Optical and Bloivpipe Examination of Chlorite. 339 



Art. XXXIV. — Optical and Blowpipe Exami7iation of the sup- 

 posed Chlorite of Chester County^ Pa,; by W. P. Blake. 



Read before the American Association for the Advancement of Science, at Albany, 



August, 1851. 



In September, 1850, Prof. B. Silliman, Jr., handed me a speci- 

 men of a beautiful green foliated mineral for optical examination ; 

 it was unexpectedly found to be biaxial; but as the locality of 

 the specimen was not known, uo further examination than the 

 measurement of the angles was made at that time. In May, of 

 this year, I received from Prof J. D. Dana specimens of the 

 hitherto supposed chlorite, of Chester Co., Pa., which I examined 

 by polarized light, and obtained results so similar to those obtained 

 ^vith the specimen first referred to, as to leave no doubt of its 

 being from the same locality.* 



The mineral occurs three miles south of West Chester, in ser- 

 pentine associated with magnesite, and is found in plates of irregu- 

 lar outline, sometimes three inches broad, and in triangular plates 

 and tabular masses, one of which is rep- 

 resented in the annexed figure. These 

 plates are equilateral triangles; and they 

 much resemble the triangular cleavage 

 specimens of the micas from Greenwood 

 furnace and Monroe, N. Y. The cleav- 

 age is perfect, parallel with the broad faces 

 pf these crystals, but is not so perfect as 

 in mica, and the laminae are more brittle. 

 A he laminae are flexible and elastic, but 

 less elastic than mica. Color, beautiful 



^erald green. Hardness of cleavage smface, 2 to 225, scale of 

 ^<^hs. Specific gravity 27 14, which is perhaps too low, as no 

 specimen could be obtained perfectly free from air. 



Optically it is biaxial, with a high angle, and the following are 

 the results obtained: 



Specimen a, examined in September, plate one decimetre long 

 and six centimetres broad, with an irregular outline. 

 . Specimen h, a triangular plate measuring one and one-fourth 

 «^ches along each side, examined in May. 



.Apparent angle between the optic axes in a, 84^-30' mean of 

 ^me measurements. 



Apparent angle in 6, 85^-59^ mean of five measurements. 



The plane of the axes is perpendicular to the cleavage surface 

 and at right angles with the base of the triangle, as indicated by 

 the arrow in the figure. I was also able to obtain evidences of 



'ana received his sDecimen from Thos 



