Crystallograpkic Notes by Penfield and Pirsson. 399 



a fair image of the signal. These yielded moderately good 

 measurements. Others however are only approximate from a 

 " schimmer " measurement. The forms observed on two crys- 

 tals which were measured are : 



&, 



010 



i-i 



k, 



021 



24 



s, 



120 



i-2 



d, 



101 



1-2 



m, 



110 



I 



e, 



111 



1 



The brachypinacoid is quite largely developed and also the 

 brachydome 2-2, and in these respects the crystals differ from 

 those of the European localities. The habit is that of a short 

 prism with pyramidal terminations as will be seen by 

 reference to the figure. This crystal whose figure is 

 given is about 3 ctm. in height, nearly 2 ctm. in 

 breadth, parallel to the brachy-axis and a little more 

 than 1 ctm. wide. 



The other crystal is characterized by the presence 

 of the unit prism, in other respects it is quite similar 

 to the one figured. The following table of calcu- 

 lated, and measured angles is appended to show the 

 identification of the forms. For calculating the 

 theoretical angles the axial ratios of vom Rath* have been 

 taken, the lengths of the vertical and brachy-axes as given by 

 him being halved, to bring them in accord with those of the 

 chrysolite group, to which the mineral belongs. 



These ratios are then, 



a: b: c :: 0-433689 : 1: 0-57569, 



and for the calculated and measured angles we have : 







Calc. • 



Meas. 





b*k, 



010 ,s 021 



40° 58£' 



41° 11' 





b^e, 



010 •> 111 



70° 53-^' 



70° 50' 





S ^.5, 



120^120 



81° 52' 



82° 07' 





5 a e, 



120^ 111 



38° 19' 



38° 19'. 



38° 22' 



b^s, 



010^120 



49° 04' 



48 c 30' 





e~d. 



111 ^ 101 



19 c 06£' 



19° 08', 



19° 08' 



m ^ m 



110^110 



46° 53^' 



47° or 





Fracture conchoidal to splintery, brittle; H. = 5. Sp. gr.= 

 3*108. From colorless to brownish white and light brown. 

 Luster vitreous on the fracture, inclining to greasy on the crystal 

 planes. Readily soluble in dilute acids, both .before and after 

 ignition. In a preliminary analysis in which the cleanest crys- 

 tal fragments were examined, it was found that they contained 

 some water, silica, ferrous oxide, manganous oxide, magnesia 

 and lime, but were entirely free from phosphoric acid. As 

 such fragments could not be obtained entirely free from calcite, 

 a quantity of the calcite, enclosing monticellite, was crushed 



* Pogg. Ann., 1871, Erg. Bd., v, 434. 



