Crocidolite

Physical characters
Veins filled with a bright-blue fibrous mineral, mixed with calcite and sphalerite, were found by the author in 1906 in blocks of waste ore on the dumps of the Trotter mine at Franklin. The mineral fuses readily to a black bead, giving a strong sodium flame, reactions which, with its blue color and fibrous form, seem to indicate crocidolite or a related sodium amphibole. The following optical characters, determined by Berman, confirm this conclusion: Biaxial and positive; 2V medium large; extinction nearly parallel to fiber length; a = 1.67 in the direction of deep-blue pleochroism and b = 1.68 in the direction of purplish-blue pleochroism.

The Stanton collection contained numerous specimens showing the association of the mineral, and more recently it has been found in abundance. It is a very pale blue variety, in excessively fine needles, more or less felted together or contained as inclusions in other minerals. It appears to have formed veinlike masses in which the chief mineral is calcite in perfectly colorless anhedrons, their exteriors rounded and coated with crocidolite.

Willemite is also found in similarly rounded crystals of considerable size, pale green or perfectly colorless, but some of it is blue from included needles of crocidolite. Sphalerite also is found in these veins in pale green to white cleavage masses. The occurrence of needlelike crystals of quartz in one specimen is described on page 36.

Composition
A sample of the crocidolite, of specific gravity 3.195 and hardness 2.5 to 2.75, was analyzed by Mr. Bauer. It showed the presence of willemite by the fluorescence, so the zinc oxide was deducted as willemite—8.91 percent—and the analysis was recomputed to 100 percent.

**Analysis of crocidolite**
	1	2	3
SiO₂	53.61	56.20	0.937	= 20 x 0.047
Fe₂O₃	18.45	20.25	0.127*	0.151 = 3 x 0.050
Al₂O₃	2.23	2.44	0.024*
FeO	2.16	2.36	0.033†	0.286 = 6 x 0.048
MgO	9.11	10.00	0.248†
CaO	0.24	0.26	0.005†
ZnO	6.50
Na₂O	4.86	5.33	0.086	= 2 x 0.043
Ignition	2.90	3.17	0.177	= 4 x 0.044
	100.06	100.01

[* Figures reflected in 0.151 value shown.]

[† Figures reflected in 0.286 value shown.]

1. Crocidolite, Franklin. L. H. Bauer (257),analyst.

2. Same recomputed to 100 percent after deducting 8.91 percent of willemite, equivalent to the ZnO found.

3. Molecular ratio computed from analysis.

From the molecular ratios, which are fairly close to, whole numbers, was derived the formula adopted (see page 70), which, although hardly a typical crocidolite formula, represents the composition of the mineral fairly well and shows that it is a metasilicate, apparently an isomorphous mixture of crocidolite and a small amount of some less sodic amphibole.


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	This page created: January 12, 2001 5:57 PM