
THE COLLECTION OF MINERALS 
stances titanium acting alone as an acid, the latter compounds being 
designated as titanates. Chief among these titano-silicates will! be 
found titanite (Case 16) represented by many varieties, some of which 
are transparent and gem-like and all of which are interesting to the 
student and the collector. Although not of economic importance, 
except as a rather rare gem species, titanite has some interest as an 
accessory rock-forming mineral. In Case 16 will also be found the com- 
pounds of the rare elements niobium and tantalum known as colum- 
bates and tantalates. These include columbite and samarskite as well 
as a considerable number of rarer minerals, all of which are very useful 
as the sources of the group of rare elements such as yttrium, cerium, 
lanthanium, didyium, ete., which are daily becoming more important 
commercially. 
PHOSPHATES 
Cases 16, J, 17 and 18 
The Phosphates, which also include the somewhat rarer arsenates, 
vanadates and antimonates, comprise a considerable and very varied 
group of minerals. Xenotine and monazite are somewhat related com- 
mercially to the columbates and tantalates of the preceding group 
because they are phosphates of the rare elements previously mentioned. 
Case 16 contains a very complete series of these. The common mineral 
apatite (Cases 16 and J) is essentially a phosphate of lime and is the 
most widely distributed of all the phosphates. As is the case with 
most of the common mineral species, apatite is found in a great many 
varieties; these differ in color and transparency, but all when crystal- 
lized exhibit the characteristic six-sided prism capped by a low pyramid 
or by a flat base. The large series exhibited shows well the varied differ- 
ence in form and color of this mineral as well as its almost universal 
distribution. Pyromorphite and mimetite (Case J) are respectively the 
phosphate and arsenate of lead with lead chloride. Both are alteration 
products occurring in the oxidized portions of lead sulphide deposits, 
and the commoner of the two, pyromorphite, is ranked as an ore of lead. 
The brilliant color and unique crystal aggregates to be found in the 
series of specimens exhibited render these minerals objects of consider- 
able attractiveness. 
Vanadinite (Case 17) is the vanadate of lead and bears the same 
relation to the more commercially important deposits of lead sulphide 
as do pyromorphite and mimetite. In the series displayed attention is 
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