52 Washington and Wright — Feldspar from Li 



tnosa. 



Art. Y. — A Feldspar from Linosa and the Existence of 

 Soda Anorthite ( Carnegieite) ; by Henry S. Washington 

 and Fred. Eugene Wright. 



The mineral described in this paper was found as loose 

 crystals, together with crystals of kaersutite, at a small parasitic, 

 cinder cone of the volcano Monte Rosso, on the island of 

 Linosa; east of Tunis. * -The chemical investigation was under- 

 taken by the senior author and the optical by the junior. 



Physical Characters. 



The dark brown, pumiceous, basaltic lava clings so tenaciously 

 to the crystal faces that it was not possible to obtain satisfac- 

 tory material for crystallographic measurements. The crystals 

 vary much in size, the largest one measuring 3'5 cm in length 

 and about 2 om in width. While some are fragmentary, others 

 are wholly bounded by crystal faces. They are elongated 

 parallel to the a axis, and are of simple crystal habit, being 

 bounded by the forms (001), (010), (110), and (110), on the 

 assumption that the mineral is a triclinic feldspar. 



Although the crystal faces do not lend themselves to gonio- 

 metric measurement, the basal cleavage is highly perfect, and 

 albite twinning lamellae are occasionally well developed. On 

 such a cleavage flake the angle between the basal cleavage 

 planes of two adjacent albite lamellae was measured on a two- 

 circle goniometer with reducing attachment, and found to be 

 8° 3'. From this the angle (001) /v (010) = 85° 59' can be 

 deduced, this angle for albite being 86° 24', for anorthite 

 85° 20', and for labradorite 86° 12'. The cleavage after (010) 

 is very imperfect, as is often the case with the soda-lime feld- 

 spars, and only indications of cleavage after the prism faces 

 (110) and (110) were noted. When the cleavage does not 

 control, the fracture is highly conchoidal. Viewed along cer- 

 tain directions, especially about normal to the front pinacoid, a 

 peculiar, milky, opalescent sheen, resembling that of moon- 

 stone, can be seen in some of the crystals. 



Fine polysynthetic twinning lamellae after the albite law are 

 not uncommon, and occasionally cross polysynthetic twinning 

 lamellae, probably after the pericline law, were observed. On 

 a plate nearly normal to the obtuse bisectrix the angle between 

 the albite and the pericline twinning lamellae was found 

 to be 94°. 



The specific gravity was determined (by H. S. W.) with the 

 pycnometer on two separate portions of the carefully selected 



* Cf. H. S. Washington, Jour. Geol., vol. xvi, p. 10, 1908 ; and Washington 

 and Wright, this Journal, vol. xxvi, p. 187, 1908. 



