Ford and Bradley — On Hydros incite. 59 



Art. VII. — On Ilydrozincite ; by W. E. Ford and W. A. 



Bradley. 



Hydrozincite is a basic carbonate of zinc that bas long been 

 recognized as a distinct mineral species. It is a secondary 

 mineral tbat commonly occurs as an incrustation on other zinc 

 minerals. Its structure is usually massive to fibrous or often 

 earthly to compact. Frequently it occurs as botryoidal crusts 

 with a concentric formation. As far as is known, it has never 

 been observed in crystals until its recent discovery at Good 

 Springs, Lincoln County, Nevada. Since no determinations of 

 the crystal or optical characters of the mineral have ever been 

 made and since the varying results of the recorded chemical 

 analyses have left its composition in doubt, it was thought 

 worth while to undertake an investigation of this crystalline 

 material. 



The specimens from Good Springs are composed chiefly of 

 a massive, earthy material of a light brown color which is 

 apparently mostly smithsonite. Occasional small crystalline 

 masses of calamine and calcite are to be observed. The hydro- 

 zincite is found lining the cracks and irregular openings in the 

 massive material. It occurs as delicate incrustations of very 

 small crystals grouped in radiating masses. Commonly under- 

 neath the crystals is found a thin layer of the same mineral in 

 a mass of interlacing crystalline needles. 



When the crystals are examined under the microscope it is 

 seen that they are exceedingly thin with a tabular, lath-shaped 

 form. They are usually sharply pointed at their free ends, 

 but the terminations were too irregular to permit of the 

 measurement of any angles. They show a pearly luster with 

 frequently an iridescent play of color on their surfaces. With 

 crossed nicols they always showed an extinction parallel to 

 their elongation. This was found to be the direction of the 

 slower ray while the direction across the crystals was that of 

 the faster ray. With convergent light they proved to be 

 biaxial in character and to have apparently their tabular 

 development parallel to the optical axial plane. Obscure dark 

 interference curves were shown which moved rapidly from the 

 field when the sections were turned from the position of ex- 

 tinction. Their character was such as would be expected with 

 sections that were parallel to an axial plane. These curves 

 moved out of the field toward the vibration direction a. If the 

 orientation of the sections was as suggested, this direction is, 

 therefore, that of the acute bisectrix and the mineral is optically 

 negative. The two extreme indices of refraction were deter- 



