SOME ADDITIONS TO THE ALUNITE-JAROSITE GROUP OF MINERALS. By W. F. HILLEBRAND and S. L. PENFIELD. Two new varieties of jarosite will be described in the present paper. One is from Nevada, and was collected by Mr. H. W. Turner, of the United States Geological Survey, and sent to the Survey laboratory at Washington for identification; the other is from New Mexico, and was sent by Mr. J. H. Porter, of Denver, Colo., to the mineralogical laboratory of the Sheffield Scientific School. Except for slight differences in color the two minerals look exactly alike, each T r C FIG. 1.-Natrojarosite. consisting of minute, isolated, tabular crystals, which, as may be seen with the microscope, are composed of combinations of a rhombohedron with largely developed basal planes. By chance it happened that the present writers discovered that they were both engaged in the investigation of compounds belonging evidently to the same group, and it was decided to bring the results together into one paper. NATROJAROSITE. The material collected by Mr. Turner was obtained on the east side of Soda Springs Valley, Nevada, on the road from Sodaville to the Vulcan copper mine. It consists of a glistening powder, made up of perfect crystals having the habit shown in fig. 1, although generally only one rhombohedron, r, is present instead of two, as shown in the figure. The largest crystals observed were 0.15 mm. wide and 0.025 mm. thick, and the general average would not be over half that size. In spite of their minuteness, however, it was possible to measure the angles of the crystals with the reflection goniometer, the chief difficulty arising not so much from their small size as from the vicinal character of the basal planes. After repeated trials a crystal was 32 found having a fairly good basal plane, and from this crystal the following angles were obtained: The crystals belonged to the rhombohedral division of the hexagonal system, and the angle c▲ r, 51° 53′, which is probably very nearly correct, has been assumed as fundamental, and from it the following axial ratio has been calculated: c=1.104. That the axial ratio as given is very near the truth is shown by the fact that the measurements of e▲ 8 and rr do not vary many minutes from the calculated values; while on a number of other crystals, measurement of the angle er, though varying considerably, was found to be not far from 52. The angles of cand ▲ r' of the ordinary potassium jarosite are 55° 16′ and 90° 45', respectively. Under the microscope the crystals exhibit normal optical properties. With a high-power lens and convergent light the thicker crystals show the dark cross and the beginnings of the first ring of the interference figure. The birefringence is negative. The color of single crystals, when seen under the microscope in transmitted light, is golden yellow. Many of the crystals show numerous brown inclusions. The color shown by a mass of the crystals is yellowish brown, and the material glistens, owing to reflections from the basal planes of the minute. crystals. The material used for the chemical analysis was the purest that could be obtained, although crystals containing the brownish inclusions just mentioned could not be avoided, and there were occasional brown ferruginous particles mixed with the crystals. The specific gravity of the material was found to be 3.18 at 30.5 C. The results of the analysis by Hillebrand are as follows: a Of the soda 0.22 per cent is not extracted by hot water after full ignition of the mineral, and hence may belong to a feldspar or some other foreign mineral. Only 5.81 per cent is assumed to belong to the jarosite and used in deriving the molecular value. Bull. 262-05-3 The ratio of Fe„O, : Na„O : SO, : H2O is evidently 3:1:4:6 as in ordinary jarosite, where the alkali is potash instead of soda. The slight excess of Fe,О, and H.O, as indicated by the ratio, is evidently due to some ferric hydroxide; probably the dark ferruginous impurities seen under the microscope are in part responsible for this, and there are also traces of some arsenate and silicate present. By attributing the excess of Fe,O, and H,O to impurities, it is found that 94 per cent of the material analyzed may be regarded as pure natrojarosite, as indicated below: That 6 per cent of impurities should be present in a crystalline powder such as was analyzed is not surprising when it is taken into consideration that it would require something like 2,500,000 crystals to make one gram of material, the estimation being based on the assumption that the crystals are 0.10 mm. in axial diameter and 0.02 mm. thick, which is certainly above their average size. Among the specimens from Cooks Peak, New Mexico, sent to the Sheffield laboratory by Mr. Porter, were some masses of a rather firmly cemented aggregate of minute crystals of a mineral of the jarosite group. The specimens are of a brownish-yellow color, and