108 Chester and Cairns — Crocidolite 



Art. XIII. — Crocidolite from Cumberland, R. Z, with a dis- 

 cussion of the composition of this and allied minerals, and a 

 method for the determination of ferrous oxide in insoluble sili- 

 cates ; by Albert H. Chester, and F. I. Cairns. 



1. Crocidolite. 



The occurrence of crocidolite in Ehode Island was announced 

 some years ago by one of the writers,* but nothing more than 

 to establish its identity was done at that time. Our recent 

 work has been undertaken not only to make a thorough analy- 

 sis of the mineral from this locality, but also to throw some 

 light, if possible, on the constitution of the species. It occurs 

 at Beacon Pole Hill, close to the well known mineral locality, 

 Diamond Hill, Cumberland, R.L, in seams in a granite ledge. 

 It is usually disseminated in fine particles through feldspar, 

 but often occurs in larger masses, up to the size of a butternut. 

 Unbroken surfaces sometimes present a botryoidal appearance, 

 and the nodules when broken show a radiated structure like 

 wavellite, but much less marked ; indeed in these specimens 

 the fibrous structure is not always very apparent to the eye, 

 though readily seen by the aid of a magnifier. The fibers are 

 short, very fine, and interlaced or matted together in an irregu- 

 lar manner, except where the radiated forms are seen. Its 

 color is usually a dark bluish gray, the radiated nodules, how- 

 ever, being darker, almost an indigo-blue, while the streak has 

 a slightly lighter shade of the same gray. It is associated with 

 dolomite, glassy quartz, and rarely with light purple fluorite. 

 The feldspar matrix is grayish white in color, except where 

 stained with iron, and coarsely granular, though occasionally 

 small cleavage planes may be seen. It is judged to be albite 

 from its easy fusibility, and from the intense yellow color it 

 gives to the blowpipe flame. 



One of the larger masses afforded a sample for analysis 

 wholly free from gangue, as proved by careful examination 

 with the magnifier, as well as from the absence of all gritty par- 

 ticles when pulverized in the agate mortar. It easily breaks 

 down under the pestle into a mass, which, though soft and feel- 

 ing perfectly smooth, is so tough as to require long and hard 

 rubbing to make it sufficiently fine for analysis. 



Tested qualitatively it was found to consist of silica, oxide 

 of iron, soda, water, and a little magnesia. Before the blow- 

 pipe it gives the usual reactions of crocidolite, including the. 

 alkaline water when heated in a closed tube. Its specific 

 gravity is 3'2. No hygroscopic water was given off when 



* Dana's Man. of Min. and Lith., third edition, p. 252, 1879. 



