MINERALOGICAL COLLECTIONS IN THE NATIONAL MUSEUM. 999 



in which the cases are to be studied, and also to facilitate reference to 

 the text of a descriptive catalogue soon to be printed. 



The arrangement of the "special collections" is essentially the same, 

 except that in place of the group label there is a general descriptive 

 label applying to the entire case, as follows: 



ZINC MINERALS OF NEW JERSEY. 



Ores of zinc in workable quantities are found in New Jersey, at Franklin and 

 vicinity, Sussex County. The region is unique in that the deposit consists almost 

 entirely of a mixture of the oxides and anhydrous silicates of zinc. The ore depos- 

 its are in beds or veins that are conformable with the stratification of the rocks in 

 which they are embedded; they jjitcli to the northeast, they dip to the southeast, 

 and they lie in a fold. The zinc minerals occur in the gangue rock associated with 

 a variety of limestone carrying manganese, and with a manganese garnet. The run 

 of the mine is usually a mixture of franklinite, willemite, and zincite. 



Franklin ite is iron black in color, having a metallic luster, a hardness varying 

 from 5.5 to 6.5, and a specific gravity of 5. It is slightly magnetic, crystallizes in 

 regular octahedrons, and is also massive, granular to compact. It has the chemical 

 composition ZnFe:0^. 



Willemite varies in color from apple green, flesh red, to manganese brown. Its 

 hardness is 5.5, specific gravity 4. It is rarely crystallized, occurring usually in 

 disseminated grains or masses. It has the chemical composition Zn2Si04. 



Zincite is of a dark red color, occurring usually foliated massive, or in coarse par- 

 ticles or grains, sometimes having, a granular structure. Its hardness is about 4, 

 and it varies in specific gravity from 5.3 to 5.7. It has the chemical composition 

 ZnO. 



THE COMPARATIVE SERIES. 



The comparative series treats of the several characteristics or prop- 

 erties of any one mineral species as applied to all other mineral si:)ecies. 

 This series is intended i)rimarily for the student of mineralogy. Here 

 the properties of minerals are illustrated and defined. In each case 

 the label containing a definition of the property under consideration 

 precedes a series of specimens, and, wherever they can be used advan- 

 tageously, a series of models illustrating that property. One example, 

 that of "composition", will serve to illustrate the methods used in this 

 series : 



COMPOSITION. 



All minerals are composed of either an element alone or two or more elements in 

 combination. Elements are said to combine when on bringing them together a new 

 substance is produced, differing from and possessing properties which, as a rule, are 

 not the mean of those of its constituents. For example, the gases hydrogen and 

 oxygen under the proper conditions combine to form water — a liquid. These combi- 

 nations are represented symbolically by the juxtaposition of the symbols of the 

 couibining elements. Thus a molecule of water, composed of two atoms of hydrogen 

 and one of oxygen, is represented by the symbol H.O. The multiplication of a 

 group of atoms is denoted bj^ placing the proper numeral to the left of the group of 

 symbols or by inclosing them in brackets and placing a small numeral at the right; 

 thus 3H:0 or (HjO).; denotes three molecules of water. The combination of groups 

 is expressed by placing their symbols in juxtaposition with a dot between them; 

 thus Fe:0:;.II.O denotes a compound of oxide of iron with water. Sometimes a comma 

 or the sign -|- is used in place of the dot. Further, the letter R is used to denote a 

 varying group of equivalent elements ; thus RCb^Os is a compound in which there is a 

 varying amount of the equivalent or isomorjjhic elements of the rare earths. 