have in places the glistening appearance of a mica-schist. They also look as though they had been subjected to pressure and had been somewhat sheared. The material is rather easily crushed, and the powder, when examined with the microscope, exhibits the proprieties of the natrojarosite just described. The crystals are associated with a little. limonite and quartz, and pure material for analysis could not be obtained. Only a partial analysis, therefore, was undertaken with the following results: 3 FeO.... KO. Pho. 55.60 4.49 .77 96 SO, and HO were present but not determined. The results are sufficient to indicate that the material is essentially natrojarosite. PLUMBOJAROSITE. It occurs as a This material is from Cooks Peak, New Mexico. glistening, crystalline powder and as loosely cohering masses which may easily be crushed by pressure between the fingers. The crystals are very symmetrical, and are exactly like those of natrojarosite (fig. 1), although generally only one rhombohedron, r, is present. On the average the crystals are a trifle smaller and noticeably thinner than those of natrojarosite. A number of crystals were measured on the reflecting goniometer, the chief difficulty arising rather from the vicinal character of the faces than from their small size. One unusually large crystal, 0.28 mm. broad and 0.015 mm. thick, was finally found, having the development shown in fig. 2, which is unusual, for generally r (1011) and not s (0221) is the prevailing rhombohedron. Fortunately the crystal was so taken up on a minute point of wax that the measurement of ss in three rhombohedral zones was possible. The results of five measurements of 8 sover the upper and lower pole edges varied between 109° 5' and 109° 30', the average being 109° 16'; while six measurements over the middle edges varied between 70° 10′ and 71° 00', the average being 70° 36'. The average of the two supplementary values gives 88, 2201^0221=109° 20', which has been assumed as fundamental, and from it the following axial ratio has been calculated: c=1.216. с B S FIG. 2.-Plumbojarosite. On the crystal from which the foregoing measurements were obtained the basal plane was vicinal and hence no reliable measurements of CAS could be had from it. On a number of other crystals, however, the angle of car was measured with varying results, the variation resulting from the uncertainty of the reflections from the basal planes. Four measurements of car, which were recorded in the notebook as derived from the best reflections, varied between 54° 15′ and 54° 44′, the average being 54° 30', while car, 00011011, by calculation from the fundamental measurement, is 54° 32'. Hence it may be assumed that the axial ratio as established is reasonably exact. The calculated value of r▲r, 1011 101, is 89° 42'. In polarized light the crystals exhibit normal optical properties and negative birefringence. Being on the average thinner than crystals of natrojarosite, it is seldom that, with the highest powers and convergent light, even the beginning of the first ring of the uniaxial interference figure is visible. Individual crystals show under the microscope in transmitted light a golden-yellow color. A mass of crystals has the appearance of a glistening dark-brown powder, the color being decidedly darker than that of natrojarosite. The analysis of the mineral was made on the very best material, having a specific gravity of 3.665 at 30° C. The results are surprising, and were wholly unlooked for, since it is found that this jarosite contains lead in the place of alkalies. The results by Hillebrand are as follows: a The presence of alumina was not definitely proved. The figures here given are the differences between the several weights of the ammonia precipitates and those of the ferric iron in them, as determined by permanganate after reduction by hydrogen sulphide. Probably somewhat high. 3 3 The ratio of Fe,O, PbO: SO, H2O is very close to 3:1:4:6, indicating that the mineral is a variety of jarosite, and the slight excess of FeO3, H2O, and PbO + alkalies may be accounted for by assuming that slight impurities are present, partly ferric hydroxide, in part some lead salt, and perhaps a soluble silicate, as shown by the complete solubility of the silica in acids. Assuming that the ratio is exactly 3:1:4:6, it is found that 4.36 per cent of impurities are present, and the remaining 95.64 per cent may then be regarded as plumbojarosite, as follows: Fe,03 SO3 Theory for PbFes 27.06 or 28.29 9. 13 or 9.55 95.64 or 100.00 Since it took probably 2,500,000 crystals of natrojarosite to make 1 gram of material, it certainly must have taken fully 4,000,000 to make a gram of plumbojarosite, for the crystals of the latter mineral, though somewhat heavier, are decidedly thinner than those of the former; hence the presence of 4.5 per cent of impurities in such a crystalline product is not to be wondered at. |
