Cahnite

Ca₄B₂As₂O₁₂.4H₂O

Tetragonal-sphenoidal

a : c = 1 : 0.615

Forms
a(100), m(110), p(111), o(111)

Habit
Single, pseudotetrahedral crystals of cahnite are very rare, as the mineral is generally found in interpenetrating twins with parallel axes, the twinning plane being the first-order prism. The characteristic feature of the crystals is the cross formed by the regular intersection of the twinned edges of the sphenoids and the coincidence of the prism faces, which are generally bright. The crystals are white and transparent, with a glassy luster. The cleavage is very perfect parallel to the prism of the first order, which increases the general resemblance of the mineral to barite, with which it is not uncommonly associated. The hardness is 3, and the specific gravity is 3.156.

Optical character
Cahnite is uniaxial and positive; w = 1.662 and e = 1.663; birefringence therefore very weak. Because of its low birefringence and considerable dispersion, cahnite shows abnormal interference colors, making the mineral easily recognizable under the microscope (Berman, 249).

Composition
Cahnite is a hydrous boroarsenate of calcium and it fuses quietly at about 3, yielding the green flame of boron. It is easily and completely soluble in dilute hydrochloric acid. In the closed tube, it yields water and becomes opaque but does not fuse. Heated with potassium carbonate and carbon it yields an arsenic mirror.

**Analyses of cahnite**
	1	2	3	4	5
CaO	38.27	37.13	37.62	0.671 = 4 x 0.168	37.64
B₂O₃	10.14	11.64	11.86	0.169 = 1 x 0.169	11.74
As₂O₅	36.79	37.47	38.05	0.166 = 1 x 0.166	38.54
H₂O	11.75	11.78	12.42	0.689 = 4 x 0.172	12.08
PbO	1.15	Trace			100.00
MgO	0.24
ZnO		1.58
CO₂	Trace
	98.34	99.60	99.85

1. Slightly impure sample, weight 0.1 gram, containing a little hedyphane and calcite. L. H. Bauer (249), analyst.

2. Slightly impure sample, weight 0.26 gram. L. H. Bauer (249), analyst.

3. Very pure sample, weight 0.5 gram. L. H. Bauer (249), analyst.

4. Molecular ratio computed from 3.

5. Composition computed from the formula 4CaO.B₂O₃.As₂O₅.4H₂O.

The close agreement of the three analyses of different samples of the mineral is striking. The molecular ratio leads to the simple formula 4CaO.B₂O₃.As₂O₅.4H₂O or Ca₄B₂As₁₂.4H₂O. Cahnite is thus an entirely new type chemically.

Occurrence
Cahnite has been found only at Franklin. The first crystals seen were on specimens that probably came from the Parker shaft. They present two distinct types of paragenesis. In one type cahnite is implanted, together with barite and pyrochroite, on the walls of cavities in beautifully crystallized axinite. In the other type crystals of cahnite, calcite, and olive-green willemite are implanted on massive friedelite and barite or on garnet. One crystal in such a specimen, exceptional in being untwinned, is a quarter of an inch in diameter.

The specimens found by Mr. Stanton in 1926 came from pillar 229 north, 36 feet above the 700-foot level of the Franklin mine. They all contain axinite, which forms veins with small open cavities. Resting on the axinite are crystals of rhodonite, barite, hedyphane (some of it being in crystals), and willemite of either the usual prismatic habit or in thin plates with the base dominant. The cahnite is later than all those minerals and is implanted upon them. The only mineral later than cahnite is datolite, in a coating of fibrous nature, like the so-called botryolite at Arendal, Norway. The datolite coats all the other minerals in the veins, although most of the crystals of cahnite are free from it and clearly belong to the same period of deposition. Cahnite has also been found in a neighboring part of the mine associated only with rhodonite, on crystals of which it is implanted.

Figure 194
Crystal of cahnite twinned on m(110), showing the forms a(100), m(11), p(111), and o(111). Franklin.

Still another association came to light early in 1927. Specimens from the picking table showed small drusy cavities in franklinite, lined with dodecahedral crystals of garnet. The garnet is pink on the free surfaces but shows successively white and yellow layers toward the walls of the cavities. Tiny glass-clear crystals of cahnite are implanted on the garnet and show to the minutest detail the complete symmetry of the twin figured above. The only associated mineral is light-brown to yellow biotite in long, slender prismatic crystals projecting into the cavities.

In 1927 a small vein containing cahnite implanted on crystallized rhodonite was found near the locality of 1926. The crystals of cahnite are symmetrical twins of a size which, in comparison with any found before, may fairly be called gigantic, the largest being three-quarters of an inch square. Figure 195 shows a photograph and drawing of these extremely symmetrical interpenetrating twins.

Figure 195 Crystal of cahnite showing the same sort of twinning illustrated in figure 194. Franklin. A, Photograph; B, drawing in the same position.

All the localities so far mentioned were in the northern part of the mine, but in 1928 cahnite was discovered in the south end of the mine also, in pillar 1239, 20 feet below the 750-foot level and 15 feet from the hanging wall. It there forms tiny crystals associated with calcite.

History
A few minute implanted crystals of cahnite were first observed and sketched by Lazard Cahn about 1911. The specimens containing them were submitted to the author for study and became part of the Holden collection at Harvard University. The tiny white glassy crystals were characteristically twinned, and their form and angles suggested strongly a relation to the barium-bearing zeolite edingtonite, but material for analysis or for any but the simplest chemical tests was lacking. However, the author believed that it could be shown that the mineral contained calcium instead of barium, and he therefore regarded it as a calcium edingtonite. The name cahnite was proposed for it in recognition of Mr. Cahn's indefatigable efforts to preserve and to make known to science the rarer Franklin minerals. The name appeared in the American Mineralogist in 1921 in the title of a paper that was neither read nor printed.

Thus the matter stood until 1926, when George Stanton, of Franklin, rediscovered the mineral in moderate abundance in veins in massive ore in the northern part of the mine. Its identity remained concealed at first, as the newly found material was poorly crystallized. Mr. Bauer had established its peculiar chemical nature before the characteristic twin crystals were again found. A spectroscopic examination of one of the original crystals was then made by Mr. Nitchie at the Palmerton laboratory of the New Jersey Zinc Co., which established the complete chemical identity of the two finds.


	Website © by Herb Yeates 1997-2001.
	This page created: January 12, 2001 5:49 PM