1873.] 



Amblygonite and Montebrasite. 



175 



numerous twin plates, even in the specimens that to all appearance are 

 the most homogeneous, has proved that the laminar masses of montebrasite 

 must be referred to the triclinic system. The optic axes are situated in 

 a plane which divides into two very unequal parts the acute angle of 

 74° 16' of the two cleavages. This direction is entirely different from 

 that found for montebrasite of Hebron and of Montebras, in which the 

 plane of the axes lies in the obtuse angle of 105° formed by the two prin- 

 cipal cleavages. 



The appearance of the bars traversing the central ring of each system 

 indicates very distinctly a twisted dispersion, as well as a small amount of 

 inclined dispersion, which is characteristic of a crystal belonging to the 

 triclinic system. 



In November 1871 the author received a specimen from the middle of 

 a mass of amblygonite from Montebras resembling the mineral from 

 Hebron. It has three principal cleavages,^?, m, t, which the author 

 recognized in the mass from Hebron, the angles between which are 

 p m = 105°, mt=13o° to 136°, p t=89° to 89° 15'. 



By means of artificial twins formed of two plates, each of which had 

 been worked perpendicular to the two cleavages p and m, and which were 

 united by their faces p, it appeared that the plane of the optic axes is 



situated in the obtuse angle p m, and traverses the edge ^ , but that it is 



m 



not quite normal to m, since it gives angles of about 82° with m and 23° 

 with p. The character of the coloured rings shows that in montebrasite 

 of Montebras, as in that from Hebron, there coexists with the horizontal 

 a well-marked inclined dispersion ; and these are peculiar to crystals of 

 the triclinic system. 



Analyses by M. Pisani. 



Hebron. Montebras. 



Fluorine 5-22 3*80 



Phosphoric acid 46*65 47'15 



Alumina 36-00 36-90 



Lithia 9-75 9-84 



Water 4-20 4-75 



101-82 102-44 



Specific gravity 3*03, Pisani. 3-01, Pisani. 



2-99, Damour. 2-977, Damour. 



Wavellite, in the form of thin coatings, forms a layer over almost all 

 the fissures that occur in the amblygonite of Montebras. In cavities in 

 these coatings are found long thin needles, which have enabled the author 

 to correct the older measurements of this min eral. 



