Calcite
Diffraction data computed using the structure from the paper listed below, 
along with the cell parameters refined from the powder pattern of R050127
      Graf D L
      American Mineralogist 46 (1961) 1283-1316
      Crystallographic tables for the rhombohedral carbonates

      CELL PARAMETERS:   4.9884  4.9884 17.0568   90.000   90.000  120.000
      SPACE GROUP: R-3c      

           ATOM        X         Y         Z     OCCUPANCY  ISO(B)
            Ca     0.00000   0.00000   0.00000     1.000     1.300
            C      0.00000   0.00000   0.25000     1.000     0.300
            O      0.25780   0.00000   0.25000     1.000     1.000

            X-RAY WAVELENGTH:     1.541838
            BOUNDS ON TWO THETA:    5.0  90.0
            LIMITS IMPOSED ON THE INDICES ARE:  +/-  3 +/-  3 +/- 15
            MAX. ABS. INTENSITY / VOLUME**2:      26.53139147    
            The INTENSITY cut off value is   1.00

               2-THETA      INTENSITY    D-SPACING   H   K   L
                23.08          8.10        3.8538    0   1   2
                29.43        100.00        3.0348    1   0   4
                31.47          2.43        2.8428    0   0   6
                36.01         13.71        2.4942    1   1   0
                39.45         20.15        2.2841    1   1   3
                43.21         14.25        2.0939    2   0   2
                47.17          6.16        1.9269    0   2   4
                47.56         19.66        1.9119    0   1   8
                48.56         20.01        1.8749    1   1   6
                56.63          3.73        1.6254    2   1   1
                57.46          9.18        1.6037    1   2   2
                60.74          5.28        1.5249    2   1   4
                61.07          2.41        1.5174    2   0   8
                61.44          3.18        1.5090    1   1   9
                63.13          2.29        1.4728    1   2   5
                64.74          6.84        1.4400    3   0   0
                65.69          3.90        1.4214    0   0  12
                69.27          1.51        1.3564    2   1   7
                70.32          2.13        1.3386    0   2  10
                72.98          2.69        1.2964    1   2   8
                77.26          1.93        1.2349    1   1  12
                81.63          2.24        1.1795    2   1  10
                83.88          1.57        1.1535    1   3   4
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       XPOW Copyright 1993 Bob Downs, Ranjini Swaminathan and Kurt Bartelmehs
  For reference, see Downs et al. (1993) American Mineralogist 78, 1104-1107.