118 S. L. Penfielcl — Mmeralogical Notes. 



The calculated angles taken from Dana's Mineralogy are 

 c a w = 65° 11' and CaO = 57° 49'. The parting parallel to m 

 was moreover observed at a number of places on the specimen 

 where the latter had been broken. Subsequent attempts to 

 produce surfaces parallel to these same planes by breaking up 

 the detached fragments with a hammer, using the customary 

 devices for obtaining cleavage in definite directions, utterly 

 failed. In other words, the partings parallel to m and o do 

 not have the properties of true cleavage. They seem to be 

 partings along definite cracks within the crystal or they were 

 produced by the peculiar strain to which the fragments were 

 subjected in breaking away from _the attachment of quartz. 

 The partings parallel to (110) and III) were readily produced 

 on this material by pressure in the direction of the h axis, the 

 blocks of feldspar being held between plates of lead and 

 squeezed in a vice. As shown by Lehmann* a parting parallel 

 to (110) results from the contraction due to sudden cooling, 

 when heated fragments of albite are thrown into water. A 

 similar_result was obtained by the writer but no parting paral- 

 lel to (111) was observed. 



Tests made by pressing the Bakersville oligoclase, and by 

 contraction from sudden cooling, did not yield flat surfaces 

 that could be referred to definite crystal directions. 



In conclusion, therefore, it should be emphasized that the 

 cohesion relations of the plagioclase feldspars may show con- 

 siderable variation. The basal cleavage is always perfect. 

 That parallel to the brachy-pinacoid (010) is usually distinct, 

 while it is sometimes imperfect and hard to produce and espe- 

 cially when poly-synthetic twin lamellae are absent. Under 

 certain conditions of pressure or tension, partings may occur 

 parallel to the prism m{110) or the pyramid (111) which re- 

 semble perfect cleavages, while, as in the case of the Bakers 

 ville oligoclase the partings may run in other directions resem- 

 bling imperfect or interrupted cleavages. 



Laboratory of Mineralogy and Petrography. 



Sheffield Scientific School, February, 1894. 



*Zeitschr. Kryst., si, p. 612, 1886. 



