276 DESCRIPTIONS OF MINERALS. 



morpliic rocks. Found in the lava of Vesuvius ; in the 

 Tyrol ; Faroe Islands, Iceland ; in imbedded crystals in some 



doleryte of the Connecticut Valley. 



At Hanover, X. H., anorthite crystals occur altered to a silicate 

 which afforded, in an analysis by Hawes, only 2 '2 of lime, and, in place 

 of the rest of this ingredient, 7 11 of potash. 3 -77 of soda, 110 of iron- 

 sesquioxide, and 2 '67 of water, with 80 05 of alumina and 52 52 of 

 silica ; which compound, if the water be made basic, has the ratio 

 nearly of labradorite, though distinct from that species in the alkalies, 

 and also in specific gravity, which is 2 96 or very nearly 3. It has some 

 relation to zoisite, and to typical saussurite, but is widely different 

 in constituents and ratio ; it is related also to iadeitc. (See page 

 263.) 



Labradorite. — Lime-soda Feldspar. Labrador Feldspar. 



Triclinic. Angle between the cleavage planes 93° 20' and 

 86° 40'. Usually in cleavable massive forms. 



Color dark gray, brown, or greenish brown ; also white or 

 colorless. Often a series of bright chatoyant colors from in- 

 ternal reflections, especially blue and green, with more or 

 less of yellow, red, and pearl-gray. 



Composition. JCaiXa,Al O^Siy=Silica 52*9, alumina30-3, 

 lime 12-3, soda 4-5 = 100. Sometimes contains a little potash 

 in place of the soda. B.B. fuses quite easily to a colorless 

 glass. Only partially decomposed by hydrochloric acid. 



Obs. A constituent of the larger part of eruptive rocks, as 

 doleryte, and amphigenyte, and many lavas ; and also of some 

 metamorphic rocks. Occurs as an ingredient in part of the 

 Archaean rocks in North America, and was named from its 

 first discovery in Labrador. 



Andesite. Triclinic. Angle between the cleavage planes 87 c -88". 

 Near labradorite in composition. The formula ^Ca|Na 2 A10 13 Si 4 = 

 Silica 59-8, alumina 25 5, lime 7"0, soda 7'7=1000. 



Hyalopltane. Monoclinic, and hence angle between the cleavage 

 planes 90"". A baryta feldspar ; the formula like that of andesite, ex- 

 cepting the substitution of Ba for Caand K 2 for Xa?. It has been found 

 in the Binnenthal, Switzerland, and at Jakobsberg, Sweden. 



A baryta-feldspar, having the ratio of andesite, 1:3:8, has been 

 described which is triclinic, and approaches oligoclase in optical char- 

 acters. 



Oligoclase. — Soda-lime Feldspar. 



Triclinic. Angle between the cleavage planes 93° 50' and 

 86° 10'. Commonly in cleavable masses. Also massive. 



Color usually white, grayish white, gravish green, green- 

 ish, reddish. Transparent/ subtranslucent. H.=6*7. G.= 

 2-5-2-7. 



