Weidman and Zenker — Marignacite. 287 



Art. XXVII. — Marignacite* a New Yariety of Pyrochlore 

 from Wausau, Wisconsin ; by S. Weidman and Victor 

 Lenher. 



In the north-central part of Wisconsin, within the general 

 pre-Cambrian area, there are widespread occurrences of igneous 

 rocks intrusive in the Huronian sedimentaries.f Chief 

 among these intrusives is a complex magma, consisting of vari- 

 ous phases of granite, quartz-syenite and nepheline-syenite. 

 Pegmatitic modifications are a characteristic feature of the 

 granite-syenite magma, both nepheline and quartz-bearing peg- 

 matites being developed in considerable quantity. In several 

 phases of the quartz-bearing pegmatite, a small octahedral 

 mineral was observed that proved upon investigation to be a 

 variety of the rare group of pyrochlore minerals, the principal 

 constituents of which are the metals of the rare earths. 



The minerals associated with the pyrochlore in the pegma- 

 tite consist mainly of quartz, alkali-feldspar and acmite. Other 

 minerals in the pegmatite veins of the locality are lithia-mica, 

 lepidomelane, varieties of acmite-pyroxene high in alumina 

 and potassium, rutile, fluorite, and several zirconium-bearing 

 minerals. The pegmatite and associated-granite and syenite 

 are closely related in composition, and are characterized by a 

 relatively high content of alumina and soda. In most respects 

 the granite and nepheline-syenite are similar to the well- 

 known occurrences of nepheline-bearing and associated rocks 

 of Arkansas, Ontario, Canada, Essex County, Mass. and the 

 Christiana region of southern Norway. 



The pyrochlore was found in the pegmatite occurring along 

 the road in the S.W. 1/4 of Sec. 22, T. 29, K. 6 E., about nine 

 miles northwest of Wausau. It occurs only in small crystals 

 of nearly perfect octahedral form, varying in form up to about 

 1/8 inch (3 mm ) in diameter. The mineral was found in small 

 quantity, and although after its identification further search 

 was made for it, only a small additional amount was secured. 

 It is likely, however, that the pyrochlore will be found else- 

 where in the pegmatite veins of the immediate locality 

 although very probably only in small quantity. 



The only form of the mineral observed was the simple octa- 

 hedron (fig. 1). Some of the crystals occur in aggregates 

 (figs. 2 and 3) and some show a slighly distorted form, but no 

 combinations with other crystal forms were observed. Under 



* Published by permission of the Director of the Wisconsin Geological 

 and Natural History Survey. 



f Geology of North Central Wisconsin, Bull. No. XVI, Wisconsin Geo- 

 logical and Natural History Survey. 



