Art. LXXIV.—Notes on a Pseudomorphous Form of Gold.
[Read before the Auckland, Institute, 5th September, 1881.]
The occurrence of gold in pseudomorphous crystals has not been hitherto observed, I believe, so that this specimen, which was obtained from a white clay, taken from a leader in a drive of the Evening Star Claim, at the Waiotahi Creek, Thames, is of great interest. It is reniform-shaped, about ½ inch in length, and covered with minute crystals and filaments studded sparsely with minute irregular-shaped grains of a yellowish-brown mineral, insoluble in acids, which are probably zircon. It is, apparently, the same mineral alluded to by Captain Hutton, in his second report, page 29, on the
Thames Goldfields. The bed-rock in this locality is a soft, grey-coloured felspathic tufa, with pyritpus leaders, weathering white and decomposing into a white clay. On page 40 of the report alluded to, an analysis of this rock is given in No. 4
The form assumed by the gold is apparently that of botryogen, a bisulphate of iron, which has been formed by the decomposition of pyrites. The measurement under the microscope of the angle of the oblique prisms is nearly 120°, the typical form of botryogen is 117° 34′; the latter angle is used in constructing figs. 1 and 2 in the diagram. Two crystalline forms are apparent, the one oblique prisms with the acute angles modified by the positive orthodome faces of the form 1 ∞ 1 (see fig. 1), the other oblique prisms with edges replaced by faces of the form 11 ∞ and ∞ 1⅔. In fig. 3 is shown a portion of the specimen, enlarged. This occurrence of gold in the form of casts of botryogen indicates that the deposition of gold in this district has extended over a great period of time, involving these changes, or of its redeposition in this form. In either case it is important evidence towards the more complete knowledge of the occurrence of gold in this district.
The following particulars are attached for convenience of reference of the crystallographic symbols:—
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Tennant and Mitchell.||Naumann.||Dana|
|1 ∞ ∞||∞P∞||ii||The ortho-pinacoid faces|
|∞1∞||OP||O||" clino " " of the oblique prism.|
|∞∞ 1||OP||O " basal " "|
|1 ∞ 1||P∞||i||The positive orthodomes of right prism on oblique rhombic base.|
|11∞||∞P||i||The faces of the oblique rhombic prism, 1st order.|
|∞1⅔||⅔P∞||⅔||i||The chinodomes of oblique prism on rhombic base, 2nd order.|
I am indebted to Dr. Purchas, the finder of the specimen, for the opportunity of making the above description.