382 Clarke and Diller—Topaz from Stoneham, Me. 
the latter species with its fluorine replaced either by oxyge™s 
by bydroxyl, or by both; and its formation is probit 
may be written structurally in seven different ways; * 
being as follows, and the others variations of these: 
I 3. 
si0 
A, 3 
*O- A120 aoe: are 
FSSsic SiO, 
O—Al=0 Nal-F Al 
S 2 
The first formula is rather improbable, the second represents 
an orthosilicate, the third falls under a metasilicate tYP* 
Which one is best supported by evidence? : 
A prime difficulty in the way of discussing the chemica? 
structure of minerals arises from the fact that we have a8 Y®& 
