46 Mr Haidinger on the Crystalline Forms 



(a little below calcareous spar ;) and sometimes, in fibrous va- 

 rieties, it is so inconsiderable as to soil the fingers, and write 

 upon paper. On the contrary, their specific gravity is higher, 

 and often approaches to 4.7. It is important to observe, 

 that the exterior strata of large crystals sometimes afford a 

 black streak, and show low degrees of hardness, while the in- 

 terior parts still offer the characters indicated in the preced- 

 ing description. It should seem, therefore, that the differ- 

 ence in several of these properties is owing to a change or 

 decomposition of the substance itself, which does not affect 

 the regular form. 



II. — Pyramidal Manganese Ore. 

 Fundamental form. Isosceles four-sided pyramid. P a 

 105» 25, 117° 54'. Fig. 11. 



a = V 2.76. 

 Simple forms, f P— 4 (a) = 139° 56', 58° 4fr ; P_l & 



114° 51', 99° IP; P (P). 

 Char, of comb, pyramidal. 

 Combinations. 1. § P— 4. P. Fig. 12. 

 2. \ P— 4. P— 1. P. 



Cleavage, P — co rather perfect ; P — 1 and P less distinct, 

 and interrupted. Fracture uneven. Surface, |P — 4, very 

 smooth and shining, P horizontally streaked and often dull. 



Lustre imperfect metallic. Colour brownish-black. Streak 

 dark-reddish, or chestnut-brown. Opaque. 



Hardness = 5.0. ..5.5. Sp. gr. = 4.722, of a crystallized 

 variety. 



Compound Varieties. — Twin-crystals: axis of revolution 

 perpendicular, face of composition parallel to a face of P — 1, 

 Fig. 13. The composition is often repeated parallel to all the 

 faces of the pyramid, Fig. 14. Generally small particles only 

 of the surrounding individuals are joined to the central one. 

 Massive : composition granular, firmly connected. 



' Observations. 



The preceding description is given by Professor Mohs.* 



* Treatise on Mineralogy, Transl. vol. ii. p. 416. 



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