'66 NEW YORK STATE MUSEUM 



Albite 



■ Albite occurs incrusting orthoclase and hornblende of a previous 

 generation often associated with calcite, secondary quartz and 

 byssolite. The crystals are small, rarely exceeding 5 mm in length, 

 clear and colorless with brilliantly reflecting faces. The strong 

 twinning tendency gives rise to numerous striations and vicinal 

 planes causing in every case a series of double images from the 

 faces in the prismatic zone. The twinning is complex, in most 

 •cases combining the albite and pericline laws. A very common 

 habit of albite twin is shown in figures i3a-i3b, the faces illustrated 

 being determined by approximate measurements, as no definite 

 angle values could be obtained by reason of the complex twinning 

 developed. Crystals of this habit occur notably associated with 

 calcite of the crystal combination shown in figure iob-ioc. They 

 are directly implanted on large hornblende masses of the first 

 generation. A twinning habit shown in figure i4a-i4.b is found on 

 the same specimen in small individuals buried in a mat of white 

 fibrous amphibole which represents an advanced stage of the 

 change from hornblende to byssolite. These rarely exceed 2 mm 

 in length along the brachi axis and are detached and completely 

 developed on all sides. They are milky white in color and pene- 

 trated with many inclusions of asbestic amphibole. In twinning 

 they resemble the crystals from Roc Tourne, Savoy. 1 Both types 

 of crystals show surfaces broken by numerous etch pits which 

 correspond in symmetry with the twinning habit. The forms 

 noted are b (010), c (001), m (no), M (no), / (130), z (130) x (101), 

 y (201), n (021), p (111), and (in). Two doubtful negative 

 quarter pyramids q (352) and r (463) in the zone [111.130] were 

 noted as narrow faces but could not be substantiated from the 

 material available. The planes of the hemiprisms (130) and (130) 

 are developed to an unusual habit for the species while those of the 

 commoner hemiprisms (no) and (no) are comparatively narrow 

 though well defined. The reading on the measured angles in every 

 instance varied materially from the theoretical value by a variable 

 difference amounting in some instances to 30' and due to the vicinal 

 twinning according to the Pericline law. 



Pyroxene 



Pyroxene occurs as the variety augite in dark green to black 

 crystals of prismatic habit associated with orthoclase and amphi- 



1 Rose, G. Poggendorff's Annalen. 1865. p. 125, 457. 



