Zircon

“ The effect of heat on zircon is remarkable. Most coloured zircons, exposed to a high temperature, either change or lose their colour, but this loss is attended by a gain in brilliancy. The “Matura diamonds” of Ceylon are zircons which have been thus artificially decolorized. Certain zircons when heated in a Bunsen-flame glow with an orange incandescence, whilst others may emit an orange glow when ground on a copper-wheel fed with diamond-dust. Even exposure to sunlight will sometimes modify the colour and lustre of a zircon. ”

“ There are also other decided examples among the transparent minerals to sustain this view; the most remarkable of which perhaps may be found in the zircon, a [Pg 94] gem which is soft as quartz; yet it ranks next to the diamond in brightness, and far surpasses in eclat every other gem, even the sapphire, which is next to the diamond in hardness. Density does not seem to have anything to do with the determination of the refractive power of gems, for the garnet, spinel, sapphire, and zircon are much heavier than the diamond, and are yet far inferior in brilliancy. ”

“ Having observed, for example, that the syenitic granite of Norway, in which the mineral called zircon abounds, has altered the Silurian strata wherever it is in contact, we do not hesitate to refer other masses of the same zircon-syenite in the south of Norway to a post-Silurian date. Some have imagined that the age of different granites might, to a great extent, be determined by their mineral characters alone; syenite, for instance, or granite with hornblende, being more modern than common or micaceous granite. ”

“ Artificial crystals of zircon have been obtained by the action of silicon tetrachloride and silicon tetrafluoride on zirconia, and by the action of zirconium tetrafluoride on silica at high temperatures.
Zircon is one of the most widely distributed minerals known, though usually it occurs in very small quantities. Good crystals have been found in New Zealand, in Ceylon, at Miask in the Urals, and in North Carolina. This last deposit has been worked commercially for the extraction of zirconia for Nernst lamps (vide p. 320) . ”

“ Brown zircon, while lacking dichroism, is frequently rich and pleasing in shade, and when well cut is very snappy, the luster being almost adamantine, the dispersion being large, and the refractive index high. It is useless to deny that by the unaided eye one might be deceived into thinking that a fine brown zircon was a brown diamond. However, the large double refraction of the zircon easily distinguishes it from diamond (use the sunlight-card method or look for the doubling of the edges of the rear facets as seen through the table) . ”

“ Zircon is a silicate of zirconium, ZrSiO4, with small quantities of other elements. Most varieties contain ferric oxide and thoria; more rarely small proportions of the yttria earths may be present. All varieties contain traces of a large number of the common metals. Traces of radium are usually present, with helium and neon, [50] and the mineral is strongly radioactive. ”

“ Some [252] dark quartz yields a nearly garnet red product, after heating.
Zircon. Slight increase in temperature causes many of the zircons from Ceylon to change markedly in color. An alcohol flame serves admirably to effect the change, care being taken to warm up the stone very gradually and to cool it slowly. Drafts should be prevented, as they might suddenly cool the stone and crack it. Some zircons become completely whitened by this treatment. At the same time they increase markedly in density and in refractive index and thus become even more snappy and brilliant than when colored. ”

“ In the crystal of zircon (fig. 88) the eight prism-faces a, m, &c. constitute a zone, denoted 573 by [a, m, a′, &c.] , with the vertical tetrad axis of symmetry as zone-axis. Again the faces [a, x, p, e′, p′, x′′′, a′′] lie in another zone, as may be seen by the parallel edges of intersection of the faces in figs. 87 and 88; three other similar zones may be traced on the same crystal. ”

“ The whitened zircon, when finely cut in the brilliant form, with truly flat facets and sharp edges and with a top angle of about 39 degrees and a back angle of about 44 degrees, so closely resembles a diamond that it will deceive almost anyone on casual inspection. The expert, even, may be deceived, if caught off his guard. The writer has a fine specimen of a little over one carat, with which he has deceived many jewelers and pawnbrokers, and even an importer or two. ”

