The space group of silicon is Fd-3m (usually written with bar over the three). There are two settings of that space group. (This situation arises because the point of highest symmetry does not coincide with the point of fewest equivalent Wyckoff positions.) Have a look at the drawings in the International Tables for that space group (#227).
Origin choice one has the origin at a point of -43m symmetry, which has multiplicity 8. This is how Si is usually drawn in the text books, as an fcc lattice with a basis of one atom at the origin and another at (1/4, 1/4, 1/4). Origin choice two has the origin at a point of -3m symmetry, which is in the middle of a bond between two Si atoms. In that choice, the atoms are located at 8 equivalent positions generated by (1/8, 1/8, 1/8). You can easily imagine that if you put the atom at the wrong position for the assumed origin choice, the intensities will be all wrong. One quick check is to look in the GSAS atom list at the number of equivalent positions for your Si atom. It should be 8, and I bet you'll see 16. Also, if the only charge in the unit cell is at the position of the atom, reflections like (222) will not be observed. However, taking account of either bond charge or anisotropic (non-spherical) thermal displacements, that reflection will acquire a very weak intensity. Your table has a nonzero observed intensity for the (222). I assume you don't actually see a peak there, do you? If there is no peak in the raw data but GSAS wants to put one in, it is probably an issue with the background. If you do observe such a strong peak, there is an issue with your diffractometer. Bremsstrahlung contamination getting by the discriminator? I think that should answer all of the problems you describe. (I see that Vincent Favre-Nicolin has written an equivalent answer in the time I was writing this.) ^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~^~ Peter W. Stephens Professor, Department of Physics and Astronomy Stony Brook University Stony Brook, NY 11794-3800 fax 631-632-8176
