THE MINERALOGICAL AND OTHER CHARACTERISTICS OF GOLD, AND THE MODE OF DISTINGUISHING IT WHEN FOUND; TOGETHER WITH THE ASSAY, REDUCTION, AND REFINEMENT OF GOLD.

For the purpose of presenting all that is connected with that precious metal, that has built up a state within a few years, and of making the reader fully acquainted with all that is necessary for the gold-seeker to know, we will now describe, in some detail, the natural history, character of gold, and the mode of determining its presence and value.

Gold invariably exhibits something of the peculiar yellow color which it is known to possess in a pure state; but this color is modified by various metals with which it may be mixed. Thus it may be described as having various shades of gold-yellow; occasionally approaching silver-white, occasionally resembling brass-yellow of every degree of intensity, and even verging on steel-gray in some specimens from South America.

The lustre of gold is highly metallic and shining, and owing to the small amount of oxidation at its surface, it preserves its shining lustre even after long exposure in contact with other substances. Thus the shining particles are often seen in sand when the quantity is barely sufficient to repay the cost of working, notwithstanding the value of the metal. Even however, if the surface is dull, the true color and appearance are easily restored by rubbing, and when polished it takes a very vivid lustre, which is preserved for a long time in the atmosphere.

Although in the division which has been introduced into gold-yellow, brass-yellow, and grayish-yellow, native gold seems with some slight modifications to agree with the geological relations of its varieties, yet this mode of arrangement deserves little serious notice. The gold-yellow varieties comprise the specimens of the highest gold-yellow colors, though there are some among them which have rather a pale color; they include most of the crystals and of the imitative shapes, in fact the greater part of the species itself. The brass-yellow native gold is confined to some of the regular and imitative shapes of a pale color (which is generally called brass-yellow,) and, as it is said, of a less specific gravity than the preceding one; but this does not seem to have ever been ascertained by direct experiment. The grayish-yellow native gold occurs only in those small flat grains which are mixed with the native platina, and possess a yellow color a little inclining to gray; they are said to have the greatest specific gravity of them all. The real foundation of this distribution seems to be the opinion that the first are the purest, the second mixed with a little silver, and the third with platina. It is not known whether the latter admixture really takes place, but it is certain that several varieties of gold-yellow native gold contain an admixture of silver.[17]

In color and lustre, inexperienced persons might mistake various substances for gold; these are chiefly iron and copper pyrites, but from them it may be readily distinguished, being softer than steel and very malleable; whereas iron pyrites is harder than steel, and copper pyrites is not malleable; for although the latter mineral yields easily to the point of a knife, it crumbles when we attempt to cut or hammer it, whereas gold may be separated in thin slices, or beaten out into thin plates by the hammer. There can thus be no possible difficulty in distinguishing these various minerals in a native state, even with nothing but an ordinary steel knife. From any other minerals, as mica, whose presence has also misled some persons, gold is easily known by very simple experiments with a pair of scales, or even by careful washing with water, for gold being much heavier than any other substance found with it (except platina and one or two extremely rare metals,) will always fall first to the bottom, if shaken in water with mud, while mica will generally be the last material to fall. This is the case, however fine or few the particles of either mineral may be.

Gold therefore can be distinguished by its relative weight or specific gravity, and by its relative hardness, from other bodies which resemble it. It is described generally as soft, completely malleable and flexible, but more accurately as softer than iron, copper or silver, but harder than tin or lead. It is useful to know facts of this kind, as a simple experiment that can be made with instruments at hand, is often more valuable than a much more accurate examination requiring materials not immediately available. Thus if it is found that a specimen (perhaps a small scale or spangle) is readily scratched by silver, copper or iron, and scratches tin and lead, it may, if of the right color and sinking rapidly in water, be fairly assumed to be gold.

