The ratio is expressed in terms of the number of silica molecules
for each alumina molecule.
The Al:Si ratio
for the example formula is 10.5. It is found by dividing the SiO2
value by the Al2O3 value i.e. 3.2 divided by 0.31 = 10.25.
This ratio can
be written as 1 : 10.25 or more simply as just 10.25.
.

Nicky's
Blue Celedon

KNO
CaO

0.3
0.7

Al2O3

0.4

SiO2

2.6

Al
: Si

6.50

Here is an example 