AMERICAN MUSEUM GUIDE LEAFLETS 
netite is another iron oxide which merits attention because of its im- 
portance as a source of iron. Specimens from many of the American 
and foreign deposits are shown in this series. Cassiterite and rutile 
are closely related oxides of this group and furnish the visitor with magni- 
ficent examples of tetragonal crystals. Many of these are so free from 
distortion as to be almost diagrammatic in their four-fold symmetry 
(Compare with models in Case M). Among the hydrous oxides in 
Case 7 are two minerals to which attention is particularly directed 
because of their economic importance as ores and because they illustrate 
in a striking way the characteristic manner in which this class of minerals 
has been deposited. Limonite, the hydrated oxide of iron, and psilom- 
elane, the hydrated oxide of manganese, give evidence of their secondary 
origin by a variety of forms both interesting and curious. Here we find 
iron oxide which has replaced and taken the crystal forms of other iron 
minerals, rounded masses deposited layer upon layer, and delicate 
thread-like stalactites of great beauty. 
CARBONATES 
Cases 7, 8, E and 9. 
‘Among the simplest of the groups of compounds derived from the 
oxides of the non-metals are the carbonates, which are combinations of 
carbon dioxide with one or more of the metallic oxides. At the head of 
this division stands the mineral calcite, important because of its very 
wide distribution and its common association with minerals of ore veins, 
and extremely interesting because of the almost infinite variety of form 
and habit shown by its crystals. There is no finer example to be found 
among mineral species of the manifold expression of the law of symmetry 
in crystallization, which in this instance among thousands of complex 
manifestations preserves a three-fold symmetry. The series of crystal- 
lized calcite in Cases 7 and 8 well illustrates the wide range of forms 
characteristic of this mineral, from the simple rhombohedra from Poretta 
and the six-sided prisms from Saxony to the highly complex modifica- 
tions from Cumberland and Michigan. 
Dolomite, the carbonate of calcium and magnesium; siderite, the 
carbonate of iron, and rhodochrosite, the carbonate of manganese, all 
belong in the same group with calcite and have many of the char- 
acteristics of form which were seen in that mineral. They are best 
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