[R-sig-Geo] plot contour map for irregular data points

[Andre Jung]

Hello,

I'm dealing with the following problem:

I have a table with x and y coordinates and corresponding values of a 
mineral concentration, let's call it z.

Can someone provide me a short step-by-step manual for the steps 
necessary to get a contour map?

How to sort and interpolate my matrix to an equidistant grid which can 
afterwards be plotted by contour(x,y,z)? (e.g. fill the missing 
locations with NAs.)

Thanks, guys.