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:--
Weight of substance in the air in grains
Deduct weight of ditto in water
--------
Difference
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.
weight of substance in air
Specific gravity ---------------------------------
weight of equal bulk of water
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:
Weight of powder and water in grains =
Deduct weight of powder alone =
-------
Difference (weight of water left in bottle) =
Weight of bottle full of water in grains =
Weight of water left in bottle =
-------
Difference (weight of water displaced } =
by, and equal in bulk to, powder) }
-------
weight of powder in air
The specific gravity = --------------------------
weight of water displaced.
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:--
Osmium 21-1/5
Platinum 19-1/2 - 22 not hammered.
Iridium 18-7/10
Gold 15-3/4 - 19-1/4 ditto
Mercury 13-1/2
Palladium 11-7/10
Lead 11-1/4
Rhodium 10-3/5
Silver 10
Copper 7-3/4 - 8
Brass 8-1/2
Lead ore (galena) 7-1/2
Copper pyrites 5
Iron pyrites 4
Diamond 3-1/2
Sand 2-3/5 - 3
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]_
------------------------------------------+-------+-------+-------+------
Locality. | Gold. |Silver.|Copper.| Iron.
------------------------------------------+-------+-------+-------+------
Auriferous sand of Schabrowski, near | | | |
Katherinenburg, Siberia (G. Rose) | 98·76 | 0·16 | 0·35 | 0·06
Boruschka, near Nijny-Tagilsk, Siberia | | | |
(Rose) | 94·41 | 5·23 | 0·39 | 0·04
Brazil (Darcet) | 94·00 | 6·85 | |
Beresovsk, Siberia (Rose) | 93·78 | 5·94 | 0·08 |
Sand near Miask, Siberia (Rose) | 92·47 | 7·27 | 0·06 | 0·08
Bogota (Boussingault) | 92·00 | 8·00 | |
Washings near Miask, Siberia (Rose) | 89·35 | 10·65 | |
Gold of Senegal (Darcet) | 86·97 | 10·53 | |
Auriferous sand, Nijny-Tagilsk, Siberia | | | |
(Rose) | 83·85 | 16·15 | |
Trinidad gold, (Boussingault) | 82·40 | 17·60 | |
Transylvanian gold (Ditto) | 64·52 | 35·48 | |
Mine of Sinarowski in the Altai (Rose) | 60·08 | 38·38 | 0·33 |
------------------------------------------+-------+-------+-------+------
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.
FOOTNOTES:
[17] Mohs' Mineralogy, by Haidinger, vol. ii. p. 438.
[18] A _very rough_ estimate of the value of specimens of native gold
may be obtained by multiplying the specific gravity by 4; the result
gives the value in shillings nearly.
[19] Abridged from Dufresnoy's "Mineralogie."