Decomposition of Iron Pyrites. 1 79 



enclosure of pyrite during crystallization ; those described under 

 pyrite may have been formed by a subsequent alteration of that 

 mineral into marcasite. It is plainly indicated that marcasite 

 possesses a remarkable force of crystallization and retention of 

 physical properties, sufficient to impress its character even upon 

 a 90 per cent, dilution with pyrite (Specimen No. 1). 



3. Color and lustre of marcasite. The observations throw- 

 further light on a question concerning which a curious uncertain- 

 ty seems still to prevail, viz., the color and lustre of marcasite. 

 On this point the following are the statements by several authors. 



Pale or grayish bronze-yellow, sometimes almost greenish gray 

 (Nicol, Manual of Min., 1849). 



Light brass-yellow, sometimes inclining to green and gray 

 (Phillips' Mineralogy, Brooke and Miller, 1852). 



Yellowish white or of a livid greenish gray (Dufrenoy, Traite 

 de Min., 1866). 



Light brass-yellow, inclining to grayish and greenish, often 

 with variegated tarnish (Leonhard, Grundzuge d. Min., 1860). 



Pale bronze-yellow or nearly tin-white, with a tinge of yellow 

 or gray (Bristow, Glossary of Min., 1861). 



Brass-yellow, inclining to grayish (Tschermak, Min., 1863). 



Brass-yellow, but somewhat more gray (Quenstedt, Handb. d. 

 Min., 1863). 



Grayish or greenish brass-yellow (Senft, Felsg., 1868). 



A much lighter yellow, and more greenish than pyrite (Egles- 

 ton, Lectures on Min., 1871). 



Pale bronze-}' ellow, livid yellow inclining to green or gray 

 (De Selle, Cours de Min. et Geol., 1878). 



Pale bronze-yellow, sometimes inclined to green or gray. 

 (J. D. Dana, System of Min., 1883). 



The observations made on my collection, however, have con- 

 vinced me, that, on a fresh fracture, unaffected by alteration, the 

 true color of marcasite is invariably grayii^li white, nearly tin- 

 white. The yellowish and greenish tinges, commonly presented by 

 its weathered surface or even by the surface of fracture, especially 

 after short exposure, are mere results of incipient decomposition, 

 and should no more be assigned to its normal color than the fol- 

 lowing stages of iridescence or rusty incrustation. The weather- 



