Siderite
Diffraction data computed using the structure from the paper listed below, 
along with the cell parameters refined from the powder pattern of R040034
      Graf D L
      American Mineralogist 46 (1961) 1283-1316
      Crystallographic tables for the rhombohedral carbonates

      CELL PARAMETERS:   4.6819  4.6819 15.3031   90.000   90.000  120.000
      SPACE GROUP: R-3c      

           ATOM        X         Y         Z     OCCUPANCY  ISO(B)
            Fe     0.00000   0.00000   0.00000     1.000     0.700
            C      0.00000   0.00000   0.25000     1.000     0.300
            O      0.27430   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 +/- 14
            MAX. ABS. INTENSITY / VOLUME**2:      47.11484112    
            The INTENSITY cut off value is   1.00

               2-THETA      INTENSITY    D-SPACING   H   K   L
                24.85         36.55        3.5827    0   1   2
                32.17        100.00        2.7826    1   0   4
                38.45         21.41        2.3410    1   1   0
                42.49         16.19        2.1276    1   1   3
                46.33         19.43        1.9597    2   0   2
                50.98         10.79        1.7914    0   2   4
                52.92         19.00        1.7300    0   1   8
                53.10         28.20        1.7246    1   1   6
                60.74          1.73        1.5249    2   1   1
                61.73         12.48        1.5027    1   2   2
                65.16          2.49        1.4317    1   0  10
                65.63         10.06        1.4226    2   1   4
                67.30          4.13        1.3913    2   0   8
                68.16          2.12        1.3757    1   1   9
                68.47          1.01        1.3703    1   2   5
                69.56          9.77        1.3515    3   0   0
                74.39          3.35        1.2753    0   0  12
                78.27          2.87        1.2214    0   2  10
                80.27          5.09        1.1960    1   2   8
                87.01          3.16        1.1199    1   1  12
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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.