Art. XLVI.—On Occurrences of Gold in the Coromandel District.
[Read before the Auckland Institute, 20th June, 1898.]
Plates XLIX. and L.
Native gold generally occurs massive or in thin plates or scales. It is sometimes, but rarely, found crystallized as octahedra or rhombic dodecahedra of the isometric system. Still more rarely do cube forms occur. The occurrence to be described possesses all three crystal forms, and was obtained from a “vugh” or cavity in a reef of clear crystalline quartz, adhering very loosely to the quartz crystals, and raised in fanciful shapes above them, resembling, indeed, nothing so much as a butterfly with outspread wings. So marked is the resemblance that even before it had been removed from the reef, and while it was still in situ, the finders had named it “The Golden Butterfly” (see Plate XLIX.).
The body of the “butterfly” is composed of irregularly shaped isometric crystals, and the “wings,” in the main, of five flat lamellar rhombic dodecahedra, so irregularly developed as to present, at first sight, the appearance of flattened monoclinic prisms. Of these five plates, three are on one side and two on the other, and the three largest present a marked similarity in crystalline form, being formed essentially of a single rhombic dodecahedron divided at one end to form two separate similar crystals (Plate L., figs. 2 and 3). On one side only of the medial line are cube crystals developed, this side being probably the lower when the crystals were in their natural position. All the plates, or “wings,” were attached very loosely to the main body by an irregular sub-crystalline projection, as shown in figs. 2 and 3. No gold was found near the specimen.
Taking one plate as typical of the rest, and considering one side of it first (fig. 2), it will be found that the basis of the form is a single rhombic dodecahedron divided at one end into two distinct smaller rhombic dodecahedra (i, i). These, as shown by the darker hatching, are surmounted by two more rhombic dodecahedral planes (1, 1), and these again by a succession of at least seven octahedral planes, denoted on the gold crystal by fine lines. These faces have plane angles of 60°, and interfacial angles of 110° (? 109° 28′ 16″), as nearly as I could determine them with the somewhat crude
goniometer at my disposal. There is no appearance of any twinning on an octahedral plane.
The reverse side (fig. 3) presents the same dodecahedral outline. Here, however, two dodecahedral planes form the basis, surmounted by octahedral planes (1, 1), and these again by a final dodecahedral plane (i). On this side, and with an edge parallel to “the edge of an octahedron, is a cube corner (H, fig. 3). This cube has the apex bevelled inwards, forming a depression, as shown in fig. 6. In none of the cubes or octahedra that I have examined have salient edges been discovered. The cube faces have plane and interfacial angles of about 90°.
An apparently similar form has been described by a previous observer* as a pseudomorph after botryogen or red iron vitriol, but, as I have been unable to detect botryogen in the reef, and as pyrites is also far from common, it is hardly likely that the gold, in this particular case at any rate, has crystallized as a pseudomorph, more especially as the angles (fig. 4) approximate much more closely to the 120° of the rhombic dodecahedron than to the 117° 24′ of botryogen, and therefore the most logical conclusion seems to me that it has crystallized in its natural system. However, as I have mentioned, unequal development of the faces will produce a strongly marked monoclinic appearance.
The specific gravity of the “gold” is about 17.6, and the fineness is therefore 0.8842, assuming the other constituent to be silver, which from the colour is probably the case.
Gold in Calcite.
Several instances of this rare occurrence have lately been brought under my notice. The following appears to be a typical mode: The calcite appears as an incrustation on the walls of a quartz reef, and the deposition of gold and of calcite appears to have been contemporaneous. The calcite is in the form of Iceland spar or double-refracting spar occurring in clear rhombohedra. The gold is scattered in thin scales and plates throughout the Iceland spar, with, apparently, no relation to the cleavage planes of the spar, for the scales cross these planes at all angles. The gold presents no crystalline faces, and from the colour appears to be much poorer in quality than that from the adjoining quartz reefs.
The very limited leisure time at my disposal has hitherto prevented me from devoting my attention to the mode of deposition of the gold in the calcite, and I must plead the same excuse for the general brevity of these notes.
[Footnote] * Trans. N.Z. Inst., vol. xiv., pp. 457, 458.