Crystal (geology)

“ Inclusions of foreign matter, accidentally caught up during growth, are frequently present in crystals. Inclusions of other minerals are specially frequent and conspicuous in crystals of quartz, and crystals of calcite may contain as much as 60% of included sand. Cavities, either with rounded boundaries or with the same shape (“negative crystals”) as the surrounding crystal, are often to be seen ”

“ Gemstones that have an orderly internal molecular arrangement are referred to as crystalline. This internal order is commonly reflected in the external shape of “rough” or uncut gemstones. The resultant shape is a polyhedral solid bounded by planes and called a crystal. Well-formed crystals are formed in nature only under relatively ideal conditions of temperature, pressure, and space. The specific temperatures and pressures involved vary with different minerals, but most crystals need space in which to form so that their “growth” is not impaired by surrounding rocks and minerals. ”

“ The shape of a crystal does not depend like that of the animal or the plant upon the life which its forefathers have led or the conditions under which they have grown.
There are indeed other features in which a mineral or any crystal may resemble a living thing in a way even more surprising than in its form. Two of the most remarkable are these: it grows out of a solution as though it were alive; and, if it is wounded or broken, it heals itself and replaces the missing part just as a living organism may do. ”

“ When a mineral occurs as a well-formed individual crystal, it has a definite, precise shape. The kind of crystal shape it has depends on its own type of crystalline internal structure. A well-formed crystal has smooth, flat, outer surfaces called crystal faces, which are arranged together to form prisms, cubes, pyramids, and many other geometric shapes. For example, quartz, a common Texas mineral, is commonly found as a six-sided, prism-shaped crystal that is topped by pyramid-like forms. Pyrite, another common mineral, occurs as cube-shaped crystals. ”

“ Imperfect crystals.—
A crystalline mineral commonly forms under conditions that do not permit it to become a well-shaped crystal. Although the mineral may show a few crystal faces, it does not have a complete crystal shape and so is described as massive, or is said to occur in masses. Some of the minerals that make up rocks occur as crystalline masses. For example, calcite is a crystalline mineral that occurs in the metamorphic rock marble without its normal crystal shape. ”

“ The crystals were all found in disintegrated crystalline rocks, consisting principally of coarse feldspathic granite, which have all decomposed to a greater depth than their position. Most of them are obtained by digging where one crystal has been found, or by driving a plow till some hard object is struck. Several dozen have been found weighing from twenty to thirty pounds each. ”

“ Plate X. Photograph of a cast made by pouring plaster over a surface of mud in which ice crystals had recently formed, in Rock Island, Ill., and where they had later been removed by natural sublimations into the atmosphere, leaving open cracks in the mud. It will be seen that the crystal growth in this case involves an x-like or radiating pattern formed of relatively very slender forms that almost everywhere are very gently curved somewhat reminding of the slender thread-like crystals sometimes seen in frost on windows. ”

“ One of the most beautiful illustrations of cohesive attraction is seen in crystallization. In every instance in which substances pass into the form of a solid, they tend to assume regular shapes called crystals. Each material has its own characteristic form, so that a crystal is a type of a species in the mineral world, even as a plant or an animal is in the organic kingdom. A crystal is a substance bounded by plain surfaces and symmetrically arranged about imaginary lines called axes. The final form depends upon certain smaller forms in its interior structure. ”

“ All mineral matters that you call dead bodies were also created as well as vegetable substances, and have been in travail to produce seeds from which others could be engendered. Crystal is not so dead but that it is given to it to know how to separate itself from other waters, and from itself, with its angles and diamond-points, in the midst of them. ”

“ Still more rare is a mineral, of which singularly enough we possess as yet no satisfactory analysis, known as Turnerite, from the Tavetsch Valley in the Alps. In this collection there is a number of perfect crystals, five if I remember correctly, each of which displays many highly-lustrous facets, and occurring both isolated and embedded in a rock made up of quartz and albite. The largest crystal is three-quarters of an inch long and five-eighths of an inch thick. It is a doubly-terminated hexagonal prism, the basal edges being regularly replaced by twelve small planes. ”

“ Plate III. Photograph of fossil casts of a close tangle of ice crystals seen in a stony calcareous layer in the Eagle Ford shale in Walnut Creek, about eight miles north of Austin, Texas. This tangle is closer than any of the recent ice crystal marks figured here, but equally closely grown crystals have been seen by the writer on frozen mud in Illinois. Natural size. ”

“ And so it is of the lithosphere, for the myriads of individuals are either the ruins of former crystals, or they are grown together in such a manner that crystal facets had no opportunity to develop.
Such mineral individuals as once possessed the crystal form may have been broken and their surfaces ground away by mutual attrition under the rhythmic beating of the waves upon a shore or in the continuous rolling of pebbles on a stream bed, until as battered [26] relics they are piled away together in a bed of sand. ”

“ I pointed out, I remember, that these delicate forms are, like the frost patterns on a window-pane, really expressions of the crystalline shape and symmetry of the mineral; each mineral consists of crystals, and therefore has its own peculiar crystalline shape which is one of its inherent properties; moreover, this persists unchanging through the ages, and under all conditions, and is in no way dependent upon the environment in which the mineral is situated. ”

“ An especially interesting type of crystal is that which grows in queer needle-like layers, somewhat suggestive of tiny stalactite growths; this variety we frequently discover in gravelly or peaty soil, while it sometimes raises and supports upon its points large sections of earth and stones. ”

