Cuspidine

Physical character
Cuspidine has been found at Franklin only as glassy-white fragments of crystals associated with nasonite, and the other peculiar silicates of the Parker shaft. Its specific gravity is between 2.965 and 2.989.

Composition
Cuspidine appears to be a calcium orthosilicate that contains fluorine in the place of one-eighth, molecularly, of the oxygen. Opinions differ regarding the place of the fluorine in the compound; hence several formulas have been proposed for the mineral, though all are derived from the empirical formula given above.

**Analysis of cuspidine**
	1	2	3
SiO₂	32.36	31.40	0. 539 = 2 x 0.270
F	9.05	9.94	0.476 = 2 x 0.238
CaO	61.37	58.66	1.094 + 0.018* = 4 x 0.273
MnO	0.71		0.018
Na₂O	0.48
K₂O	0.27
	104.24	100.00
O = F₂	3.81
	100.43

[* value 0.018 here taken from the (MnO) line in the table.]

1. Crystal fragments, Franklin. C. H. Warren (195), analyst.

2. Composition computed from the formula adopted.

3. Molecular ratio of no. 1.

Inspection of the analysis shows that the material, though rather low in fluorine, had practically the composition of cuspidine, and it is believed to have been that mineral.

Occurrence
The author is indebted to Dr. C. H. Warren for the analysis of Franklin cuspidine (Palache, 195) and for the facts here given regarding its discovery. The material was isolated by hand picking and by heavy solution and was analyzed by Warren when Penfield and he were studying the new silicates from the Parker shaft. At that time lie found no satisfactory interpretation of the analysis, the identity of the mineral remained hidden, and his notes were laid aside. Of the 1.5 grams of material separated, none was saved from the analysis, and no physical characters were determined except the specific gravity.

On learning that the author was studying Franklin minerals, Dr. Warren kindly sent him the record of the analysis and has accepted the interpretation of it that identifies the mineral as cuspidine. The discovery of cuspidine among the minerals from the Parker shaft is interesting, especially as confirming the pneumatolytic origin of that peculiar assemblage of species. The only other known occurrences of cuspidine—in metamorphosed limestone blocks elected from Monte Somma and in the peperino of the Alban Hills—are typical examples of pneumatolytic genesis.


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