PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 437 



Copper.] 



edges, showing a monoclinic form, in which the clinopinacoid is reduced to a very 

 narrow surface, or is obsolete, as shown by figures 27 (page 436) and 28. It is seen 

 occasionally as a narrow face running the whole length of the crystal. These 

 crystals are about 1 or 1.5 millimeters in transverse section. 



From a coarse powder, mounted in balsam, the double refraction is seen to be 

 rather low. A bisectrix ( p ) is in the acute optic angle, making the sign of the 

 mineral najativc. A fragment, represented herewith (figure 28), shows 

 extinction at an angle of about 9 from the principal cleavage. 



By means of a good thin section, the following characters were 

 determined: The refraction and double refraction are low, about that 

 FIG. as. FRAG- O f the feldspars. The acute bisectrix is n r and the optic angle is so 



MENT OFADU- 



LARIA. small that the mineral appears almost uniaxial. The easy cleavage is 



perpendicular to ?? and another is about perpendicular to n e . The crystals are made 

 up of a number of parallel columns, caused apparently, by the two cleavages, which 

 allow them to be dislodged, and they do not all extinguish at the same moment, though 

 essentially in their true positions. The optic plane is parallel with the diagonal of 

 the transverse section. There is frequently a distinct division of the transverse 

 section into four or more sectors, by planes of parting, running from the centre to 

 the angles, and their somewhat irregular and striped manner of extinction seems to 

 be due to a prevalent structure of that kind. There are also many subordinate 

 divisions of the sectors into quadrilateral small areas which extinguish sooner along 

 their edges than at their centres, or at one side sooner than at the opposite side. 

 These conditions are probably peculiar to the multiple manner of development and 

 interpenetration. 



Micro-chemical test by the Boricky process gave evidence of much potash, and 

 a trace of both soda and lime. A similar examination by the method of Behrens 

 showed only potash and also the presence of alumina.* 



The specific gravity of these crystals is 2.544. 



A goniometric measurement of the surface prism angles gave the following: 



TTO A 110=61 30'. 



These characters conspire to show that the mineral is near orthoclase, of the 

 form a </nI a r in. 



*The micro-chemical search for alumina by the Behrens method is very direct and simple, viz. : a powdered portion of the 

 pure crystals \V;LS dissolved in hydrofluoric acid and evaporated todryness on a sand bath, in a small platinum crucible. The 

 residue is dissolved in sulphuric acid, and again evaporated todryness, bringing the mineral into the state of sulphate, soluble 

 in water. After adding a few drops of water, and warming, a small portion of the solution may be placed on a glass slide, and a 

 grain as large as half a pin's head of chloride of ca'sium. in powder, added to the solution on the slide, when immediately are 

 formed crystals of sulphate of alumina and ciesium which can be examined under the objective of the microscope. If the crystals 

 form too rapidly they are simply crystalliths in form of a cross i-(- ) with ragged arms; but with further time they develop into 

 octahedrons. 



The test for potassium is equally direct : a small portion of the original solution is neutralized (/. e., Its excess of sulphuric 

 acid) by u grain of acetate of soda. On a ^,'lass slide, liy the addition of a drop of chloride of potassium, are formed immediately 

 yellow octahedrons of chloride of potassium and platinum, which can be examined as they float in the liquid. 



A test for soda was made as follows, without result, viz. : A drop of the original solution was evaporated to dryness on a 

 glass plate, and to it was added first a drop of acetie acid and afterwards a drop of acetate of uranium. Had soda been present 

 there would have formed tetrahedral crystals of acetate of uranium and sodium. 



