Hi Joanie

If I understood your question right, I might have the answer and I think
others might be interested as well. Last year I struggled with a similar
problem and found a way of plotting 3-component data on an X-Y scatter
diagram in Excel.

Suppose you have 3 components A, B and C. Let's say you have mineral
analyses expressed in terms of these components only (or projected onto
these 3 components). This gives a triangle with apices A, B and C. The
trick is now to transform the triangular coordinates into (x, y)
coordinates. If you locate the A apex at (x, y) = (0, 0), the B apex at (1,
0) and the C apex at (0, 0.5 SQRT3), the transformation required is:

x = 0.5 * C + B
y = 0.5 SQRT3 * C

(in the above lines, SQRT3 is the square root of 3)

After transforming the data, you can plot the datapoints plus the 3 apices
onto an X-Y scatter diagram. You can then print the diagram and draw the
sides of the trangle by hand, or you export the data to a drawing program
compatible with Excel (I use MacDraw) and finish the triangle there.

I hope this helps you.

Best regards,


 Leo M. Kriegsman
 Professor of Geology and Mineralogy
 Department of Geology                          fax:     +358 2 333 6580
 University of Turku                               tel:     +358 2 333 5481
 20014 Turku,  FINLAND                        e-mail: [log in to unmask]