2348. MINERALS FROM TRUMBULL, CONNECTICUT— SHANNON. 479 



MARGARODITE. 



A foliated and radiated pearly mica from this locality has been 

 widely distributed in old collections, labeled margarodite. This mica 

 is very abundant in the limekiln vein, where it occurs as an alteration 

 product of the topaz as aggregates of scales often grouped in 

 spherical or fan-shaped bunches. In color the margarodite ranges 

 from pale yellow to smoky brownish gray. The yellowish variety 

 occurs replacing the large crystals of topaz. The grayish type 

 is coarser and occurs in open spaces lined by topaz crystals and as 

 narrow veins in which the crystals grow from either wall and inter- 

 lock loosely in the center of the vein, the spaces of which are filled 

 with colorless to purple fluorite. Some of the coarsest of the mica 

 is deep blue, apparently from thin layers of deep blue fluorite 

 inserted between the laminae. The margarodite is in plates which 

 uniformily show the structure found in commercial mica deposits and 

 known as feathering evidently due to twinning. In appearance this 

 material is more pearly than ordinary muscovite and laminae are 

 more brittle. 



Upon analysis of selected cleaned plates the results, given in 

 column I below, were obtained. In columns II and III are given 

 the results obtained upon analysis of margarodite from this locality 

 by J. Lawrence Smith.^ 



Analyses of margarodite from Trumhull. 



a Not determined. 



Fluorine was present in sufficient quantity to give a distinct reac- 

 tion, but was not estimated. Chemically this differs from normal 

 muscovite only in the amount of water which replaces the potash, 

 and this difference is not great. That the water is basic is estab- 

 lished by the high temperature required to liberate it. When the 

 mineral was heated over a blast flame until the hard glass tube 



1 Smith, J. Lawrence, Amer. Joum. Sci., ser. 2, vol. 16. pp. 46, 47. 



