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The structure of aragonite was first determined by Lawrence Bragg in 1924 in what is the now standard space-group setting Pnma (No. 62). Subsequent studies have all taken his structure as their starting points, despite Bragg's own stated doubts and some earlier etching studies which indicated that the underlying symmetry may really be polar. We have reinvestigated the structure and found that there are many reflections with significant intensity among those that should be systematically extinct in Pnma. Some of these reflections have been subjected to further experimental analysis and have been shown not to be due to Renninger effects. A possible model that satisfies these observations is one where the true structure is in space group P\overline 1 and the structure is twinned about the three axial twofold rotation axes of Pnma. The space group P1 cannot be ruled out. Evidence for these conclusions is presented. The crystal chemistry of aragonite is revisited and described in terms of the stuffed alloy CaC. The carbonate group is confirmed to be non-planar in the crystal.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768102003294/br0111sup1.cif
Contains datablock I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108768102003294/br0111Isup2.hkl
Contains datablock I

Computing details top

Data reduction: Xtal DIFDAT ABSORB ADDREF SORTRF; program(s) used to solve structure: .; program(s) used to refine structure: Xtal LSLS; molecular graphics: Xtal PIG ORTEP; software used to prepare material for publication: Xtal BONDLA CIFIO.

calcium carbonate top
Crystal data top
CaCO3V = 226.96 (2) Å3
Mr = 100.08Z = 4
Triclinic, P1Dx = 2.929 Mg m3
Hall symbol: -P 1Mo Kα radiation, λ = 0.71073 Å
a = 5.7394 (4) ÅCell parameters from 25 reflections
b = 4.9616 (2) Åθ = 22.4–39.1°
c = 7.9700 (5) ŵ = 2.47 mm1
α = 89.999 (4)°T = 293 K
β = 90.012 (5)°Hexagonal prism, colourless
γ = 89.996 (4)°0.54 × 0.15 × 0.13 mm
Data collection top
CAD4
diffractometer
Rint = 0.02
θ/2θ scansθmax = 39.9°
Absorption correction: gaussian
(Davenport et al., 1995)
h = 1010
Tmin = 0.675, Tmax = 0.730k = 88
5690 measured reflectionsl = 1414
2794 independent reflections3 standard reflections every 120 min
2750 reflections with F2 > 0 intensity decay: none
Refinement top
Refinement on F2 w = 1/[σ2(Fo2) + (0.04Fo2)2]1/2
Least-squares matrix: Full matrix(Δ/σ)max = 0.001
R[F2 > 2σ(F2)] = 0.026Δρmax = 2.14 e Å3
wR(F2) = 0.056Δρmin = 1.89 e Å3
S = 1.81Extinction correction: (Becker & Coppens, 1975)
2750 reflectionsExtinction coefficient: 867 (62)
32 parameters
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Ca10.74101 (3)0.24923 (5)0.58465 (2)0.008421 (18)
Ca20.23969 (3)0.24995 (5)0.91478 (2)0.007976 (18)
C10.41592 (16)0.7540 (2)0.76281 (10)0.00583 (8)
C20.91378 (16)0.7482 (2)0.73876 (10)0.01006 (9)
O10.40398 (12)0.7519 (2)0.92178 (8)0.01237 (8)
O20.90486 (12)0.7500 (2)0.57719 (8)0.01184 (8)
O30.4128 (2)0.52529 (17)0.68209 (10)0.01140 (8)
O40.9109 (2)0.97523 (17)0.81800 (10)0.01231 (9)
O50.41298 (18)0.97498 (16)0.68004 (10)0.00951 (8)
