Ling,

I have no tricks for K-feldspar, although I have tried to run it with a  
CO2 laser several times. The problem is that K-feldspar reacts with BrF5  
in the chamber at room temperature. One thus obtains variable delta-18  
values as a function of time. There are two potential solutions that I can  
think of at present (and which I have not yet tried my self). One is to  
gold coat the samples to prevent them from reacting at low temperature in  
the chamber. The other is to load only one grain at a time and do not  
prefluorinate.

Let me know how it goes.

-Ed Young