Notes on Phlogopyte. 7 



found that in such altered specimens, and principally in those which change to 

 steatite, scarcely a lamina can be separated, which is thin enough for a micro- 

 scopical examination. Its flexibility is likewise lost, as it will break to pieces 

 by bending. 



On some specimens of phlogopyte, and principally on those of Pope's Mills, 

 St. Lawrence County, the basal plane (o) is marked with two systems of par- 

 allel lines of separation or cleavage, which cross at an angle of 60 degrees. 

 Another series of parallel lines sometimes is noticeable, crossing the first two 

 series at an angle of 60 degrees. A hexagon, crystallographically regular, will 

 be formed by three pairs of the three series. 



In order to show in a graphical manner these lines of separation, and also the 

 phenomenon of Asterism, which will be discussed in the following- lines, figure 

 1 is given: a, b, c, d, e,f, represents the crystallographic hexagon in its equili- 

 brium, a d, b e, cf, are its diagonals or axes, which all meet iu the central point 

 (0). They give the direction of the separation marks. 



The whole hexagon being rotated in its plane for 90 degrees, will be in the 

 position a , b , c, d', e , /'. Then its diagonals (a' d') (b' e') (c' f) will be the 

 perpendicular axes of the first one, and the diagonals of the first one will repre- 

 sent the direction of the perpendicular axes of the second. 



The various angles formed by the diagonals of the two hexagons, will be met 

 with on specimens on base (<>) as lines of separation. The directions (a d) 

 (be) {''/) or (a' d') (b 1 e') (c r f) give the angles at which the small crystals or 

 base cross each other. Figure 1 will be referred to again. 



V. Rensch's " Schlagfigures "' could be produced in some cases with unaltered 

 laminae of phlogopyte of Warwick and Edenville, as those specimens were the 

 least altered, having kept their original elasticity. It was found best to use a 

 thin lamina, and a fine round point of a needle. The figures or lines caused by 

 the quick piercing of the laminae presented a fine round aperture caused by the 

 point of the needle around which three short lines were visible, diverging 

 under angles of 120 degrees. These lines correspond with (bO) (f'O) and (tlO'j, 

 or {b'O) {f'O) ('i/O), and if continued they would answer to the diagonals of the 

 hexagon, or to the direction according to which laminae of phlogopyte can be 

 separated. 



The existence of an indistinct cleavage, according to those lines, in phlogopyte, 

 can, therefore, be regarded as established. 



Lustre. — Base (<>) has on fresh surfaces a submetallic bronze lustre. If 

 exposed for sometime to the air, it changes to a pearly one. The lateral faces 

 are usually of a fatty or pearly appearance. 



C'dor. — Base (o) in compact masses has usually a brownish-red, yellowish- 

 red or brownish color. 



Specimens which are wholly or partly altered have on base (o) a whitish, 

 greyish color, sometimes stained with a number of dark-brownish patches, 

 which spreading over the whole surface give to it a dentritic appearance. The 

 lateral faces are of a brownish or greyish color. 



Asterism. — Thin laminae of phlogopyte are transparent, and allow a yellowish 

 light to pass through them. 



Specimens from Edwards and Edenville, which are of a white silvery color, 

 do not give a very sharp picture of the object looked at, and are surpassed by 

 the brownish-red variety. The cause is in the alteration of the whitish phlo- 

 gopyte, which doubtless formerly has been of a brownish .color. 



Rose, in his Monatsberichten der Berliner Akadvinie, 1869, drew attention 

 to the fact that some mica has the optical property of decomposing a ray of light 

 into six or twelve beams, when looked at through a thin lamina. It appears to 

 the observer as if the light would throw out from its nucleus six or, in sonu 



