196 ALUMINA. 



6. Combination of a Silicate and Sulphate, 

 lapis-lazuli. — Ultramarine. 



Monometric. In dodecahedrons. Cleavage imperfect. 

 Also massive. Color rich Berlin or azure 

 blue. Luster vitreous. Translucent to opaque. 

 H=5-5. Gi— 2-3— 2-5. 



Composition : silica 45*5, alumina 31*8, 

 soda 9*1, lime 3*5, iron 0*8, sulphuric acid 

 5*9, sulphur 0-9, chlorine 0*4, water 0*1. 

 Fuses to a white translucent or opaque glass, and if calcined 

 and reduced to powder loses its color in acids. The color 

 of the mineral is supposed to be due to sulphuret of sodium. 



Dif. Distinguished from azurite by its hardness and by 

 giving no indications of copper before the blowpipe ; and 

 from lazulite by its fusibility, hardness, and not giving the 

 reaction of phosphoric acid. 



Obs. Found in granite and granular limestone, and is 

 brought from Persia, China^ Siberia, and Bucharia. The 

 specimens often contain scales of mica and disseminated 

 pyrites. 



Uses. The richly-colored lapis lazuli is highly esteemed 

 for costly vases, and for inlaid work in ornamental furniture. 

 Magnificent slabs are contained in some of the Italian cath- 

 edrals. It is also used in the manufacture of mosaics. 

 When powdered it constitutes the most beautiful and most 

 durable of blue paints, called ultramarine, and has been one 

 of the most costly colors. The late discovery of a mode of 

 making an artificial ultramarine, quite equal to the native, 

 has afforded a substitute at a comparatively cheap rate. 

 This artificial ultramarine consists of silica 45*6, alumina 

 23*3, soda 21*5, potash 1*7, lime trace, sulphuric acid 3*8, 

 sulphur 1-7, iron 1*1, and chlorine a small quantity unde- 

 termined. It has taken the place in the arts, entirely, of the 

 native lapis-lazuli. 



Hauyne, (including nosean and spincllane.) In dodecahedrons, and 

 allied to the preceding. Color bright blue, occasionally greenish. 

 Transparent to translucent. H=6. Gr=228 — 25. Composition, 



What is the crystalline form of lapis-lazuli 1 What is its color ? its 

 ardness 1 its composition 1 How is it distinguished from apatite and 

 azulite ? How does it occur 1 What are its uses 1 What is said of 

 he artificial ultramarine ? 