“ Zircon is well known in Ceylon, which supplies the world with the finest specimens, and is highly appreciated by the inhabitants of that sunny isle, but it scarcely finds a place in jewellery elsewhere. The colourless stones are cut as brilliants, but brilliant-cut fronts with step-cut backs is the usual style adopted for the coloured stones. ”

“ Such stones originally were reddish brown zircon pebbles from Indochina, but they have been converted by being subjected to temperatures 57 approaching 1800° F. for periods of up to two hours. When the original zircons are heated in a closed container, the stones become blue or colorless; when a flow of air is allowed through the container, the stones become golden yellow, red, or colorless. In most of these converted stones the color remains quite stable, but in some it may revert to an unattractive greenish or brownish blue after a period of time. ”

“ The colourless jargoon may be obtained by heating certain coloured stones. When zircon is heated it sometimes changes in colour, or altogether loses it, and at the same time usually increases in density and brilliancy. ”

“ Moreover, the luster of zircon is nearly adamantine, so the expert does not miss the cold metallic glitter as he would with any other white stone. The color dispersion, too, is so high (86% as great as in diamond) that the zircon has considerable "fire," and thus the casual handler is again deceived. A fine white zircon is really prettier than a poor diamond. It cannot compare, however, with a fine diamond. It would never do to let an expert see your zircon beside even a fair diamond. The zircon would look "sleepy." ”

“ Zir"co- (?) . (Chem.) A combining form (also used adjectively) designating zirconium as an element of certain double compounds; zircono-; as in zircofluoric acid, sodium zircofluoride.
Zir`co*flu"or*ide (?) , n. (Chem.) A double fluoride of zirconium and hydrogen, or some other positive element or radical; as, zircofluoride of sodium.
Zir"con (?) , n. [F., the same word as jargon. See Jargon a variety of zircon.] (Min.) A mineral occurring in tetragonal crystals, usually of a brown or gray color. It consists of silica and zirconia. A red variety, used as a gem, is called hyacinth. ”

“ Under the blowpipe, zircons are infusible, but the coloured stones when heated strongly become heavier, and as they are contracting, their colour fades, sometimes entirely, which changes are permanent, so that as they possess the adamantine lustre, they are occasionally cut like a diamond, and used as such, though their deficiency in fire and hardness, and their high specific gravity, make them readily distinguishable from the diamond. ”

“ But neither the transparency of the quartz nor the silvery lustre of the mica can be expressed in the engraving.
The uniform mineral character of large masses of granite seems to indicate that large quantities of the component elements were thoroughly mixed up together, and then crystallised under precisely similar conditions. There are, however, many accidental, or “occasional,” minerals, as they are termed, which belong to granite. Among these black schorl or tourmaline, actinolite, zircon, garnet, and fluor spar are not uncommon ”

“ Their mineralogical value gives them no small commercial importance. Of such are magnificent zircon crystals, occurring as individuals up to fifteen pounds in weight, and many finer ones weighing a pound, as well as beautiful twin crystals of the same mineral ”

“ There is not one in a hundred jewellers who is acquainted with the physical properties of the gems; and very few can distinguish the diamond from the white zircon or the white topaz, the emerald from the tourmaline of similar hue, the sapphire from iolite, or the topaz from the Bohemian yellow quartz. Jewellers are governed generally by sight, which they believe to be infallible, whilst hardness and specific gravity are the only sure tests. ”

“ A few zircons, a fraction of a millimeter in size but perfect in form, were found associated with others rounded on their solid angles and edges. The crystals are of the common short form and bear the usual faces in a greater or less degree of development. Pyramids of the first and second order alternate in magnitude; pinacoid encroaches upon prism, and vice versa.
Quartz constitutes by far the largest proportion of the minerals, both in bulk and in weight. It is always fragmental; sometimes water-clear, but chiefly occurs in opaque grains of different colors. ”

