  Early in the eighteenth century some children of Holland, playing on a warm summer’s day in a court yard with a few bright colored stones, noticed that these possessed a strange power when warmed by the heat of the sun. They attracted and held (just as a magnet attracts iron) ashes, straws and bits of paper. On reporting this strange discovery to their parents the latter, it is said, could give no explanation of the curious property, but a relic of their knowledge of it is left in the name of “aschentreckers” or “ash-drawers” which they gave the stones and by which they were known for a long time. Such was the method of introduction to the civilized world of the mineral now known as Tourmaline, a mineral which in variety of color, composition and properties is one of the most interesting in Nature. The lapidaries who had given the Dutch children the stones for playthings did not recognize them as different from the other gems in which they were accustomed to deal. So to the present day, although Tourmaline is considerably used in jewelry, it is rarely ever called by that name. The green varieties are often known as Brazilian Emerald, Chrysolite or Peridot, some varieties of blue as Brazilian Sapphire, others as Indicolite, the colorless as Achroite, and the red as Rubellite, Siberite, and even as Ruby. It is only somewhat recently that these different stones have been recognized as being varieties of a single mineral species which is known by the name Tourmaline. This name comes from a Cingalese word (turamali) which was applied to the first Tourmaline gems sent from Ceylon to Holland. At one time the name Schorl was chiefly applied to the species. This was before the means of distinguishing mineral species were as well understood as they are now, and a large number of minerals and even rocks were included under the name Schorl. One by one, however, they were distinguished by separate names until Schorl included only Tourmaline, and shortly afterward the name Schorl was dropped altogether. In its opaque form, colored either black or brown, Tourmaline is a comparatively common mineral. It accompanies many so-called metamorphic rocks, i. e., rocks which have been changed by heat and pressure from their original condition, and is also common in granite and other eruptive rocks. As a rock forming mineral it often occurs as long, slender prisms, frequently about the size of a darning needle and radiating in all directions. The only mineral for which it is likely to be mistaken in this form is Hornblende. It can be distinguished from this in the following manner: On fusing the powdered mineral with a mixture of bisulphate of potash and fluor spar (best done on a little loop of platinum wire) Tourmaline will color the flame green, while Hornblende will produce no coloration. The black opaque crystals often reach a large size, as some are known to be four feet in length. Both black and brown Tourmaline are usually opaque, and hence have no value as gems. The Tourmalines available for gems are transparent and have a great variety of color. The gem Tourmalines are to be found in only a few localities. They occur in Maine, Connecticut and California in our own country, and also in Brazil, Russia and Ceylon. The crystals are usually in the form of long, slender prisms. They often have the peculiarity of being differently colored in different portions. Thus a crystal may be green at one end and red at the other, and in cross section may show a blue center, then a colorless zone, then one of red and then one of green. Some of the crystals from Paris, Me., change from white at one termination to emerald green, then light green, then pink, and finally are colorless at the other termination. In some crystals again the red passes to blue, the blue to green and the green to black. TOURMALINE.
Exactly what produces these differences of color is not known. It is known that black Tourmaline has an excess of iron, the red and green an excess of sodium and lithium, and the yellow and brown an excess of magnesium in their composition. These same differences of composition characterize similar colors in portions of the same crystal as well as separate crystals. Hence the evidence is quite conclusive that the color in some way depends on the composition. Many transparent Tourmalines, while appearing of a uniform color when viewed in any one direction, exhibit different colors when viewed in different directions. Thus, one of the long, slender crystals may appear green when held lengthwise in front of the eye, but when looked at from the end appears brown. Again, some crystals appear perfectly transparent when viewed perpendicularly to the sides of the prism, but when viewed from the end are perfectly opaque. This may be true even when the thickness is less in the latter direction. Both these properties are due to the arrangement of the molecules of Tourmaline, which is such as to make the power of absorbing light different in different directions. The form of crystals of Tourmaline is usually that of a three-sided prism. The sides of the prism are usually marked by narrow parallel lines called striÆ, and the prism may be more or less rounded by the addition of other planes. If a doubly terminated crystal be examined carefully, it will be seen that the planes on the two ends are not alike, either in number or inclination. On one end there may be three planes, on the other six, or even twelve. If the planes on one end make a blunt termination, those on the other may make a sharply pointed one. Such a peculiarity of crystal form is possessed by but few minerals. Those possessing it