70 E. V. Shannon — Mullanite. 



microscope by means of a vertical illuminator, had chisel 

 shaped terminations which suggested monoclinic sym- 

 metry, but none of these terminated crystals was large 

 enough for measurement, It is not improbable that the 

 compound 5PbS.2Sb 2 S 3 is, like the similar compound, 

 5(Pb,Ag 2 )S.2Sb 2 S 3 , dimorphous, and both forms may be 

 present in the Gold Hunter ores. 



Physical Properties. — Specific gravity, 6.274 on coarse 

 fibers from Iron Mountain, 6.407 on fine fibers from the 

 Gold Hunter Mine. The last is probably the true value 

 for the mineral as the former may have contained some 

 included air. Color steel-gray, of free crystals some- 

 what darker due to tarnish ; hardness 3.5 ; luster metallic 

 to metallic-adamantine; opaque; streak, when rubbed 

 upon unglazed porcelain, gives a somewhat shining 

 brownish-black streak which when rubbed fine with a 

 blunt end of a test tube or some other hard object, 

 becomes first more metallic and then a distinct brown. 

 This constitutes the only ready method of distinguishing 

 this mineral from the associated epiboulangerite, the 

 streak of which is dull grayish-black. Habit, prismatic- 

 fibrous; cleavage, c(001) and 6(010) distinct, a(100) and 

 probably w(110), imperfect. The thick fibers from Iron 

 Mountain are brittle. The flexibility of the Gold Hunter 

 needles depends directly upon their thickness, the thinner 

 fibers being very flexible while the thickest ones are quite 

 brittle. 



Composition. — The composition of mullanite is shown 

 in the following table : 



I II III Ratios IV V 



Sb 25.73 25.69 25.71 .2140 or 1.00 or 4 25.725 24.67 



Pb 55.125 54.926 55.046 .2664 1.25 5 55.405 53.331 



S 18.665 18.970 18.817 .5869 2.74 11 18.869 38.112 



As .249 .249 641 



Cu 0.00 0.00 0.00 



Pe trace trace 1.47 



Total99.77 99.835 98.563 99.999 98.224 



I, II. Mullanite, Iron Mountain Mine, Superior, Mont. 



III. Mullanite, average of I and II. 



IV. Mullanite, values calculated for the formula 5PbS.- 

 2Sb 2 S 3 . 



V. Mullanite, Gold Hunter Mine, Mullan, Idaho. The 

 iron represents siderite (3.04%) present as impurity. 

 Pure needles of mullanite contain no iron. 



West Haven, Conn. 



