Art. IV. On the Tourmalines and other Minerals found at Chesterfield and Goshen, Massachusetts.
Art. IV. On the Tourmalines and other Minerals found at Chesterfield and Goshen, Massachusetts, by Col. George Gibbs.
(For the American Journal of Science.)
The schorl of the mineralogists of the last century united a variety of substances which subsequent observations have separated into several species. The green schorl is now the epidote, or the Vesuvian, or the actynolite. The violet schorl, and the lenticular schorl, are the axinite. The black volcanic schorl is the augite. The white Vesuvian schorl is the sommite. The white grenatiform is the leucite. The white prismatic is the pycnite, a species of the topaz, and another is a variety of feldspar. Of the blue schorl, one variety is the oxyd of titanium, another the sappare, and another the phosphate of iron. The schorl cruciform is the granatite. The octahedral schorl is the octahedrite, or anatase. The red schorl of Hungary, and the purple of Madagascar, are varieties of the oxyd of titanium. The spathique schorl is the spodumen.
The black schorl, and the electric schorl, only remained, and to avoid the confusion created by the use of the term schorl, the name of tourmaline was given to this species by that celebrated mineralogist, the Abbé Haüy.[50]
The tourmaline is found of almost every colour, and this variety of colour caused at first a number to be formed into new species; which are now considered only as varieties of the tourmaline: such as the rubellite, the tourmaline apyre, and indicolite.
The different analyses of the tourmaline, however, affords a greater variety of results than is known in almost any other mineral.
| The specific gravity of the | black varies | from 3.08 to 3.36 |
| Green | from 3.15 to 3.36 | |
| Red | from 2.87 to 3.10 |
| Analysis gives | Silex from | 35 to 58 |
| Alumine | 20 to 48 | |
| Magnesia | 0 to 10 | |
| Iron | 0 to 23 | |
| Manganese | 0 to 13 | |
| Alkali | 0 to 10 | |
| Water | 0 to 4 |
These differences must be in some measure ascribed to a defect in the accuracy of some of the analyses. But it appears that iron has not been discovered in the red tourmaline. It is not unworthy of notice, that the red tourmaline is considered as infusible, but the others fusible.
The red tourmaline has been the most valued, from its scarcity, its employment in jewelry, and the beauty of its crystals. It has been discovered in Siberia, in Moravia, in the East-Indies, and in Massachusetts. In Siberia it is found in a vein of decomposed feldspar in a fine-grained granite, with black tourmaline. In Moravia with quartz and lepidolite (or rose-coloured mica) in gneiss. In the East-Indies, at Ava and Ceylon, but its geological situation is not known, though it is probably in gneiss or granite.
The red or rose tourmaline of Massachusetts, is found chiefly at Chesterfield, in a subordinate bed of granite, contained in mica slate. The mica slate is the predominant rock of the country. It is fine grained, and contains an abundance of small garnets. Direction of the strata north and south, varying a little easterly; inclination perpendicular. The bed of granite is about three hundred feet long, and from five to twenty feet broad. It is contained in a narrow ridge of mica slate, which descends into, and is lost in, a valley. The sides are precipitous; the highest part is about forty feet high. On the east side a considerable part of the granite has been destroyed by natural causes, leaving the granite bare. The granite consists chiefly of granular feldspar, with grains of white quartz, and a little light coloured mica, is moderately fine grained, and of a grayish white colour. In addition to tourmaline, it contains also emerald, some of the crystals of which are from three to five inches in diameter. I succeeded in getting one out of its matrix, which is three and a half inches in diameter, and its summit (which is a plane without any additional facettes) is perfect.
The tourmalines are contained chiefly in a false vein of silicious feldspar and quartz, which begins in the centre of the upper edge of the bed of granite, and passes obliquely, descending to the northeast, about twenty feet, where it is intercepted from sight by the mica slate. The vein is about one and a half foot thick in the upper part, and not more than six or eight inches where it is lost. This vein of silicious feldspar contains also a vein of bluish white transparent quartz, which is from three to eight inches thick, and passes through the centre of the vein of feldspar.
When I first examined this rock, soon after its discovery by Dr. Hunt, of Northampton, I determined the feldspar to be a new variety, which has been since confirmed by Professor Hauffman, and now ranks as a new sub-species, under the name of silicious feldspar. (P. 41, of the Mineralogical Table.)