The weight of gold, as of all substances, it is convenient to estimate relatively, and in comparison with the weight of an equal volume of water. The relative weight, or specific gravity, as it is called, of gold, is remarkably high, the lightest varieties being twelve times heavier than water, and pure gold nineteen times. This is expressed by saying that the specific gravity of native gold is 12-19, and the number determined by comparing the weight of the mineral in water and air.

As the value of gold depends almost entirely on its specific gravity, and this test, therefore, is of the greatest practical importance, it will not be out of place if we here explain some very simple apparatus for the determination of this point.

If the specimen then is large enough to be suspended conveniently by a thread, weigh it first in air by a fine balance, expressing the result in grains, and taking care previously to remove dust or loosely adhering particles. Then suspend it by a horsehair from the scale-pan (it is convenient to have a hook attached to it for this purpose,) and thus suspended, immerse it and re-weigh it in water, taking care that it is covered on all sides by at least half an inch of water, and carefully brushing off with a feather any bubbles of air that adhere to the surface. The results may then be noted as follows:—

This result gives the weight of a bulk of water equal to that of the specimen, and by dividing the weight of the specimen in air by this number, the specific gravity is obtained.

If, however, the substance is in the form of fine sand, or very small lumps, it is better, after weighing it carefully, to take a small dry phial furnished with a stopper; counterpoise this phial accurately in the weight-scale by shot or strips of lead, then fill it completely with pure water, taking care that no bubbles of air are left in, and weigh the quantity of water it contains: afterwards empty the bottle and dry it inside.

Next fill the bottle about two-thirds full of the powder to be examined, weigh this and record the weight. Then fill the bottle once more with water, taking care, as before, that all bubbles are expelled and none of the powder washed out. Once more weigh it.

We have then to make the following calculation:

It may be useful to know the specific gravity of various substances at all resembling gold in weight or appearance, and we therefore append the following short table. The specific gravity of water is assumed to be unity:—

By the help of this table the value[18] of auriferous sand may also be in some degree estimated, since, as will be seen, the specific gravity of most of the sands is under 3, while that of the most impure gold is 12; so that if the specific gravity of the sands themselves, when experimented on, is much greater than that of ordinary sand, it is likely that the excess will be for the most part gold, in a district otherwise known to be auriferous: the greater the specific gravity, too, the greater probability there is, of this being the cause. It may also be worth while to mention here, that the specific gravity of those pepitas or lumps of gold which present a fine yellow color varies generally from 14-7/10 to 18-8/10; but when much paler they may range as low as 12-1/2, which is that of a mineral called electrum, which will be described presently, and which is a mixture of silver and gold.

When a piece of gold is broken (which is not done without difficulty—greater in proportion to its purity,) the fractured edges are very uneven and torn, exhibiting a peculiar fibrous appearance, known to mineralogists as "fine hackly." This fracture indicates that the mineral is torn asunder and not really broken, and is a proof of considerable toughness.

The form in which gold is found is various. It is sometimes crystalline, in eight or twelve-sided regular figures, passing into cubes, but the crystals are generally small and rare. In case of such crystals being found, it is well worth knowing that they possess a value as mineral specimens far beyond that of the gold which they contain.

More frequently the metal is found in lumps or grains, called by the Spaniards pepitas, varying in size from that of a pin's head to masses weighing, as has been already mentioned, nearly one hundred pounds troy. The term pepita is only applied to grains of some magnitude, and the most common limits of size are from that of a small pin's head to that of a nut or gooseberry.

When much smaller and still rounded, they are called gold dust, and when flattened, scales or spangles. In nature, and when seen in veins of quartz, gold often occurs foliated, or in leafy expansions of extreme thinness, or in branchy (dendritic) forms, probably made up of minute crystals. It is in the form of very minute grains that the metal is generally disseminated through rocks and auriferous ores of various metals, and these are reduced according to circumstances in methods that will be alluded to in a future chapter. In pepitas and small grains it is carried down by streams and deposited in their beds, the pepitas being usually most abundant where there is reason to suppose considerable disintegration of the surface, and where the action of denuding causes to a great extent is evident. The coast of Africa and the rivers of Europe are examples of the former case, while the Siberian deposits and those of California would appear to belong to the latter.

