Decomposition of Iron Pyrites. 193 



coating of clustered pyritohedra ; in clay. Decomposition : a 

 copper-red tarnish and reddish brown ochreous film on the sur- 

 face of the crystals, with interstices occupied by rusty deposits ; 

 freshly broken fragments soon assume a yellow tinge and iri- 

 descence. 



No. 70. Marcasitic pyrite. Weehawken, Hudson County, 

 New Jersey. Glittering yellowish sharp octahedra, with pol- 

 ished faces, and angles slightly bevelled by faces of the pyrito- 

 hedron, pale brass-yellow and splendent on fracture. One octa- 

 hedron was found, on fracture, enclosing symmetrically a crys- 

 tal of chalcopyrite. Associated with diabantite and cbalco- 

 pyrite, in calcite-veins through diabase. Decomposition : a 

 beautiful iridescence rapidly ensuing on exposure, often with 

 crossing lines of blue and red running parallel to the edges of 

 the triangular faces. These colored stripes seem to imply the 

 symmetrical disposition of the intermixed impurity which 

 causes the ready tarnish. The alteration continues and may be 

 seen in all stages of ochreous decomposition, to the hepatic tur- 

 gite described below. 



No. 71. Turgite after pyrite. Weehawken, New Jersey. 

 Dull to shining octahedra, often with sharp edges and retaining 

 the pyritohedral modifications as in No. 70. They consist of 

 black turgite, sometimes reddish black, generally with loose par- 

 ticles of brownish red turgite-ochre attached, or diffused as a 

 colored crust or border immediately around the octahedron, or 

 through the interstices of the calciferous veinstone or adjacent 

 trap. It is significant that the action has almost always gone 

 on irregularly within the grains, so that many show an inter- 

 mixture of particles of unaltered pyrite and of reddish turgite- 

 ochre. From this unequal progress of the alteration, the sur- 

 face does not generally retain its perfect lustre, as in hepatic 

 limonite-pseudomorphs elsewhere, but is more or less rough- 

 ened. On the smooth faces of some crystals, little deposits of 

 soft reddish white material are not uncommon, which, on 

 examination under the microscope, appear to be made up of al- 

 ternating laminae of red turgite-ochre and white gypsum ; a water 

 solution of the rusty crystals always gives the chemical reaction 

 for calcium-sulphate. 



No. 72. Pyrite. Sussex County, New Jersey. Octahedra 



