72 Thirty-fib st Report on the State Museum. 



NOTES ON PHLOGOPYTE. 



By R. Fritz-Gaertner, Ph. D. 



The mineralogical characteristics of Phlogopyte have been repeatedlj 

 described by Breithaupt, Dana, Zirkel, and many other autoors. 



In bringing this short notice before the public, I would state that the follow- 

 ing lines will treat mainly of phlogopyte of the State of New York, with par- 

 ticular reference to phlogopyte of Warwick, Orange county, and Pope's Mills, 

 St. Lawrence county. 



Examples of phlogopyte were also examined and compared, which were 

 derived from Oxbow and Antwerp, Jefferson county ; O'Niel Mine, Orange 

 county ; Edenville, Orange county ; Edwards, Somcrville, Natural Dam, 

 and Gouverneur, St. Lawrence county. 



The specimens which the writer examined belong to the mineralogical collec- 

 tion of New York State, and are exhibited in the Museum at Albany. 



The main object of these notes is to give some contribution to our knowledge 

 of the microscopical structure of phlogopyte ; but as the microscopical habitus is 

 closely allied to some microscopical features of this mineral, it will be necessary 

 to treat, also, of its general characteristics. 



Crystallographic system. — Phlogopyte of the above localities has a hexago- 

 nal appearance, and usually occurs in six-sided prisms, with base {<>) perpendicu- 

 lar to the lateral faces. Ease (n) is uneven and bent, principally when it 

 assumes a large extension. 



On a specimen from Pope's Mills, (o) is covered with small crystals, which lie 

 entirely in the plane of the laminae. These crystals, though minute, can be 

 detected without the aid of a magnifier. Thev are of a light-bronze color, and 

 cross each other at an angle of sixty degrees. 



The lateral sides of the prism are uneven and corroded; only in a few speci- 

 mens could polished faces be perceived. Usually the lateral faces are tapering 

 towards the base, 



Thin laminae, parallel to the base, produce, in Norenberg's Polariscope, or in 

 the tourmaline, tongue lenmiscates, which are symmetrical on the loft and right , 

 and which differ from the symmetrical front and back. This indicates the 

 orthorhombic system of phlogopyte. 



Having no apparatus at hand to measure the angle of the optical-axial 

 divergence, I refer to Dana's Mineralogy (1870), where it is reported as varying 

 between 3 and. '20 degrees. The optical-axial angles of biotite vary from 1 to 

 5 degrees. It will be seen that, in optical characters, phlogopyte and biotite, 

 both magnesia micas, approach each other very closely, and in some special cases 

 this will cause difficulties in their discrimination according to t^jeir optical 

 appearance. 



Quenstedt, in his Mineralogy, states that thin laminae of biotite present, in 

 the Norenberg's Polariscope, an hexagonal appearance in their lenmiscates, 

 whilst thicker laminae of the same mica give the orthorhombic one. 



Thick laminae of phlogopyte, under the micro-polariscope, polarize in a bright, 

 yellowish color, in the darkened field, whilst thin laminae give a very taint 

 luminous effect. 



Cleavage. — Parallel to base (o) considerable. On specimens which are in a 

 state of methamorphic alteration, the cleavage is nearly obsolete. It will be 



