124 Genth and Pen field — Lansfordite, Nesquehonite, etc. 





ftA« 



d*b 



m a m 



to ^ to 



Calculated 



49° 6' 



65° 27' 



65° 36' 



114° 24' 



Measured 



49 6 



65 28 



66 30 



113 30 





48 57 



65 8 

 65 15 

 65 10 



65 51 



66 



66 



66 4 



115 5 



The prismatic angles vary considerably from the funda- 

 mental values, which would be expected from the nature of • 

 the crystals, and consequently the value of the a axis 0*64-5 

 can only be regarded as approximate. 



The optical properties indicate decidedly orthorhombic 

 symmetry. The plane of the optical axes is the base, the 

 brachy-axis being the acute bisectrix and axis of greatest elas- 

 ticity ; the double refraction is therefore negative and is rather 

 strong. From a small plate l^X4 mm cut parallel to the macro- 

 pinacoid the divergence of the optical axes was measured on a 

 large Fuess axial angle apparatus and found to be : 



2E = 83° 55' for red Li flame. 

 2E = 84 15 for yellow Na flame. 

 2E = 84 22 for green Th flame. 



Five reading for yellow varied between 83° 56' and 84° 37'. 

 As fche green and red lights were not so brilliant it was harder 

 to bring the hyperbolse on the cross-hairs of the instrument 

 and the values can not be regarded as very accurate; they 

 serve, however, to show the small dispersion of the optical 

 axes, which is f><Cv. Small cleavage chips of the mineral 

 show under the polarizing microscope with crossed nicols very 

 little change in color on revolving the stage, but in convergent 

 polarized light an optical axis is seen almost in the center of 

 the field. The optical axes are therefore almost at right angles 

 to the prismatic faces. Indices of refraction were not deter- 

 mined. The hardness of the mineral is about 2*5. The crystals 

 readily scratch gypsum but cannot be made to scratch calcite.. 

 The specific gravity was determined by just floating the crys- 

 tals in the Thoulet solution and found to be 1*83. The solution 

 had a slight solvent action on the mineral, carbon dioxide being 

 liberated, bubbles of which attached themselves to the crystals 

 and buoyed them up. The specific gravity had to be taken 

 therefore very carefully after washing the crystals and taking 

 the observation before any appreciable chemical action had set 

 in. A specific gravity determination made in alcohol gave 

 1'852, agreeing closely with the above. 



For the chemical analysis only the best crystals were used, 

 which after drying for three days over sulphuric acid lost 

 only 0*10 per cent and showed a few opaque spots resulting 

 probably from a slight contamination of lansfordite. The 

 analysis yielded : 



