S. Weidman — Irvingite, a New Variety of Lithia-mica. 451 



Art. XLIII. — Irvingite* a New Variety of ' Lithia-mica / by 



S. Weidman. 



Among the minerals of the pegmatite veins which form 

 abundant large dike-like masses in the quartz-syenite and neph- 

 eline-syenite near Wausau, Wisconsin, was observed a nearly 

 colorless mica, the analysis of which shows it to be, apparently, 

 a new variety of lithia-mica. This mica was found in the 

 quartz-bearing pegmatite in the N.W. J of section 22, T. 29 

 N. Range 6 E., the same locality in which the new variety 

 of pyrochlore, marignacite,f was discovered. 



The most abundant minerals:}: occurring in the pegmatite of 

 this locality are quartz, alkali-feldspar, crocidolite, riebeckite, 

 acmite containing an appreciable quantity of A1 2 3 and K 2 0, a 

 pyroxene higher in A1 2 3 and Na 2 than jadeite, and lepidome- 

 lane. Some of the less abundant minerals are rutile, fluorite, 

 the pyrochlore, marignacite, and zircon containing much 

 alumina. 



Physical Characters. — The lithia-mica was observed in the 

 pegmatite in crystals varying from a fraction of an inch to over 

 an inch in diameter. Many of the larger crystals have, besides 

 the well-developed basal cleavage of mica, a very prominent 

 prismatic parting. This parting enables it to separate into 

 laths and needles of variable width. The color varies from 

 grayish white to yellowish and pinkish white. It is extremely 

 tough and elastic and possesses easy fusibility. The interference 

 figure in convergent light appears to indicate an axial angle 

 somewhat larger than that of the lithia-micas lepidolite and 

 zinnwaldite. 



Chemical Composition. — The analysis made by Prof. Victor 

 Lenher and the molecular ratio are shown in the accompanying 

 table 



There was no difficulty in obtaining sufficient pure material 

 for analysis. The analysis shows the mica to contain a consider- 

 able amount of lithia and fluorine and a relatively large amount 

 of silica and soda. 



* Published by permission of the Director of the Wisconsin Geological 

 and Natural History Survey. 



fThis Journal, vol. xxiii, p. 287, 1907. 



± For a general description of the minerals of the pegmatite veins see " The 

 Geology of North Central Wisconsin," Bulletin 16, Wis. Geol. and Nat. His- 

 tory Survey, pp. 275-331, 1907. 



