222 Pirsson and Wells — Occurrence of JLeadhillite. 



They are not sufficiently large, however, to prevent a simulta- 

 neous production of cleavage over the twinned individuals as 

 a whole. 



In some cases it appears as if the composition face in twin- 

 ning was the base (001). Certain cleavage plates examined in 

 convergent light display two axial images superimposed upon 

 each other, the lines joining the focal points of the lemniscate 

 figures being at angles of 60°. Such a method of twinning is 

 like that of the micas and chlorites which indeed leadhillite 

 with its pronounced basal cleavage and simulation of hexagonal 

 symmetry resembles. 



To this may be due as suggested by Groth* the optical char- 

 acter of the doubtful mineral susannite which was determined 

 to be uniaxial by Bertrandf on material from Matlock and 

 which we have confirmed by examination of a cleavage plate 

 from a single specimen from Leadhills in the Brush collection. 



Physical properties. — The cleavage parallel to the base is 

 very perfect and easily produced. The luster of the natural 

 basal plane is pearly, while that of cleavage plates is sometimes 

 pearly and sometimes adamantine like that of the faces in the 

 prismatic zone. The luster of the ortho-pyramids is often 

 dull or lacking, resulting from innumerable microscopic pit- 

 tings. The hardness is nearly 3. The specific gravity is 6*54, 

 which agrees almost exactly with Hintze's;}; determination, 

 6'5-iT, for the Sardinian mineral. Our determination was 

 made on a chemical balance at 25° C. with a pure solid fragment 

 weighing over 2 grm. The precaution was taken to remove 

 air before weighing in water, by placing the fragment under 

 water in a vacuum. To use boiling water at 100° for this pur- 

 pose is inadmissible, because when the mineral is heated in 

 water to this temperature, it becomes filled with a multitude 

 of cracks and afterwards falls to powder upon the application 

 of a slight pressure. 



Optical propjerties. — Cleavage plates being clear and color- 

 less furnish excellent material for optical investigation. The 

 axial plane is, as in all known occurrences, parallel to a (100) 

 and the acute bisectrix practically perpendicular to c (001). 

 The optical character is negative. The angle of the optic 

 axes is small and the dispersion sensibly large, p < o. This was 

 confirmed by measurement of a plate with the large Fuess 

 axial angle apparatus at 25° C. with these results. 

 2 EtI =20° 21' 

 2 Exa =20 08 

 2 Eli =18 52 



*Tabel. Uebesicht. Min., 1889, p. 62. 

 tCompd. Read.. Ixxxvi. p. 348, 1876. 

 X Posg. Ann., clii, p. 256. 



