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Diffraction data computed using the structure from the paper listed below, 
along with the cell parameters refined from the powder pattern of R050128
      Graf D L
      American Mineralogist 46 (1961) 1283-1316
      Crystallographic tables for the rhombohedral carbonates

      CELL PARAMETERS:   4.9901  4.9901 17.0690   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:      26.50040133    
            The INTENSITY cut off value is   1.00

               2-THETA      INTENSITY    D-SPACING   H   K   L
                23.07          8.09        3.8555    0   1   2
                29.42        100.00        3.0364    1   0   4
                31.45          2.43        2.8448    0   0   6
                36.00         13.70        2.4951    1   1   0
                39.43         20.15        2.2850    1   1   3
                43.19         14.25        2.0947    2   0   2
                47.14          6.16        1.9277    0   2   4
                47.53         19.68        1.9132    0   1   8
                48.53         20.02        1.8758    1   1   6
                56.61          3.73        1.6260    2   1   1
                57.44          9.18        1.6043    1   2   2
                60.71          5.28        1.5255    2   1   4
                61.03          2.41        1.5182    2   0   8
                61.41          3.19        1.5099    1   1   9
                63.10          2.29        1.4734    1   2   5
                64.71          6.84        1.4405    3   0   0
                65.64          3.91        1.4224    0   0  12
                69.23          1.51        1.3571    2   1   7
                70.28          2.13        1.3394    0   2  10
                72.94          2.69        1.2970    1   2   8
                77.20          1.93        1.2357    1   1  12
                81.58          2.24        1.1801    2   1  10
                83.84          1.57        1.1539    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.