i Merwin — Simultaneous Crystallization of Calcite, etc. 357 



crystals enclosed in calcite. There are enlargements upon the 

 slender filaments which could scarcely have been sustained in 

 a free space. Such a structure would not be likely to result 

 from replacement. Therefore, it seems extremely probable 

 that the wurtzite and calcite were deposited simultaneously. 



While at temperature above the ordinary, slight or decided 

 acidity is a dominating factor in determining that marcasite 

 and wurtzite rather than pyrite and sphalerite come out of 

 their solutions,* there are conditions, of which the specimens 

 just described are representative, under which the first named 

 minerals are produced from solutions which are practically 

 neutral. In case of the marcasite one of the conditions, as in- 

 dicated by geological relations, must have been ordinary 

 temperature. Recent laboratory studiesf have shown that a 

 little above ordinary temperatures much less acidity is neces- 

 sary in solutions from which these minerals are depositing to 

 permit the appearance of marcasite and wurtzite than is neces- 

 sary at higher temperatures. 



Crystallographic study of the pyrite and marcasite. — The 

 pyrite in the geode above described appears in crystals 1 to 3 mm 

 in diameter, having the form of both the rhombic dodeca- 

 hedron with faces striated strongly parallel to the long diagonal, 

 and rarely of the octahedron striated parallel to the edge. 

 None of the dodecahedral faces give reflections, but from 

 positions corresponding to the octahedron, cube, and the 

 pyritohedrons (230) and (210), reflections with accompanying 

 light figures are obtained. The character of the light figures 

 is shown in fig. 2, A. They are almost identical with those 

 from the diamond having similar striations. Fersmann and 

 Gold schmidt;): have shown that such striations on the diamond 

 accompany growth. 



The dodecahedron is not a common form on pyrite, yet a 

 few cases are known in which it is the dominant form.§ 



It has been observed both bright and dull,§ but I have found 

 no definite statement as to the character of its striation except 

 that given by Boeris.§ He says the faces are grooved and 

 striated parallel to the edge of the cube, and that they vary 

 from bright to dull. The crystals occur in nodules of calcite. 

 The striation on these faces parallel to the cube is in marked 

 contrast to those on the crystals here described. 



The pyrite began growing on the flat sides of the blades of 

 marcasite, c(001). All stages between this and single crystals 



* This Journal, xxxiii, 193, 1912. 



f Ibid., xxxiv, 358-359, 1912. 



X A. von Fersmann and V. Goldschmidt, Der Diamant, figs. 30, 35 and 

 others, 1911. 



§E. F. Ayres, this Journal, xxxvii, 236, 1880. A. Lacroix, Mineralogie, 

 2, 540. G. Boeris, Zs. Kryst. (abs.), xxxv, 504. 



