[R] 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.)
  
Thank you.