298 REPORT ON NATIONAL MUSEUM, 1886. 



ARAGONITE. — Composition : Same as calcite. Hardness, 3.5 to 4. 



This mineral has the same chemical composition as calcite, but differs 

 in its crystalline form and specific gravity. It sometimes occurs in de- 

 posits of sufficient extent to be quarried as marble. The beautiful 

 "onyx marble" of San Luis Obispo is nearly pure aragonite. 



DOLOMITE.— Composition : (CaMg) C0 3 = Calcium carbonate, 54.35 per cent.; inag^ 

 nesium carbonate, 45.65 per cent. Hardness, 3.2 to 4. 



This mineral closely resembles calcite, but can be readily distin- 

 guished from the same by its greater hardness and from its being acted 

 upon but little, if at all, by a dilute acid. Like calcite, it frequently 

 occurs in compact crystalline massive forms, and is quarried for build- 

 ing material or for making lime. Many of our marbles are dolomites, 

 as for instance those of Cockeysville, Md., and Pleasantville, 1ST. Y. 



GYPSUM. Calcium Sulphate.— Composition: CaS0 4 -f-2aq = sulphur trioxide, 

 40. 5 per cent. ; lime, 32.6 per cent. ; water, 20.9 per cent. Hardenss, 2. 



Gypsum rarely occurs in crystalline rocks, but forms extensive beds 

 among stratified rocks such as limestones and beds of clay. The fine 

 translucent variety is used for ornamental purposes, and is known as 

 alabaster. It is soft enough to be readily cut with a knife or scratched 

 with the thumb-nail, and it is not at all acted on by acids. It is there- 

 fore readily distinguished from calcite, which it somewhat resembles. 



SERPENTINE. — Composition: A hydrous silicate of magnesia, Mg 3 Sij0 7 -f-2aq = 

 silica, 43.48 per cent. ; magnesia, 43.48 per cent.; water, 13.04 per cent. Hard- 

 ness, 4. 



This mineral occurs mixed with calcite or dolomite, forming the so- 

 called verdantique marble or ophiolite. As a secondary product it is 

 sometimes found resulting from the alteration of olivine and other mag- 

 nesian minerals in various eruptive rocks, such as basalt, diabase, dun- 

 ite, and lherzolite. It often occurs in extensive deposits, usually mixed 

 with more or less chromite, magnetite, enstatite, or similar minerals, and 

 is of value as a building or ornamental stone, as will be noticed later. 



Serpentine can usually be recognized from its green or yellowish color, 

 slightly soapy feeling, lack of cleavage, and softness, it being readily 

 cut with a knife. It is, however, not so soft as talc, with which it might 

 possibly be confounded by any but a mineralogist. 



TALC. Steatite. — Composition: A hydrous silicate of magnesia = silica, 63.49 per 

 cent. ; magnesia, 31.75 per cent. ; water, 4.76 per cent. Hardness, 1. 



This is a common mineral, occurring as an essential constituent of 

 talc schist or as an alteration product, replacing hornblende, augite, 

 mica, and other magnesian minerals. The common form is that of 

 small, greenish, inelastic scales. It often occurs massive, and is known 

 by the name of soapstone, and is used extensively in stoves and fur- 

 naces. The finely granular crypto -crystalline variety is known as French 

 chalk, used by tailors and others. In its common form this mineral 

 might be mistaken for a mica, but for its soapy feeling and softness, 

 which is such that it can be readily scratched by the thumb-nail. 



