134 Hess and Wells — Occurrence of Struverite. 



are nearly square, and the thickness ranges from about one- 

 eighth to two-thirds of the length, Even in most of the very 

 small crystals the tabular form may be distinguished. 



Near a point where columbite crystals were especially thickly 

 sprinkled through the dike and where the general texture was 

 comparatively fine-grained, little aggregates of a black, opaque 

 metallic mineral whose luster and color were indistinguishable 

 from columbite but whose crystal form was less distinct, were 

 found rather thickly impregnating the dike for several feet. 

 It was evidently an original mineral in the dike and occurred 

 completely imbedded in microcline, beryl, and muscovite. In 

 the specimens examined none appeared to be entirely sur- 

 rounded by qitartz and none was found in spodumene. From 

 a hasty field examination the mineral was thought to be 

 another form of columbite, but later the surrounding gangue, 

 which in the piece used was mostly microcline, was dissolved 

 with hydrofluoric and sulphuric acids and it was found that 

 the crystals showed no resemblance to the crystal habit of 

 columbite as it occurs in the Etta dike. A slight movement in 

 the dike had crushed most of the crystals so that good speci 

 mens for optical measurements were hard to obtain. The sep- 

 arated crystals had been exposed to the action of the acids for 

 about six weeks, and while to the unaided eye they were bright 

 and smooth, W. T. Schaller, to whom they were referred for 

 crystallographic determination, found them to be too badly 

 etched to give a good reflection. He therefore extracted fresh 

 crystals from the matrix and upon these made the crystallo- 

 graphic determinations which are quoted in the next para- 

 graph. The largest crystals collected are about 5 mm long by 

 1*8 to 2 mm across the exposed cross sections. The largest 

 aggregate is 16 mm across. The powder and streak are nearly 

 black with a slightly greenish tinge. The hardness is 6-6*5 

 and the specific gravity 5"25. The mineral is opaque in thin 

 section and neither cassiterite nor rutile appears to be enclosed 

 in it, although such a possibility was suggested by the analyses 

 to be described. 



Mr. Schaller remarks : 



"The small crystals, generally from 1 to 3 millimeters in 

 length and hardly as thick, closely resemble twinned and dis- 

 torted crystals of rutile, mossite, tapiolite, etc. They are 

 tetragonal, twinned on the e (101) face and elongated in the 

 direction of the (111) : (111) intersection edge. The crys- 

 tals are very poorly adapted for measurement, the faces being 

 rough and reflecting light poorly. The forms present are 

 a\100\, <?{101[, sjlll}. The habit of the crystals resembles 

 very closely a so-called black rutile described by Headden and 



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