“ This article deals with the plainly crystalline varieties. These include, among other varieties, rock crystal, amethyst, rose quartz, smoky quartz, and sagenitic quartz. These varieties all occur in well formed crystals, and all have a vitreous luster, i. e., luster like that of glass. The differences between them are almost exclusively differences of color.
Rock Crystal—This is quartz in its purest form. Typical rock crystal is perfectly transparent and colorless, but the mineral is often more or less clouded and opaque. ”

“ We encounter, in thin sections of both, volcanic and metamorphic rocks, microscopic crystals arrested in every stage of their growth, and it is not true that these earlier forms are mere epitomes of the perfected individual. We have the fundamental globulite and the complicated and fantastic "growth-forms," which are as different from the finished crystal as is the larva from the butterfly. ”

“ Here again we need none of Vulcan's aid, for we all know that when once crystallization commences each little crystal will have its growth in spite of almost any resistance. Witness the growth of ice crystals in our water pipes in zero weather. In other words, the force exerted by growing crystals is known to be at least of the same order of magnitude as the crushing strength of the grown crystal. ”

“ I reminded him of his having spoken of crystals of ice, and asked how that term could be applied to any thing but mineral bodies.
“The term crystal,” he replied, “came from the Greek word for ice—it was afterwards applied to rock crystal, which the ancients imagined to be water converted into stone; but it now signifies the regular figure in which the particles of any substance arrange themselves in passing from the liquid to the solid state.—Each of those sub{86}stances has a figure for its crystal peculiar to itself, and from which it never varies. ”

“ However, some minerals, such as garnet and tourmaline, can grow in metamorphic rocks by recrystallization of minerals in the metamorphic rocks. The size of crystals varies from microscopic to tens of feet. Any one mineral usually has one or two typical crystal forms or arrangements of plane surfaces that aid greatly in the identification of the mineral when it occurs in good crystals (fig. 1) . Frequently gemstones are found as abraded stream-rolled pebbles, fragments, or masses that do not show crystal form. ”

“ The compound structure of such stones becomes manifested when the mass gradually thaws. In some cases these stones are coated with crystals of ice in six-sided prisms and pyramids, as perfectly formed as the specimens of quartz or calc-spar crystals which are to be seen in mineral collections. It is very hard to believe that such beautifully formed crystals as these can be the product of any instantaneous process of formation. ”

“ There are crystals as big as cathedral pillars, delicate as mildew and sharp as needles; plain, blue, green like nothing in the world, of fiery colours or black; mathematical, perfect, like the contrivances of queer and bewildered sages; or recalling livers, hearts, gigantic human organs and animal fluids. There are crystalline caves or spectral bubbles or mineral dough; there is mineral fermenting, grilling, growth, architecture and engineering; I vow that a Gothic church is not the most complicated of crystals. Even within us there persists a crystalline power ”

“ No more correct estimates of the comparison of crystal individualities would be obtained by the study of outward forms alone of two minerals than would be gained by a judgment of persons from the cut of their clothing. Too often this outward dress tells only of the conditions by which both men and crystals have been surrounded, and but little of the power inherent in the individual. Many a battered mineral fragment with little beauty to recommend it, when placed under suitable conditions for its development, has grown into a marvel of beauty. ”

“ Having obtained a good Crystal, as free as possible from blemish, care must be taken to keep it is much as possible in a dark place when not in use. The best covering therefore is a black one of soft material, such as velvet, which will not scratch the polished surface of the quartz. [*] Exposure to the sun's rays not only scores the faces of the crystal, but also puts the odylic substance into activity, distributing and dissipating the magnetic force stored up therein. It must not be understood that the visions are in the crystal itself. They are in the soul of the seer. ”

“ As rock-crystal, the typical form of quartz, is an hexagonal prism terminated at each end by a rhombohedron, when broken it will be seen to have a conchoidal or splintery fracture. Rock-crystal is very widely distributed, being found in rocks of all ages. The most beautiful and perfect specimens are usually obtained from large cavities or geodes in the older igneous rocks, and also from veins in these and other rocks. The size and color of quartz-crystals vary greatly; some are so small as to be microscopical, while others are of very considerable bulk. ”

“ Etym., kryptos, concealed, and gamos, marriage.
Crystals. Simple minerals are frequently found in regular forms, with facets like the drops of cut glass of chandeliers. Quartz being often met with in rocks in such forms, and beautifully transparent like ice, was called rock-crytal, krystallos, being Greek for ice. Hence the regular forms of other minerals are called crystals, whether they be clear or opake. ”

“ In the early stages of development the crystal will begin to cloud over, first becoming dull, then suffused with milky clouds, among which sparkle a large number of little specks of light like gold dust in the sunlight. The focus of the eyes is inconstant, the pupil rapidly expanding and contracting, the crystal at times disappearing entirely in a haze or film which seems to pass before the eyes. Then the haze will disappear, and the crystal will loom up into full view again, accompanied by a lapse of the seer into full consciousness. ”

“ At other times a crystal which is imbedded in rock has been attacked upon its surface by the percolating solutions, and the dissolved materials have been deposited in place as a crown of new minerals which steadily widens its zone until the center is reached and the original crystal has been entirely transformed (Fig. 14) . It is sometimes possible to say that the action by which these changes have been brought about has involved a nice adjustment of supply of the chemical constituents necessary to the formation of the new mineral or minerals. ”

“ This eutectic proportion, when attained by two mineral substances in a magma, brings about a complete interlocking of their crystals, as is seen in the quartz and alkali-felspar of the rock known as "graphic granite." The order of crystallisation of minerals from an ordinary non-eutectic magma is profoundly affected by the proportions in which their constituents are present in the mass. ”