162 ALUMINA. 



Color white or bluish. Adheres to the tongue, and small 

 pieces become transparent in water. Gr=l*8 — 2*1. 



Composition : silica 39*5, alumina 34*0, water 26*5. Dis- 

 solves in sulphuric acid, yielding a jelly. Becomes milk- 

 white before the blowpipe. 



Obs. From Liege and Bayonne, France. Named in 

 honor of the geologist, Omalius d' Holly. 



Note. — There are several other hydrous silicates of alumina allied to 

 halloylite, having the following names : Pholerite, kollyrite, cimolite, 

 bole, fetlbol, rock soap,rositc,groppite, malthacite, and smelite. They 

 are in general soft and earthy, often clay-like, and are distinguished 

 from similar magnesian species by the blowpipe test for alumina. 



There are also stalactitic hydrous silicates, found in volcanic and other 

 igneous rocks, and formed by the decomposition of feldspar or other in- 

 gredients. Such silico-aluminous stalactites are not uncommon in the 

 Pacific Islands. They are of mixed composition, as necessarily results 

 from their mode of origin. Gibbsite is in some cases of this character. 

 When containing an alkali they become' zeolites. 



Allophane. Reniform and massive, occasionally with traces of crys- 

 tallization ; sometimes almost pulverulent. Color pale blue ; sometimes 

 green, brown or yellow. Luster vitreous or resinous. Splendent and 

 waxy internally. Streak white. H=3. Gr=l85 — 1-90. Compo- 

 sition, alumina 292, silica 21-9, water 442, mixed clay 47. Becomes 

 opaque, colorless and pulverulent before the blowpipe, intumesces a lit- 

 tle and tinges the flame green. Forms a jelly with acids. In marl in 

 Thuringia and Saxony, and in chalk at Beauvais in France. 



The name allophane is from the Greek alios, other, and phaino, to 

 appear, alluding to its changes of appearance before the blowpipe. 



Schr tetter ite, or opal allophane, resembles allophane ; it consists of 

 silica .120, alumina 46*3, water 362, with some iron, copper and 

 lime. 



fahlunite . — Chlorophyllite — Finite. 



In six and twelve-sided prisms, usually foliated, parallel to 

 the base. Folia soft and brittle, of a grayish-green to dark 

 olive. green color, and pearly luster. Gr=2*7. 



Composition : of Fahlunite, silica 449, alumina 30*7, per- 

 oxyd of iron 722, potash 1*38, magnesia 6*04, water 865, 

 protoxyd of manganese 1*90, lime 0*95. (Wachtmeister.) 

 Of Chlorophyllite, silica 45*2, alumina 27*6, magnesia 9*6, 

 protoxyd of iron 8*2, protoxyd of manganese 4*1, water 3*6, 

 (Jackson.) Yields water before the blowpipe and becomes 

 bluish -gray, but fuses only on the edges. 



Dif. It is distinguished from talc by affording water be- 



Of what does halloylite consist ? 



