Magnesite
Diffraction data computed using the structure from the paper listed below, 
along with the cell parameters refined from the powder pattern of R040114
      Markgraf S A, Reeder R J
      American Mineralogist 70 (1985) 590-600
      High-temperature structure refinements of calcite and magnesite
      Sample: T = 24 C

      CELL PARAMETERS:   4.6345  4.6345 15.0228   90.000   90.000  120.000
      SPACE GROUP: R-3c      

           ATOM        X         Y         Z     OCCUPANCY  ISO(B)
            Mg     0.00000   0.00000   0.00000     1.000     0.422
            C      0.00000   0.00000   0.25000     1.000     0.455
            O      0.27780   0.00000   0.25000     1.000     0.543

            X-RAY WAVELENGTH:     1.541838
            BOUNDS ON TWO THETA:    5.0  90.0
            LIMITS IMPOSED ON THE INDICES ARE:  +/-  3 +/-  3 +/- 13
            MAX. ABS. INTENSITY / VOLUME**2:      16.76624859    
            The INTENSITY cut off value is   1.00

               2-THETA      INTENSITY    D-SPACING   H   K   L
                32.65        100.00        2.7423    1   0   4
                35.87         13.21        2.5038    0   0   6
                38.86          6.12        2.3173    1   1   0
                43.01         50.90        2.1030    1   1   3
                46.86         13.39        1.9388    2   0   2
                51.64          5.35        1.7700    0   2   4
                53.90         18.37        1.7009    0   1   8
                53.91         23.97        1.7007    1   1   6
                61.43          5.00        1.5093    2   1   1
                62.46          7.67        1.4870    1   2   2
                66.47          6.35        1.4066    2   1   4
                68.42          2.18        1.3712    2   0   8
                69.39          6.95        1.3544    1   1   9
                69.40          2.96        1.3542    1   2   5
                70.37         13.44        1.3379    3   0   0
                76.02          4.92        1.2519    0   0  12
                76.97          1.93        1.2388    2   1   7
                79.74          2.03        1.2026    0   2  10
                81.58          3.30        1.1801    1   2   8
                88.84          1.30        1.1014    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.