“ Second, zirconia ore proper, or zirkite (a trade name) , which ranges in shade from a light gray to a blue black, the lighter colored material carrying a higher percentage of zirconium silicate, as evidenced by analyses, which in some cases show a minimum of 73 per cent. zirconium oxide. The blue-black ore generally carries from 80 per cent. to 85 per cent. zirconium oxide. By careful sorting, however, a uniform grade carrying about 80 per cent. is produced. ”

“ Viewing the whole of this compound mass of schist and limestone as a single rock, and the granite as another, it is easily seen on the most superficial glance, from the strong contrast between the dark grey of the former and the red of the latter, that the dark rock is intersected and disturbed by innumerable veins of granite. These traverse it in every possible direction, and are of various sizes, the smallest not exceeding that of a thread, and reticulating the dark rock in a most intricate and amusing manner. ”

“ We are thus to presume that that crystalline texture of rock which we see exemplified in granite is the condition into which the great bulk of the solids of our earth were agglomerated directly from the nebulous or vaporiform state. It is a condition eminently of combination, for such rock is invariably composed of two or more of four substances—silica, mica, quartz, and hornblende—which associate in it in the form of grains or crystals, and which are themselves each composed of a group of the simple or elementary substances. ”

“ Black granite rises in sheer cliffs, mountain-high above you. The Reuss thunders seventy-five feet below the arch on which you are standing. You are wet with the spray. A sense of personal nothingness, of annihilation, comes over you. You feel as a shrimp might between the jaws of a whale.
The granite appears absolutely naked; but a closer inspection shows you a few humble Alpine plants in such crannies and crevices as their roots could find a hold in, together with a little mould from decayed lichens to feed upon. ”

“ When magnified and examined with the polariscope, a thin section of granite is a very beautiful object, and its different constituent parts stand revealed with the greatest distinctness: we at once learn to see the crystals of feldspar, somewhat opaque and cloudy as they usually are in granite, but now and then clear and beautifully striped, and also the crystals of mica, imbedded in the clear quartz, which will be at once known by its bright clear colors and by the margin of rainbow-like tints which border its patches. ”

“ Again shafts of hard granite may make tall spires and turrets upon a mountain peak, a vein of quartz may bulge out in a white or yellow or rose-colored band; and a ridge of black lava, reaching down the side of a foot-hill, may creep and heave like the backbone of an enormous dragon. ”

“ When discussing the ages of the Plutonic rocks, we have seen that examples occur of various primary, secondary, and tertiary deposits converted into metamorphic strata near their contact with granite. There can be no doubt in these cases that strata once composed of mud, sand, and gravel, or of clay, marl, and shelly limestone, have for the distance of several yards, and in some instances several hundred feet, been turned into gneiss, mica-schist, hornblende-schist, chlorite-schist, quartz rock, statuary marble, and the rest. ”

“ Now, if we examine a fragment of the oldest or fundamental gneiss or granite, we shall see glassy grains of quartz, reddish or white flat-surfaced crystals of felspar, and dark-coloured prisms of hornblende. When destitute of any arrangement in layers, the rock is granite; when arranged more or less in flakes or laminæ, it is gneiss, the structure of which may arise either from its having been formed in successive beds, or from its having been flattened or drawn out by pressure. ”

“ Now, when we examine our granite beneath the microscope, which can be done by having thin slices prepared, we shall learn something about it which we could hardly hope to have discovered without this aid. There has been much speculation as to the origin of granite, whether it is a plutonic—that is, an old volcanic rock—or whether it is only a deposit from water consolidated and altered during the lapse of long ages by heat and pressure: the microscope will help us to the truth. ”

“ Metamorphic, or Stratified Crystalline Rocks.—The fourth and last great division of rocks are the crystalline strata and slates, or schists, called gneiss, mica-schist, clay-slate, chlorite-schist, marble, and the like, the origin of which is more doubtful than that of the other three classes. They contain no pebbles, or sand, or scoriæ, or angular pieces of imbedded stone, and no traces of organic bodies, and they are often as crystalline as granite, yet are divided into beds, corresponding in form and arrangement to those of sedimentary formations, and are therefore said to be stratified. ”