CONTRIBUTIONS PROM THE MINERALOGIC LABORATORY 9 



Seventh type. Crystals of the seventh type are rhomhohedral in 

 aspect and present scalenohedrons' of the zone [0112.1011.1120] as 

 shown in figure 13. Of these the faces composing the middle band 

 belong to the form v (6175) while those which, with the rhombo- 

 hedron e (01 12), form the termination are built up faces, probably 

 belonging to D (8. 1.9. 10) though the presence of vicinal planes and 

 striations render the measurements obtained from these faces vague 

 and the form uncertain. The rhombohedron r (1011) and the 

 prism a (11 20) also occur. The presence of pyrite inclusions ar- 

 ranged on the phantom faces of the rhombohedron a (1011) suggest 

 that crystals of this type were produced by a "building up" process 

 from secondary calcareous solutions upon primative, rhombohedral 

 crystals. Small amounts of galena and sphalerite were found as- 

 sociated with this phase of the Rondout calcite. 



Eighth type. Crystals of this type are notably larger than those 

 heretofore described and are characterized by rather dull faces. 

 The combination shown in figure 14 which represents this phase 

 consists of the positive scalenohedrons A (3142) Y (5382) and 

 © (10. 3. 13. 2) terminated with the rhombohedron (0112). Of these 

 the scalenohedron X (3142) is represented by dull and roughened 

 faces and the scalenohedron y (5382) is frequently absent from 

 crystals of this type. Pyrite inclusions are present on, or just 

 below, the surface of e (01 12), as distinct bands bisecting the sym- 

 metry along the edges of r (1011) and often terminating in brushes 

 [fig. 1 5] ; in some cases noted these bands were connected by lateral 

 extensions along the basal edges of r (1011). Phantoms of opaque 

 white calcite which are shown on the cleavage and take the form 

 of the rhombohedron r (ion) suggest the secondary derivation of 

 this type from a simpler primitive crystal. 



Ninth type. The rhombohedron r (1011) which gives to crystals 

 of this type a distinct rhombohedral habit, is represented by large 

 dull faces. The rhombohedron e (01 12) which modifies the ter- 

 minal edges, and the prism a (1120) which modifies the basal edges 

 of r (ioit) are present as narrow bright faces. The scalenohedron 

 A (15. 4. 19. 3) is occasionally present as a modification represented 

 by small faces of medium brilliancy [fig. 16]. 



TWINNING 



Twin crystals are quite frequent among the calcite forms from 

 Rondout, the common type being that formed with the twinning 

 plane parallel to c (0001). Several instances were noted of crys- 

 tals of the second type twinned parallel to e (01 12). Figure 17 

 shows a crystal of the first type twinned parallel to c (0001) in 



