Prof. Johnston 07i the Dimorphism of Baryto-calciie, 3 



instance. The ordinary form of the carbonate of barytes is 

 the right rhombic prism. The carbonate of lime in arrago- 

 nite crystallizes in tlie same form ; there is nothing very un- 

 expected, therefore, in a combination of the two assuming 

 the same form. Neither will it be very unexpected when we 

 meet, as we probably hereafter shall, with a similar compound 

 of the two carbonates in the form of the rhomboid in which 

 calc spar ordinarily crystallizes. It will be an interesting fact 

 when observed, but we are in some measure prepared for it 

 by the knowledge of the form and composition of the plumho- 

 calcite. But what is remarkable in the second form of the 

 baryto-calcite is, that it is neither a rhomboid nor a right 

 rhombic prism, though, as I have already stated, it partakes of 

 the characters of both. 



It is well known that the carbonates of lime, iron, man- 

 ganese, and zinc are only plesiomov)^hous, the oblique angles 

 of their respective rhomboids being 105° 5'— 107° — 107° 20', 

 and 107° 40^ Now, the rhombic base of Mr. Brooke's baryto- 

 calcite is 106° 54'; it is considerably within the limits, there- 

 fore, of the dimensions which these carbonates assume. Is it 

 unlikely that this rhombic base, approaching so nearly to that 

 of rhomboidal carbonate of lime, should be directly derived 

 from it? 



There are two ways in which we may suppose this oblique 

 rhombic form to arise, or two principles with which we may 

 suppose it to be connected. If we suppose that each of the 

 carbonates is capable of crystallizing when alone^ in the oblique 

 rhombic prism, then we know three incompatible forms of 

 carbonate of lime and two of carbonate of barytes, and we can 

 understand perfectly why they should crystallize together in 

 this form as we find them in the compound mineral in ques- 

 tion. The carbonate of lime also would on this supposition 

 form the connecting link between two isodimorphous groups, 

 being itself /amorphous. Thus, 



Rhomboid. Right Rhombic Prism. Oblique Rhombic Prism. 



C+Ca in calc spar. C-fCa in arragonite. C+Ca in oblique 



C+Pb in plumbo-calcite. C+Pb in carbonate of lead. ^ 



C-f-Fe in brown spar. C+Fe in junckerite*. 



C+Ba in witherite. C+Ba in oblique 



baryto-calcite. 

 But if we suppose that in the oblique baryto-calcite the car- 

 bonate of lime retains its conmion rhomboidal form, as the 

 dimensions of the crystal would at first sight indicate, and 



*For an account of this interesting mineral, see Ann, deChimie et dePhys.y 

 June 1834, p. 11)8. 



B2 