The analysis of Professor Stromeyer, of Gottingen, gives,
| Silex | 70.68 |
| Alumine | 19.80 |
| Soda | 9.05 |
| Iron, Mag. and Lime | .38 |
| —— | |
| 99.91 |
The chief difference between this and the adularia is, that one contains fourteen potash and the other nine soda. Between this and the saussurite, or tenacious feldspar, the one contains eleven of lime, and the other only a trace.
The silicious feldspar, which I suspect to be the basis of the granite, crystallizes in thin rhomboidal tables. They are very frangible, and have one clivage perpendicular to the faces of the tables. Sometimes the tables have one lateral edge or more truncated. In one fragment of a crystal I observed a very obtuse acumination on the table, which appeared to be diedral, the sides being placed on the obtuse lateral edges of the tables. On account of the extreme frangibility of the crystals, it is certainly extremely difficult to seize their characters. Specific gravity only 2.333, probably owing to interstices between the tables. The colour is white, translucid, passing to semi-transparent; lustre sometimes dull, at others shining. The tables are sometimes so aggregated that their edges being exposed, offer wedge-shaped and stelliform figures. The tourmalines are chiefly contained in this vein. They are red, or green, rarely blue or black.
The green tourmalines vary from one-eighth of an inch to one inch in diameter; they are sometimes four inches in length, and are entirely confined to the inner vein of quartz. They are triedral prisms, with convex faces, striated longitudinally, and generally traversed perpendicularly to the axis, with very small fissures filled by some silicious substance, probably feldspar. These green crystals are opaque. The red tourmaline is frequently enclosed in the green. In certain parts of the vein almost every green crystal encloses a red one, which always corresponds by its sides and angles with the exterior crystal. Sometimes a thin layer of talc intervenes between the outer and inner crystal. In one specimen I found three crystals of the red aggregated together, and enclosed in one of the green. In another crystal I found pyrites in the place of the red tourmaline. The largest crystal of the red was one quarter of an inch in diameter, and four inches long. The red tourmalines vary in intensity of colour, and frequently (particularly in the interior) pass into violet. They pass from translucid to semi-transparent. I have found some that were terminated by triedral pyramids. The crystals are generally perpendicular to the sides of the vein. Small crystals of the red often run from the vein of quartz into the adjoining feldspar. The granite also contains minute crystals of dark and light blue tourmaline, and pale green emerald, with a very few garnets and pyrites. In the lower part of the vein, five to six feet from its interruption by the mica slate, the red tourmaline scarcely appears, and the vein contains chiefly bluish amorphous quartz and green tourmaline. It is therefore probable that this vein will not afford henceforward a great supply of this beautiful mineral.
About six miles from Chesterfield, in Goshen, is found the rose mica, with tourmalines and emeralds interspersed in the granite. Unfortunately the bed of granite has not been discovered, and the specimens we possess are taken from loose rocks, scattered over a small extent of ground in a valley, in the neighbourhood of mica slate. The rose mica is found in a large grained granite with amorphous quartz and silicious feldspar, crystallized and amorphous. The mica is generally of a rose red, sometimes yellowish green. It crystallizes in rhomboidal tables, rarely truncated on the acute angles, passing into the hexaedral table. The tourmalines are light and dark green and blue, of various shades of intensity, frequently acicular and stellated. The black, the red, and the violet tourmalines also occur, but more rarely. Sometimes the green prisms enclose others of blue and black. Specific gravity of these varieties from 3. to 3.1. The green and blue crystals in this locality are translucid or semi-transparent. The feldspar is generally white, rarely light blue. There are some emeralds in the granite. Among some specimens which Mr. Weeks of New-York, who discovered this locality, was so good as to give me, I found a beautiful rose emerald in its matrix. It is a hexaedral prism, about one and a quarter inch in diameter, the summit a plane, one of the lateral edges has a truncature. About half of the diameter of the prism is free from the matrix, and half an inch of the prism. The colour is a pale rose, rather more transparent than the emerald.
The colour of the mica of the Goshen granite calls to mind the lepidolite or lilalite, which (formerly considered as a distinct species) has now been united to mica. The lepidolite of Rosena is also accompanied by the tourmaline apyre, now the red tourmaline.