Notes on Crystals of Metallic Cadmium.
The measurements of the cadmium crystals were made by Dr. Williams who has very kindly furnished me with his results.
No reliable crystallographic description of the element cadmium seems thus far to have appeared—a fact due to the difficulty in obtaining suitable material. The crystals examined, although not capable of yielding entirely satisfactory results are nevertheless such as to make them of interest.
In 1852 G. Rose noted the fact that distilled cadmium collected at the neck of the retort in drops which solidified as complex polyhedral aggregates[13] similar to those formed by zinc[14]. In 1874 Kammerer encountered the same aggregates which he explained as complicated isometric combinations[15]. This opinion was cited in 1881 by Rammelsberg[16]. In 1884 Brögger and Flink stated that in their opinion zinc, magnesium and probably cadmium were from analogy with beryllium which they had studied, hexagonal and holohedral.[17]
This supposition has already been substantiated in the case of the two former elements[18] while the present material leads to the same result for the last named.
The cadmium crystals were produced in the same manner as were those of zinc and magnesium measured before, viz; by distillation in a vacuum. The appearance of the tubes thus obtained was closely like that in the other cases.
The polyhedral aggregates were abundant and reached considerable dimensions. The crystallizing power of the cadmium however, seems to be less, so that the only crystals suitable for measurement were extremely minute. The largest individuals were barrel-shaped, like those of zinc and resembled little piles of basal plates. Their side planes are not infrequently uneven and bent, probably as the result of the softness and great ductility of the metal.
Only the most minute crystals show pyramidal planes of comparative perfection. These are well suited for a microscopic examination, but their small size renders their measurement on a reflecting goniometer a matter of difficulty. After a careful search two crystals were secured which, although they had a diameter of only one third of a millimeter, from their microscopic appearances promised good results. Their planes however were found to give compound reflections and a somewhat disappointing variation in corresponding angles. On the best crystal three zones were measured as follows: (normal angles)
| Zone I | Zone II | Zone III | |||
|---|---|---|---|---|---|
| 0001 : 0111 = | 62° 35′ | 0001 : 1011 | 62° 4′ | 0001 : 1101 | 62° 29′ |
| 0001 : 0110 = | 89° 50½′ | ||||
| 0001 : 0111 = | 118° 57′ | 0001 : 1011 = | 118° 28′ | ||
The second crystal was much less satisfactory, since values for the angle between the base and pyramid (0001): (0111) were obtained which varied all the way from 61° 2′ to 63° 43′. These measurements must therefore be regarded as of little or no value. If we average the readings for this angle on the first crystal we obtain 62° 23′, from which
a : c = 1 : 1.6544
A comparison of the axial ratios of the four rhombohedral and four holohedral hexagonal elements gives the following:
| Rhombohedral. |  | Bismuth | a : ̲c = 1 : 1.3035 | (G. Rose, 1849). |
| Antimony | a : ̲c = 1 : 1.3235 | (Laspeyres, 1875). | ||
| Tellurium | a : ̲c = 1 : 1.3298 | (G. Rose, 1849). | ||
| Arsenic | a : ̲c = 1 : 1.4025 | (Zepharovich, 1875). | ||
| Holohedral. | | Zinc | a : ̲c = 1 : 1.356425 | (Williams and Burton, 1889). |
| Beryllium | a : ̲c = 1 : 1.5802 | (Brögger, 1884). | ||
| Magnesium | a : ̲c = 1 : 1.6202 | (Williams, 1890). | ||
| Cadmium | a : ̲c = 1 : 1.6554 | (Williams, 1891). | ||
Zinc appears from its axial ratio to belong rather to the rhombohedral group and this is the only one of the last four elements upon which the faintest indication of any divergence from a holohedral development of all of its forms has been observed. On crystals of this substance there is an occasional rhombohedral alternative of the faces of certain of the pyramids, although the crystals otherwise appear to be holohedral.[19]
The crystals of cadmium like those of magnesium show only the three forms OP (0001), P (1011)₂, and ∞P (1010). Brögger and Flink observed on beryllium the additional forms ∞P₂ (2110) and ½P (2021); while upon zinc a large number of forms in the zone of the unit pyramid occur.
Not infrequently the cadmium crystals show a tendency toward a hemimorphic development. This is plainly seen when a large number of them are examined together under the microscope. The little barrel-shaped crystals are mostly attached by their sides and yet one of their ends is often broader than the other. Sometimes they taper nearly to a point, quite like greenockite crystals.