Merwin — Simultaneous Crystallization of Calcite, etc. 355 



Art. XXXI — The Simultaneous Crystallization of Calcite 

 and Certain Sulphides of Iron, Copper and Zinc. A 

 Crystallographic Study ; by H. E. Merwin. 



Through the kindness of Mr. C. E. Siebenthal I have been 

 able to study some specimens in which marcasite, pyrite and 

 chalcopyrite are enclosed within large crystals of calcite. 



In one occurrence from Roach's Crystal Cave, Joplin, Mis- 

 souri, within a group of large crystals of calcite smaller crystals 

 are outlined by included marcasite and chalcopyrite, and also 

 by slight color differences. The outer zones contain little or 

 no sulphides and are differently colored by included matter. 

 The middle zones contain sulphides and are nearly colorless ; 

 the inner zones contain a little sulphide and are tinted red. 



The crystal forms represented during the various stages of 

 growth are different. The unit rhombohedron is the dominant 

 form outlined by the earliest sulphides. In the pink zone 

 — \R is present, and in the colorless zone the scalenohedron 

 (2130). The latter is dominant to the exclusion of the others 

 in the outer part of the crystal. During one period the rates 

 of growth of the unit rhombohedron and (2130) are clearly 

 outlined by the marcasite. The rhombohedron grew from 2 

 to 20 times as fast as the scalenohedron (fig. 1, A). 



The marcasite developed in very thin branching blades and 

 spicules having forms represented in fig. 1, B. The long axes 

 of the blades are very nearly normal to the faces of the 

 rhombohedron, which they outline, but the blades are flattened 

 in not nearly parallel planes. On the scalenohedron faces also 

 the orientation is normal to the unit rhombohedron (fig. 1, A). 

 The maximum dimensions of the blades are 5 X 1 X 0'l mm . 



A small amount of chalcopyrite was identified only after 

 dissolving the calcite. It is spicular to pyramidal in habit and 

 coarser than the marcasite. 



The presence of the structure along xy shown in fig. 1, C is 

 conclusive proof of the simultaneous deposition of the calcite 

 and the marcasite. The successive broadening and narrowing 

 of the blades of marcasite indicates changes in the relative 

 rates of deposition of the two minerals. Also, while at certain 

 periods marcasite formed at nearly equal rates on both rhombo- 

 hedral and scalenohedral faces the calcite was precipitated 

 much more rapidly on rhombohedral faces, thus producing dif- 

 ferences in the relative amounts of the two minerals in unit 

 volumes. The form of the chalcopyrite is evidence that it also 

 was deposited simultaneously with the calcite. 



