216 Pirsson — Note on the Occurrence of Astrophyllite. 



The astrophyllite was found in a rock of the usual Quincy 

 type consisting of riebeckite, aegirite, microp'erthite and quartz^ 

 with zircon as the most common accessory mineral. It is in 

 minute, elongated laths grouped into bunches and associated 

 with the riebeckite. It was also observed intergrown with, 

 the riebeckite in such a way that the axis of elongation parallel 

 to the cleavage was parallel with the vertical axis of the rie- 

 beckite ; hence in section it appears as if wedged in between 

 the hornblende cleavages. It was noticed that the riebeckite 

 in this case had on its margin with the feldspar what the 

 writer has drawn and figured as the interdented texture,* indica- 

 tive of crystallization from eutectic conditions. 



The astrophyllite was found to have the following proper- 

 ties : cleavage, excellent micaceous, and accepting this cleavage 

 as the pinacoid o (010) the direction of elongation of the crys- 

 tals is on the c axis and the directions of elasticity are a=c; 

 o = a and c = B ; strongly pleochroic, a, red-orange, c, lemon- 

 yellow ; absorption a > c ; refractive index >1"7 ; extinction 

 parallel to the cleavage cracks; birefringence high >0*04. 

 In convergent light a single biaxial optic axis was obtained on 

 the edge of the field ; the limited number of crystals and their 

 nearly parallel orientation did not permit of further investiga- 

 tion of the optic scheme. 



These are the properties of astrophyllite and definitely 

 determine it ; from the micas, which it resembles in thin sec- 

 tion, it is easily distinguished by the much higher relief, the 

 reversal of the absorption scheme referred to the cleavage and 

 the wide optic angle indicated. 



There is yet much to be learned concerning this interesting 

 mineral, whose formula is still uncertain, but which Broggerf 



believes to be R 4 K 4 Ti(Si0 4 ) 4 in which R = H, JSTa, K and E = Fe, 

 Mn, and a new occurrence w T ell investigated might be expected 

 to throw much light upon its composition. 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., Nov. 1909. 



* Contributions to the Geology of New Hampshire. No. Ill, On Bed Hill, 

 Moultonboro ; this Journal, vol. xxiii, p. 273, 1907. 

 fZeitschr. fur Kryst., xvi, p. 212, 1890. 



