196 Mr. R. P. Greg on the Crystalline Form of Rhodonite. 



form of the jet its origin ; and to judge whether any and, if 

 any, what irregularities are in the vessel. The conditions also 

 for the penetration of air into the jet are laid down ; the con- 

 traction of the jet is fully discussed, and the differences in its 

 form from a square or a circidar aperture are shown ; so that I 

 believe I may assume that these investigations will not be with- 

 out success in determining the velocity of efflux. 



XXIV. On the Crystalline Form of Rhodonite. 

 By R. P. Greg, Esq., F.G.S.'^ 



CILICIFEROUS Oxide of Manganese, or Rhodonite, is de- 

 scribed by W. Phillips (see his ' Mineralogy,' 4th edition, 

 published by R. Allan) as having for its primary form a doubly 

 oblique prism, with cleavage apparent in two directions perpen- 

 dicular to each other, that parallel to P highly perfect; say 

 MT=121°0', TP = 112°30', MP =93° to 94°. 



Dufrenoy, in his ' Mineralogy,' states that this mineral has 

 four cleavages ; two of about 87*5, and two others perpendicular 

 to these, and that it has an oblique rhombic prism for its primary. 



Rose, Dana, and Brooke and Miller in their recent edition of 

 Phillips's ' Mineralogy,' agree in stating that rhodonite is isor- 

 morphous with augite, having three cleavages : viz. a, perfect ; 

 b, less perfect ; m, imperfect ; 



ab = 90 6 

 mm'= 93 54 

 ma =136 37 

 Ac = 90 



That rhodonite is isomorphous with augite has perhaps been 

 too hastily assumed to be the case, arising from the coincidence 

 of each mineral giving cleavages of about 87° 5', as well a§ from 

 a similarity in their chemical formula ; rhodonite having for its 

 formula 3MnO 3810^, where part of the MnO is replaced by 

 FeO,ZnO,CaO,MgO ; while augite is represented by 3R03Si03, 

 where RO consists essentially of MgO and FeO. 



Hitherto perfect crystals of rhodonite, having also brilliant 

 faces, have been quite a desideratum among mineralogists. I 

 have, however, very lately received specimens from the Paisberg 

 iron mine near Phillipstadt, in Sweden, on which are implanted 

 some very perfect and brilliant ciystals, from the examination of 

 which I have been enabled, I think, without doubt to make out 

 its true form and principal cleavages, and to give correct angles. 



The following form, as given in the figure, I have observed to 



* Communicated by the Author. 



