Aikinite from the Ipchul Deposit (Khakassia)

G. A. Ivankin

(Presented by Professor K. V. Radugin)

In the summer of 1958, at the Ipchul deposit, located in the basin of the Bely Iyus River, we discovered a lead-gray mineral whose studied properties allow it to be assigned to aikinite. The present article gives preliminary information about this interesting and comparatively rare mineral.

A characteristic feature of the Ipchul deposit is the abundant development of gilbertite. In places, gilbertite and quartz almost completely replace the granitoid rocks. For example, in the dumps of adit No. 1 there is a very large quantity of blocks of rock consisting chiefly of gilbertite and quartz. It was here, in one of the blocks of gilbertite–quartz rock containing disseminations and veinlets of pyrite and molybdenite, that we discovered aikinite.

Aikinite forms elongated crystals with longitudinal striation on the prismatic faces. The largest of them reach 1 cm in length and about 1.5 mm across. Where thin crystals occur in clusters with subparallel arrangement, coarsely fibrous aggregates of aikinite are formed. Very rarely, granular aggregates of this mineral are found; these may be observed at the boundary between small quartz–calcite lenses and the quartz–gilbertite rocks enclosing them.

Aikinite from the Ipchul deposit has a lead-gray color and a strong metallic luster; it does not tarnish in air, but under the action of hydrochloric acid becomes covered with a dark-gray tarnish film (turns black). The streak on biscuit porcelain is grayish black and lustrous. The mineral conducts electric current comparatively well. When an electric current is passed through it, the mineral heats up and turns black. Cleavage is imperfect. The mineral is brittle.

In strong nitric acid the mineral dissolves vigorously without heating; at first the “solution” acquires an intense yellow coloration, caused by the liberation of sulfur. With time, a bubbly aggregate of sulfur accumulates on the surface of the solution, a white powdery precipitate of lead sulfate falls to the bottom of the test tube, and the solution itself acquires a greenish tint. Testing the nitric-acid solution for copper content by adding caustic ammonia gives a positive reaction—indigo-blue coloration. The reaction with KJ indicates the presence of Bi in the mineral.

Spectral analysis of the mineral gave the following results: Bi, Pb, and Cu—very much; Ag—0.3–0.6%; Sb and Co—0.01–0.6%; Be—0.001%; As, Zn, Te, Au, and Sn—absent.

For the purpose of a more precise determination of the mineral, several oscillation X-ray photographs of the mineral were obtained about the third crystallographic axis, parallel to the elongation of the crystals. The values of the identity period in this direction range from 4.01 to 4.03 Å. According to the data of M. A. Peacock, cited by V. I. Mikheev in The X-ray Determinative Guide to Minerals, for aikinite \(c_0 = 4.00 \pm 0.02\) Å.

In order to estimate the quantitative content of the principal components in the mineral by the spectral method, samples of aikinite strongly diluted (from 1:50 to 1:200) with Iceland spar and quartz were prepared. The approximate average weight percentages of Bi, Pb, and Cu in the mineral, calculated from the results of seven spectral analyses, proved to be 35, 37, and 12%, respectively.

Thus, the approximate quantitative ratios are \(Bi:Pb:Cu \approx 3:3:1\), which in general corresponds to the formula \(BiPbCuS_3\).

The mineral was studied under the microscope. The heterogeneous structure of its crystals is noteworthy: their central part consists of pure aikinite, whereas in the peripheral part an aggregate of aikinite, bismuth, galena, and wittichenite is observed. The structure of this aggregate is graphic, of the myrmekitic type.

The time of deposition of aikinite is determined by the following relationships with other minerals observed at the deposit.

1) Aikinite occurs in close intergrowth with calcite and quartz, which form small lenses and veinlets in quartz–gilbertite rock.

2) These veinlets cut quartz veinlets with inclusions of pyrite, molybdenite, and chalcopyrite.

3) Well-faceted short-prismatic crystals of transparent quartz in the base sometimes contain inclusions of aikinite.

Thus, in its properties the mineral studied corresponds to aikinite and is, in general, syngenetic with quartz and pinkish calcite composing veinlets and lenses in quartz–gilbertite rock; that is, it was deposited later than pyrite, molybdenite, and chalcopyrite.