On the structure of aragonite

High-resolution synchrotron powder diffraction measurements were carried out at the 32-ID beamline of the Advanced Photon Source of Argonne National Laboratory in order to clarify the structure of geological aragonite, a widely abundant polymorph of CaCO₃. The investigated crystals were practically free of impurity atoms, as measured by wavelength-dispersive X-ray spectroscopy in scanning electron microscopy. A superior quality of diffraction data was achieved by using the 11-channel 111 Si multi-analyzer of the diffracted beam. Applying the Rietveld refinement procedure to the high-resolution diffraction spectra, we were able to extract the aragonite lattice parameters with an accuracy of about 20 p.p.m. The data obtained unambiguously confirm that pure aragonite crystals have orthorhombic symmetry.

Computing details

Figures

Fig. 1.

(PMCN_SF43P_phase_1)

Crystal data

CCaO3	c = 5.742852 (15) Å
Mr = 100.09	V = 227.08 (1) Å^3
Orthorhombic, Pmcn	Z = 4
a = 4.961830 (13) Å	? radiation, λ = 0.49581 Å
b = 7.96914 (2) Å	?, ? × ? × ? mm

Data collection

Radiation source: synchrotron, APS beamline 32-ID

Refinement

Least-squares matrix: full	? data points
Rp = ?	78 parameters
Rwp = ?	0 restraints
Rexp = ?	(Δ/σ)max = 0.04
χ^2 = 6.864

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ca	0.25	0.41502 (2)	0.75985 (4)	0.00764
C	0.25	0.76194 (12)	−0.08240 (14)	0.01523
O1	0.25	0.92238 (8)	−0.09453 (8)	0.01161
O2	0.47499 (7)	0.68012 (5)	−0.08725 (7)	0.01035

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca	0.00450 (9)	0.00677 (9)	0.01166 (10)	0.0	0.0	−0.00195 (14)
C	0.0198 (6)	0.0210 (6)	0.0049 (5)	0.0	0.0	−0.0005 (5)
O1	0.0127 (4)	0.0033 (3)	0.0188 (4)	0.0	0.0	0.0037 (3)
O2	0.0004 (2)	0.0107 (2)	0.0200 (3)	0.00669 (18)	0.0003 (2)	0.0016 (2)

(PMCN_SF43P_phase_2)

Crystal data

O2Si	V = 113.06 (1) Å^3
Mr = 60.08	Z = 3
Trigonal, P3221	? radiation, λ = 0.49581 Å
a = 4.91427 (12) Å	?, ? × ? × ? mm
c = 5.4058 (2) Å

Data collection

Radiation source: synchrotron, APS beamline 32-ID

Refinement

Least-squares matrix: full	? data points
Rp = ?	78 parameters
Rwp = ?	0 restraints
Rexp = ?	(Δ/σ)max = 0.04
χ^2 = 6.864

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Si1	0.4723	0.0	0.66667	0.00754
O1	0.416	0.2658	0.7881	0.01747

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Si1	0.0075	0.0095	0.0063	0.00475	−0.002	−0.004
O1	0.026	0.02	0.0124	0.016	−0.003	−0.005

(PMCN_SF43P_phase_3)

Crystal data

CCaO3	V = 367.41 (4) Å^3
Mr = 100.09	Z = 6
Trigonal, R3c	? radiation, λ = 0.49581 Å
a = 4.9870 (6) Å	?, ? × ? × ? mm
c = 17.058 (2) Å

Data collection

Radiation source: synchrotron, APS beamline 32-ID

Refinement

Least-squares matrix: full	? data points
Rp = ?	78 parameters
Rwp = ?	0 restraints
Rexp = ?	(Δ/σ)max = 0.04
χ^2 = 6.864

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ca1	0.0	0.0	0.0	0.01057
C1	0.0	0.0	0.25	0.00997
O1	0.2465 (7)	0.0	0.25	0.01767

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca1	0.0108	0.0108	0.0101	0.0054	0.0	0.0
C1	0.0094	0.0094	0.0111	0.0047	0.0	0.0
O1	0.0123	0.0234	0.021	0.0117	−0.0039	−0.0078