118 SALTS IF LIME. 



cement or plaster is made that " sets" und«r water. See 

 further, the chapter on Rocks, for the uses of limestone. 



ARAGONITE. 



Trimetric. In rhombic prisms, (see fig. 8, page 26) ; 

 M : M=116° 10'. Cleavage parallel with M. Usually 

 in compound crystals having the form of a hexagonal prism, 

 with uneven or striated sides, or in stellated forms consisting 

 of two or three flat crystals crossing one another. Also in 

 globular and coralloidal shapes ; also in fibrous seams in 

 different rocks. 



Color white or with light tinges of gray, yellow, green 

 and violet. Luster vitreous. Transparent to translucent. 

 H=3-5— 4. Gr=2-931. 



In composition, it is identical with calcareous spar, and in 

 its action before the blowpipe it differs only in falling to 

 powder readily when heated. Effervesces also with the 

 acids. Phosphoresces when heated. Some varieties con- 

 tain a few per cent, of carbonate of strontia, but this is not 

 an essential ingredient. 



Dif. The same distinctive characters as calcareous spar, 

 except its crystalline form and superior hardness, and its 

 falling to powder before the blowpipe. 



Obs. Aragonite occurs mostly in gypsum beds and de . 

 posits of iron ore ; also in basalt and other rocks. The 

 coralloidal forms are found in iron ore beds, and are called 

 flos-ferri, flowers of iron. They look like a loosely inter- 

 twined or tangled white cord. VW ^.ocxV; 



The flos-ferri variety occurlf'aTTockport with gypsum ; 

 also at Edenville, at the Parish iron ore bed in Rossie, and 

 in Chester county, Pennsylvania. Aragon in Spain affords 

 six-sided prisms of aragonite, associated with gypsum. This 

 locality gave the name to the species. 



6. dolomite — Magnesian Carbonate of Lime. 

 Rhombohedral. R : R=106° 15'. Cleavage perfect 



# parallel to the primary faces. Faces of rhom- 

 bodedrons sometimes curved, as in the annexed 

 figure. Often granular and massive, constitu- 

 ting extensive beds. 

 Color white or tinged with yellow, red, green, 



What are the usual forms of arragonite 1 Does it differ in composi- 

 tion from calcite ? What are its colors and luster 1 What effect is 

 produced by the blowpipe ? 



