana, Arizona or New Mexico rubies.” These are of fine quality and are mostly collected by the Indians from the ant hills and scorpion’s nests of those regions.
Garnets are among the earliest stones mentioned in ancient languages, as would be expected from the way these hard and beautiful crystals weather out of the much softer metamorphic rocks, like schists. In the past they, with most any other translucent red stone, were included under the name carbuncle. This, however, is not the name of any mineral, but refers rather to a mode of cutting, en cabochon or with a convex surface.
Glucinum is a rare metal, silvery-white in color, malleable, and melting at a fairly low temperature. It is found in the mineral beryl, from which has come the alternative name beryllium. The name comes from the sweet taste of its salts. Except for beryl its minerals are rare, and the metal has found but few uses for man.
Occurs in hexagonal crystals in granites, gneisses and mica schists; hardness, 7.5; specific gravity, 2.7; color usually some tint of green; luster vitreous; transparent on thin edges.
When this mineral occurs in coarse hexagonal prisms, with or without faces on the ends, it is known as beryl; when the crystals are clear and perfect and of a dark-green color, they are of gem value and are termed emerald; when of a light-green color, they are aquamarine; and when bright-yellow in color, they are the golden beryl. There is little difficulty in determining beryl, for only apatite occurs in such crystals, and is green, and this latter mineral has a hardness of only 5. There is an imperfect basal cleavage.
Ordinary beryl is fairly common in granites of the pegmatite sort, and less common in gneisses and mica-schists. This type often furnishes crystals of large size, up to two and three feet in diameter.