27 



8. Rliodoclirosite. 



On the mineral specimens from Narsarsuk small black 

 rhombohedral crystals resembling siderite crystals are not in- 

 frequently found; the black colour, however, indicates that 

 they are not fresh. They are only 2—3""" in size and gene- 

 rally irregularly grown together into druses. As to the combina- 

 tion of their faces they are of two kinds, partly such as show 

 only the planes of the fundamental rhombohedron (Fig. 9, Plate 1), 

 and partly such as also show the prism of the second order 

 (Fig. 1, Plate II). The former always have dull faces, sometimes 

 they are curved and saddle-like. The latter, on the contrary, 

 are generally rather bright so that they can be used for gonio- 

 meter measurement. By a couple of such measurements the 

 following values were obtained : 



r:r= lb°r and 75° 5'. 



This is a rhombohedral angle somewhat larger than the 

 one (74° 55') adopted for calcite. But no great significance can 

 be attached to the values found, as the rhombohedral faces are 

 not quite even. In detaching these crystals from the specimens 

 one finds them to consist externally of a very thin bright shell 

 that breaks very easily. This shell encloses a black earthy 

 mass. There is consequently no trace left of the original 

 carbonate. 



The other sort of crystals are so dull, that' their faces 

 give no reflexions at all. They, too, are often in their interior 

 loose and earthy. Sometimes, however, they are of a somewhat 

 firmer consistency, and traces of rhombohedral cleavages may 

 be observed on them. Individuals have also been found fresh 

 enough to allow of a tolerably accurate measurement of rhom- 

 bohedral cleavage fragments. The following values were obtained: 

 r:r= 74° 13', 73° 46'. 



The allied rhombohedral carbonates have the following 

 rhombohedral angles. 



