Dana and Wells — New mineral, Beryllonite. 25 



the same angle, are not identical. Of the two positions sug- 

 gested by these facts it has seemed best to follow the usage in 

 most similar cases and make the twinning plane the unit prism. 

 Adopting this, the second cleavage corresponds to the macro- 

 pinacoid («), the Imperfect- pinacoidal cleavage is brachydiago- 

 nal (b) and the cleavage prism has the symbol 130 (^-3). 



The material at hand for exact measurement is very scanty. 

 "With very few exceptions the planes have lost their original 

 luster, and give no reflections at all. A few angles, however, 

 could be measured, and with sufficient exactness to yield a 

 satisfactory axial ratio. For fundamental angles the following 

 were accepted : 



001 a 111 = 47° 5U', 001 ~ 021 = 47° 40£'. 



Each of these is the mean of two independent angles on 

 different crystals of equal degrees of accuracy, not involving a 

 probable error of more than ±1/ ; these are : 



47° 51' and 47° 52', also 47° 40' and 47° 41'. 



The axial ratio obtained is : 



_ i 



a:b:c=. 057243 : 1 : 0-54901 ; also the angles 

 100 a 110 = 29° 47' 17", 001 « 101 = 43° 48' 13", 001 <* 011 = 28° 46' 2". 



The measured angles, the symmetry in arrangement of the 

 planes and optical characters all conform to the orthorhombic 

 system. As confirming the accuracy of these elements we 

 have : 



Measured. Calculated. 



021 ,.021 = 84° 41' 84° 39' 



023 a 023 = 139° 46' 139° 48' 



100 ~ 130 = 59° 45' Cleavage. 59° 47' 



On two crystals the angle aa was measured between cleavage 

 faces and the result 120° 22' obtained in each case. This 

 would give as twinning plane, if coinciding with the compo- 

 sition plane, the angle on 100 of 29° 49', or if at right angles to 

 the composition face 60° ll'. The former is the more j>roba- 

 ble relation and is shown to be the true one by the fact that 

 the calculated angle for 100 a 110 is 29° 47', whiie for 100 ^ 130 

 it is 59° 47'. 



In habit the crystals vary from short prismatic to tabular, as 

 shown in figs. 1 to 6 ; the aspect changes considerably with 

 the change in relative size of the pyramids ; of these w (121, 2-2) 

 is usually most prominent. The crystals are remarkable for 

 the number of planes which they present. The jDrismatic and 

 brachydome zones are both highly developed, and it is not 

 uncommon to note the presence of eight or more distinct 

 planes in each zone on a single crystal. It is also interesting 



