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.
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′ |
[Orthoclase]
KAlSi₃O₈
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.
[Microcline]
KAlSi₃O₈
[Pl. 35]
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.
[Albite]
NaAlSi₃O₈
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.
[Oligoclase]
(NaCa)AlSi₃O₈
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.
[Labradorite]
(NaCa)AlSi₃O₈
[Pl. 35]
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 [Pyroxene] Group
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.
[Enstatite]
MgSiO₃
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.
[Hypersthene]
(MgFe)SiO₃
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.
[Augite]
CaMg(SiO₃)₂, MgAlSiO₆ + Fe₂O₃
[Pl. 36]
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 [Amphibole] Group
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.
[Tremolite]
(CaMg)₃(SiO₃)₄
[Pl. 37]
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.
[Actinolite]
(CaMgFe)₃(SiO₃)₄
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.
[Hornblende]
(CaMgFe)₃(SiO₃)₄CaMgAl₂(SiO₄)₃
[Pl. 37]
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 [Garnet] Group
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.