Xonotlite

Crystallography
Xonotlite is found at Franklin in long needlelike crystals with striated prismatic zones and tapering terminations showing no measurable faces. Through X-ray study by Mr. Harry Berman the dimensions of the unit cell were provisionally determined to be: a_o = 8.55 angstroms, b_o = 7.34 angstroms, c_o = 7.03 angstroms, b = 90° ±. From these data were computed the following linear crystallographic elements: a : b : c = 1.165 : 1 : 0.958, b = 90° ±.

The needles are elongated parallel to the b-axis, oscillation X-ray photographs around which show that it is an axis of symmetry. The monoclinic character was established by study of the planes (h0l) in the prismatic zone, which was found to possess no plane of symmetry. As far as could be detected the angle b does not differ sensibly from 90°, but more precise measurements may establish a slight obliquity.

Physical properties
Most of the clear, colorless crystals of xonotlite are minute needles, and no cleavage could be observed megascopically, but the microscope reveals a cleavage normal to a and hence in the elongated zone. The hardness is 6; the specific gravity is 2.71 as determined by floating some crystals in a solution of that density.

Optical properties
Xonotlite is biaxial and positive, with 2V small; Z = b ; the extinction angle was not measured as the positions of the crystallographic axes a and c are not known; a = 1.586, b = 1.586, g = 1.594.

Composition
The composition of the Franklin xonotlite is shown by the following analysis, for the use of which the author is indebted to Mr. L. H. Bauer, chemist of the New Jersey Zinc Company:

**Analysis of xonotlite from Franklin**
(By L. H. Bauer)
	1	2	3
SiO₂	48.60	0.8092	3 x 0.2697
CaO	44.64	0.7962
MnO	0.49	0.0069 0.8093*	3 x 0.2698
MgO	0.25	0.0062
H₂O	3.97	0.2203	1 x 0.2203
	97.95

[* Represents sum of 0.7692+0.0069+0.0.0062]

1. Percentage by weight.

2. Molecular equivalents.

3. Molecular ratio.

The analysis, made on 0.0856 gram of material, shows a slight deficiency, which is probably in large part water. The water was driven off only at red heat and is surely basic.

From the analysis and the X-ray data the formula for xonotlite may be given as Ca₃Si₃O₈(OH)₃, with two molecules in the unit cell. This formula fits the X-ray data and the specific gravity determination very well.

Occurrence
Xonotlite forms parallel bundles of slender needles associated with a spongy mass of clinohedrite in a vein cutting the typical hancockite garnet-caswellite replacement product of the ore. In the clinohedrite vein, together with the xonotlite, are barite, clear-green willemite, and a few somewhat altered crystals of rhodonite. The specimen supplying the material for study was found in 1933 in a drift in the north end of the 500-foot level of the mine at Franklin.


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