292 DESCRIPTIONS OF MINERALS. 



The name staurolite is from the Greek stauros, a cross. 



Schorhmite. Black, and often irised tarnished. Streak grayish-black. 

 H. z=7-7'5. G. =8 80. Fuses readily on charcoal. Easily decomposed 

 by the acids, and gelatinizes. Contains much titanium, with iron, lime, 

 and silica. From Magnet Cove, Arkansas, and Kaiserstuhlgebirge. 

 Breisgau. 



B. HYDROUS SILICATES. 



f 



The three sections under which the Hydrous Silicates 

 are arranged are the following : 



I. General Section. Under this section there are in- 

 cluded : (1) Bisilicates — Pectolite, Laumontite, Apophy- 

 lite, etc. ; (2) Unisilicates — Prehnite, Calamine, etc. ; and 

 (3) Siibsilicates — as Allophane, and some related species. 



II. Zeolite Section. The minerals included are feld- 

 spar-like in constituents, and apparently so in quantivalent 

 (or oxygen) ratio ; the basic elements being, as in the feld- 

 spars, (1) aluminum, and (2) the metals of the alkalis K, 

 Xa, and of the alkaline earths Ca, Ba, with also Sr, to the 

 almost total exclusion of magnesium and iron. 



III. Margarophyllite Section. This section em- 

 braces species having a micaceous or thin-foliated struc- 

 ture when crystallized, with the surface of the folia pearly, 

 and the plane angle of the base of the prism 120°. Whether 

 crystallized or massive the feel is greasy, at least when 

 pulverized. It comprises (1) Bisilicates : including Talc and 

 Pyrophyllite, which are atomically and physically similar 

 species, although the former is a magnesium silicate, and 

 the latter an aluminum silicate ; (2) Xon-alkaline Uni- 

 silicates, including Kaolinite and Serpentine, which have a 

 similar difference in constituents to the preceding with 

 the same likeness in composition, and also some related 

 species ; (3) Alkaline Unisilicates : as, Pinite and the Hy- 

 drous Micas, which are species containing potassium or 

 sodium as an essential constituent ; (4) the Chlorite Group, 

 the species of which are mostly Siibsilicates. 