O60.91495 (18)0.52853 (15)0.82219 (9)0.01270 (9)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ca10.007750.008830.008230.000180.000420.00001
Ca20.007700.008860.008270.000140.000890.00002
C10.005620.008140.009550.000120.000660.00024
C20.005610.008150.009790.000050.000690.00008
O10.012270.017020.007860.000110.001150.00024
O20.012220.017330.007810.000130.000030.00024
O30.012430.008420.013730.000260.000330.00260
O40.012650.008180.013480.000070.001870.00296
O50.012540.008480.014060.000040.000550.00294
O60.012620.007990.013710.000340.001780.00269
Geometric parameters (Å, º) top
Ca1—O22.6572 (11)Ca2—O62.4347 (10)
Ca1—O32.4553 (10)Ca2—O12.6453 (11)
Ca1—O62.5496 (8)Ca2—O52.5202 (9)
Ca1—O22.6503 (11)Ca2—O12.4245 (7)
Ca1—O42.5013 (9)Ca2—O42.5557 (9)
Ca1—O52.4444 (10)Ca2—O62.5281 (8)
Ca1—O32.5592 (9)C1—O11.2690 (10)
Ca1—O52.5433 (9)C1—O31.3044 (13)
Ca1—O22.4074 (7)C1—O51.2798 (13)
Ca2—O12.6638 (11)C2—O21.2888 (10)
Ca2—O32.5086 (9)C2—O41.2917 (14)
Ca2—O42.4524 (10)C2—O61.2766 (13)
O2—Ca1—O375.94 (3)O6—Ca2—O1144.04 (3)
O2—Ca1—O650.94 (2)O6—Ca2—O5112.61 (3)
O2—Ca1—O2138.40 (3)O6—Ca2—O1144.15 (3)
O2—Ca1—O4112.76 (3)O6—Ca2—O4103.99 (3)
O2—Ca1—O5143.14 (3)O6—Ca2—O674.67 (3)
O2—Ca1—O372.22 (3)O1—Ca2—O550.90 (2)
O2—Ca1—O5120.89 (2)O1—Ca2—O171.61 (3)
O2—Ca1—O271.79 (3)O1—Ca2—O472.22 (3)
O3—Ca1—O676.25 (3)O1—Ca2—O6120.92 (2)
O3—Ca1—O2143.20 (3)O5—Ca2—O193.66 (3)
O3—Ca1—O4111.53 (3)O5—Ca2—O4121.03 (3)
O3—Ca1—O567.73 (3)O5—Ca2—O6171.73 (3)
O3—Ca1—O375.78 (3)O1—Ca2—O480.56 (3)
O3—Ca1—O5103.88 (3)O1—Ca2—O681.51 (3)
O3—Ca1—O2144.73 (3)O4—Ca2—O651.70 (3)
O6—Ca1—O2112.70 (3)O1—C1—O3119.00 (10)
O6—Ca1—O465.85 (3)O1—C1—O5121.38 (10)
O6—Ca1—O5111.91 (3)O3—C1—O5119.40 (8)
O6—Ca1—O3120.94 (3)O2—C2—O4118.82 (10)
O6—Ca1—O5171.74 (3)O2—C2—O6121.77 (10)
O6—Ca1—O293.81 (3)O4—C2—O6119.34 (8)
O2—Ca1—O450.98 (2)Ca2—O1—C190.33 (7)
O2—Ca1—O576.11 (3)Ca2—O1—Ca2138.31 (3)
O2—Ca1—O3120.81 (2)Ca2—O1—Ca2107.84 (4)
O2—Ca1—O572.29 (3)C1—O1—Ca289.47 (7)
O2—Ca1—O271.94 (3)C1—O1—Ca2119.39 (6)
O4—Ca1—O576.48 (3)Ca2—O1—Ca2108.39 (4)
O4—Ca1—O3171.70 (3)Ca1—O2—C289.15 (7)
O4—Ca1—O5120.95 (3)Ca1—O2—Ca1138.40 (3)
O4—Ca1—O294.03 (3)Ca1—O2—Ca1108.21 (4)
O5—Ca1—O3103.66 (3)C2—O2—Ca189.89 (7)
O5—Ca1—O575.34 (3)C2—O2—Ca1120.13 (6)
O5—Ca1—O2144.88 (3)Ca1—O2—Ca1108.06 (4)
O3—Ca1—O551.86 (3)Ca1—O3—Ca2103.53 (3)
O3—Ca1—O281.10 (3)Ca1—O3—C1129.10 (8)
O5—Ca1—O281.35 (3)Ca1—O3—Ca1104.22 (3)
O1—Ca2—O350.66 (2)Ca2—O3—C196.58 (6)
O1—Ca2—O4143.01 (3)Ca2—O3—Ca1135.69 (5)
O1—Ca2—O675.32 (3)C1—O3—Ca191.96 (6)
O1—Ca2—O1138.31 (2)C2—O4—Ca196.64 (6)
O1—Ca2—O5112.46 (3)C2—O4—Ca2128.95 (8)
O1—Ca2—O172.16 (3)C2—O4—Ca291.75 (6)
O1—Ca2—O4120.73 (2)Ca1—O4—Ca2103.39 (3)
O1—Ca2—O672.57 (2)Ca1—O4—Ca2136.50 (5)
O3—Ca2—O4112.03 (3)Ca2—O4—Ca2103.96 (3)
O3—Ca2—O676.70 (3)C1—O5—Ca1128.85 (8)
O3—Ca2—O1112.53 (3)C1—O5—Ca294.96 (6)
O3—Ca2—O565.77 (3)C1—O5—Ca193.28 (6)
O3—Ca2—O193.73 (3)Ca1—O5—Ca2103.51 (3)
O3—Ca2—O4171.16 (3)Ca1—O5—Ca1104.66 (3)
O3—Ca2—O6121.02 (3)Ca2—O5—Ca1135.68 (4)
O4—Ca2—O668.36 (3)Ca1—O6—C294.31 (6)
O4—Ca2—O176.21 (3)Ca1—O6—Ca2102.48 (3)
O4—Ca2—O576.67 (3)Ca1—O6—Ca2135.61 (4)
O4—Ca2—O1144.67 (3)C2—O6—Ca2130.27 (8)
O4—Ca2—O476.04 (3)C2—O6—Ca293.38 (6)
O4—Ca2—O6103.43 (3)Ca2—O6—Ca2105.33 (3)
 

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