are said to be hemimorphic, i. e., half formed. In such minerals it is evident that the forces of attraction by which the molecules were arranged differed in character at one end from those at the other. In other words, a separation of the molecular forces seems to have taken place, one kind going to one end and the other force to the opposite end. Now, it is a curious fact that most crystals which exhibit this peculiarity of form are also pyroelectric, i. e., become electric on heating. It was this development of electricity which caused the stones with which the Dutch children played, to pick up ashes, paper, etc., when the stones were warmed by the heat of the sun. Anyone can repeat their observation by gently heating crystals or even fragments of Tourmaline and applying them to bits of paper. The electrical attraction will often be found to be very strong, though it varies with different crystals. The fragments should not be overheated, the electricity being most strongly developed between 100 and 200 degrees Fahrenheit. A study of the kinds of electricity developed by the heat shows that positive electricity is produced at one end and negative at the other. Now, this exactly corresponds with what we have seen of the form of the crystal, and shows that the polar arrangement of the molecules producing different shapes at the two ends, also produces corresponding electrical properties. Crystal form, heat, electricity, and even light, are therefore seen to have intimate connection, and it may be that this interesting mineral will furnish us a means of learning more about these forces. In composition Tourmaline is a complex silicate chiefly of aluminum and boron. Iron, magnesium, the alkalies, and water also enter in varying amounts into it. In fact, so complicated is its chemical nature that perhaps no other mineral has been so often analyzed or had its analyses so much discussed. Ruskin, in his “Ethics of the Dust,” thus describes its composition: “A little of everything; there’s always flint and clay and magnesia in it; and the black is iron according to its fancy; and there’s boracic acid, if you know what that is, and if you don’t, I cannot tell you to-day, and it doesn’t signify; and there’s potash and soda, and, on the whole, the chemistry of it is more like a mediÆval As to its hardness and specific gravity, Tourmaline may be said to be both harder and heavier than quartz, its hardness being 7-7.5 in the scale of hardness of which the diamond is 10. Its specific gravity is 2.98-3.20. These qualities fit it admirably for use as a gem. It is, however, quite brittle and even at times friable. Cracks therefore frequently cut across good crystals and spoil what would otherwise make a good gem. It is very common to find tourmalines in the rocks broken into a number of pieces and the fragments “mended” together with quartz or calcite. This has been true of the black Tourmaline shown in the accompanying plate. Scarcely any other mineral exhibits this change so often as Tourmaline, a result due probably to its brittleness and the character of the rock in which it occurs. I have shown how one of the most remarkable properties of Tourmaline was discovered by children. It is also interesting to know that the locality of the finest Tourmalines in the world was discovered by two boys named Elijah L. Hamlin and Ezekiel Holmes. They were interested in the study of minerals and spent much of their leisure time searching for them. One day in the fall, having been out many hours hunting for new minerals, they were about to return home when a gleam of something green at the roots of a tree caught their eye. Eagerly bringing it to light, they found it to be a beautiful green Tourmaline. A fall of snow that night prevented their obtaining more of the crystals, but the following spring they returned and secured many fine gems. For many years thereafter the locality furnished gems of purest ray serene which have gone to adorn the coronets of kings and enriched the mineral cabinets of the world. It is estimated that fifty thousand dollars’ worth of Tourmalines have been taken from this one locality. Auburn and Rumford, Me., are two other neighboring localities where good gems have been found. At Haddam Neck, Conn., fine transparent Tourmalines occur, generally green in color, and many of them of gem quality. They occur in a granite rock. The red Tourmaline (Rubellite) from California, illustrated in the accompanying plate, is found in San Diego County of that State. The matrix in which it occurs is a lithia-bearing mica (lepidolite) of a delicate violet color. In this matrix the Tourmaline usually occurs in radiating masses. The rose color of the Tourmaline contrasting with the violet of the lepidolite makes an object which is quite a favorite with mineral fanciers, although the former is not sufficiently transparent to be used as a gem. At two other localities in the same State large transparent Tourmalines of varying colors have been found. The Brazilian Tourmalines are chiefly green in color. They occur in connection with blue and white topaz. They are the source of the gem known as Brazilian Emerald, which has not, however, the value of the true emerald. The Tourmalines from the island of Elba are generally red in color, transparent and well crystallized. They are, however, too small to use as gems. The Ceylon Tourmalines occur in the gem gravels of that locality accompanying rubies, sapphires, etc., while those of Russia, chiefly red in color, come from the Ural Mountains, being found in company with amethyst, topaz, beryl and other gems. Oliver Cummings Farrington. |
|