The following are examples of the constituent parts of various specimens of gold obtained from different gold districts, and will form a useful guide for comparison.

Table showing the Composition of Native Gold.[19]

The gold from California, according to the assay of Mr. Warwick of New York, yields 89·58 per cent, pure gold, and is therefore, about equal to that obtained from the washings of Miask (the richest district in Western Siberia, and that producing the largest pepitas,) and superior, as the assayer remarks, to the gold dust from Senegal.

There is a remarkable mixture of native gold with silver occasionally found in Siberia, and known under the name of electrum. Its color is pale brass-yellow, passing into silver-white. It occurs in small plates and imperfect cubes, and possesses many of the characters of gold, but it consists only of 64 per cent. of that metal, and 36 per cent. silver. It is at once known by its low specific gravity, which does not exceed 12.

Other mixtures of gold are (1) a rhodium-gold found in Mexico, and containing 34 to 43 per cent. of rhodium, having a specific gravity of 15½—16·8, and a clear, dirty yellow color; and (2) a palladium-gold (containing 9·85 per cent. palladium, and 4·17 per cent. silver) found in Brazil and elsewhere in South America, in small crystalline grains of pale yellow color. The auriferous ores of tellurium, including silver, have hitherto only been found in Transylvania. Their color is steel-gray, and they tarnish on exposure. The variety called graphic-gold, or graphic tellurium, consists of about 60 per cent. of tellurium, 30 per cent. gold, and 10 per cent. silver, and is worked chiefly as an ore of gold. Another variety, "yellow gold glance," yields somewhat less tellurium, gold and silver, and as much as 20 per cent. of lead.

Having now explained at some length the more manifest characteristics of gold, namely, its color, hardness, and specific gravity, it is necessary, before explaining the mode of separating it from associated minerals, that we should here give some account of the behavior of this metal under the blowpipe, and when exposed to simple chemical tests. The assay of gold and its accurate analysis, we postpone for the present.

The method of blowpipe analysis, although exceedingly useful, is not absolutely necessary in the case of gold, because of the many readier ways of determining the metal, but it seems advisable to state the appearances presented. All the varieties are readily fusible into a globule, which when the gold is pure, is unaltered by the continuance of the heat. In this respect it differs entirely from iron and copper pyrites, which, on being exposed to the flame, give off sulphur fumes and undergo considerable change. In the case of gold containing other metals, these, with the exception of silver; may generally be got rid of by continuing the heat in the exterior flame with the addition of a little nitre. Before the oxy-hydrogen blowpipe, the metal is volatilized in the form of a purple oxide.

Gold is not acted on by any of the acids alone. When exposed to the mixture of nitric with hydrochloric acid (in the proportion of one part nitric to four of hydrochloric) called aqua regia, it dissolves without residue, the solution giving a purple precipitate with protochloride of tin, and a brown precipitate with protosulphate of iron. Electrum, the mixture of silver with gold above alluded to, is only partially soluble in aqua regia, giving a residue of chloride of silver. The solution is acted on by protosulphate of iron, as already explained.

The following simple mode of detecting attempts at imposition in gold dust is worthy of being recorded in this place.

Place a little gold dust in a glass tube or earthenware saucer, and pour nitric acid upon it; then hold the glass or saucer over a flame, or upon a few embers, until red flames (nitric vapors) arise; if it be pure gold, the liquid will not become discolored; but if pyrites or brass-filings should have been mixed with it, the acid will become turbid, green, and black, discharging bubbles of gas. After the ebullition has ceased, the residue should be washed with water, and acid again poured upon it, when the same effect may be observed, but in a less degree; and if the experiment be repeated till all effervescence ceases, it will finally leave the gold dust pure.