EXPLORATION OF THE MINERAL WORLD BY X-RAYS 439 
is situated at the centre. All the silicon is in this situation, and by far the 
greater part of the aluminium. 
(b) Elements forming the centre of an octahedral group. Six oxygen 
atoms are grouped at the corners of an octahedron, with the element at the 
centre. This is the characteristic situation for magnesium and iron, and 
also for the remainder of the aluminium. Aluminium is peculiar in that it 
can play a double role, generally grouping itself with silicon, but sometimes 
behaving like the metals iron and magnesium. 
(c) The bulky elements sodium, calcium, and potassium. These elements 
are too large to be placed in tetrahedral or octahedral groups. ‘They are 
accommodated in large, often irregular, holes in the structure. 
ofl 
= 
Most Al. a) 
K 
Ca, Na 
Covities in Oxygen 
8% Lattice 50% 
Fig. 2.—Association of common elements with oxygen in mineral structures. 
The types of group are illustrated in Fig. 2. The tetrahedral and octa- 
hedral groups are the fundamental units of pattern, the stitches of which 
the mineral fabric is composed. All the common minerals, however com- 
plex their patterns, are a framework of these tetrahedral and octahedral 
groups. It must be realised that the groups are not distinct units, for there 
are not enough oxygens for each central atom to have its complete group 
belonging to it alone. The oxygen atoms of one group also form part of 
the next. It is very convenient to use the tetrahedra and octahedra in 
describing the structures, but it must be remembered that these units have 
common corners, edges, or even faces, because an oxygen atom of one also 
belongs to another. In this way the whole structure is knitted together. 
(4) The common minerals are divided into certain large groups, and in 
making his classification the mineralogist has in the past been guided by 
physical properties and form rather than by chemical constitution. A study 
of the structure of minerals has amply justified this allegiance. It is now 
seen that the basis of the classification is a kind of skeleton of the mineral 
structure, composed of the linked tetrahedral groups. These links are 
stronger than the octahedral links, and very much stronger than the links of 
