Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270105000338/bc1063sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270105000338/bc1063Isup2.hkl |
Single crystals were extracted from rocks by mechanical separation, followed by a bath in deionized water. The aluminosilicate minerals were dissolved by repeated treatment with hot HCl and HF solutions. The residue was rinsed with distilled water and the purity of crystals was checked by powder X-ray diffraction.
The microprobe analysis result (Mg0.45 (8) Al0.41 (5) Cr1.49 (4) Fe0.65 (6))O4.00 was used as a first approximation for cation substitution. In all refinements, different atoms on the same site were constrained to share the same anisotropic displacement parameter. Models with an inverse distribution or a partial degree of inversion were rejected for chemical reasons and on the basis of the refinement results. Four normal spinel models have been tested:
(1) Model 1: (Mg2+0.52 (1), Fe2+0.48 (1))[Cr3+1.57 (3), Al3+0.43 (3))O42−, with R(Fo) = 0.0163, wR(Fo2) = 0.0456, S(Fo2) = 1.477 and NP = 12.
(2) Model 2: on the basis of the Mössbauer data, Fe3+ was added to the octahedral site, keeping the total amount of iron given by the electron microprobe analysis. The refined formula is (Mg2+0.51 (1), Fe2+0.49 (1))[Al3+0.47 (3), Cr3+1.37 (3), Fe3+0.16 (1)]O4, R(Fo) = 0.0167, wR(Fo2) = 0.0470, S(Fo2) = 1.478 and NP = 13. From a statistical point of view, this model is not better than model 1.
(3) Model 3: according to O'Neill & Navrotsky (1983, 1984), Al3+ in spinels can be found on both octahedral and tetrahedral sites. A refinement with an additional constraint on the total Al content yielded the formula (Mg2+0.26 (5), Al3+0.30 (5), Fe2+0.44 (1))[Al3+0.11 (5), Cr3+1.49 (3), Fe3+0.21 (1), Φ0.19]O4. with R(Fo) = 0.0155, wR(Fo2) = 0.0409, S(Fo2) = 1.406 and NP = 14. This refinement clearly indicates that Al3+ is present at both sites, but the Mg2+ content disagrees with the chemical analysis.
(4) Model 4: the total tetrahedral site occupancy is relaxed, which yields the formula (Mg2+0.40 (11), Al3+0.28 (5), Fe2+0.39 (4))[Al3+0.13 (5), Cr3+1.42 (6), Fe3+0.26 (4), Φ0.19]O4, with R(Fo) = 0.0155, wR(Fo2) = 0.0408, S(Fo2) = 1.402 and NP = 14. Although it does not seem to be statistically different from model 3, this is the preferred model since electroneutrality is reached with a total positive charge of +7.9 (3), against +7.7 (2) for model 3.
Data collection: XSCANS (Siemens, 1991); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL-PC (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: PLATON (Spek, 2003).
Al0.41Cr1.42Fe0.65Mg0.40O4.00 | Dx = 4.512 Mg m−3 |
Mr = 194.94 | Mo Kα radiation, λ = 0.71073 Å |
Cubic, Fd3m | Cell parameters from 98 reflections |
Hall symbol: -F 4vw 2vw 3 | θ = 20.0–32.8° |
a = 8.31058 (5) Å | µ = 8.67 mm−1 |
V = 573.98 (1) Å3 | T = 298 K |
Z = 8 | Octahedron, black |
F(000) = 746 | 0.07 × 0.07 × 0.07 mm |
Siemens P4 diffractometer | 219 reflections with I > 2σ(I) |
Radiation source: Fine-focus sealed X-ray tube | Rint = 0.049 |
Graphite monochromator | θmax = 56.4°, θmin = 4.3° |
θ/2θ scans | h = −1→19 |
Absorption correction: analytical (Lundgren, 1982) | k = −19→19 |
Tmin = 0.385, Tmax = 0.445 | l = −19→19 |
5083 measured reflections | 3 standard reflections every 297 reflections |
220 independent reflections | intensity decay: 0.5% |
Refinement on F2 | Primary atom site location: heavy-atom method |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.015 | w = 1/[σ2(Fo2) + (0.0138P)2 + 0.6778P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.041 | (Δ/σ)max < 0.001 |
S = 1.40 | Δρmax = 0.96 e Å−3 |
220 reflections | Δρmin = −0.69 e Å−3 |
14 parameters | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
2 restraints | Extinction coefficient: 0.0152 (10) |
Al0.41Cr1.42Fe0.65Mg0.40O4.00 | Z = 8 |
Mr = 194.94 | Mo Kα radiation |
Cubic, Fd3m | µ = 8.67 mm−1 |
a = 8.31058 (5) Å | T = 298 K |
V = 573.98 (1) Å3 | 0.07 × 0.07 × 0.07 mm |
Siemens P4 diffractometer | 219 reflections with I > 2σ(I) |
Absorption correction: analytical (Lundgren, 1982) | Rint = 0.049 |
Tmin = 0.385, Tmax = 0.445 | 3 standard reflections every 297 reflections |
5083 measured reflections | intensity decay: 0.5% |
220 independent reflections |
R[F2 > 2σ(F2)] = 0.015 | 14 parameters |
wR(F2) = 0.041 | 2 restraints |
S = 1.40 | Δρmax = 0.96 e Å−3 |
220 reflections | Δρmin = −0.69 e Å−3 |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
O | 0.26254 (5) | 0.26254 (5) | 0.26254 (5) | 0.00650 (14) | |
CrM | 0.5000 | 0.5000 | 0.5000 | 0.00477 (9) | 0.71 (3) |
AlM | 0.5000 | 0.5000 | 0.5000 | 0.00477 (9) | 0.063 (27) |
Fe3M | 0.5000 | 0.5000 | 0.5000 | 0.00477 (9) | 0.13 (2) |
Fe2T | 0.1250 | 0.1250 | 0.1250 | 0.0062 (3) | 0.39 (4) |
MgT | 0.1250 | 0.1250 | 0.1250 | 0.0062 (3) | 0.40 (11) |
AlT | 0.1250 | 0.1250 | 0.1250 | 0.0062 (3) | 0.28 (5) |
U11 | U22 | U33 | U12 | U13 | U23 | |
O | 0.00650 (14) | 0.00650 (14) | 0.00650 (14) | −0.00079 (9) | −0.00079 (9) | −0.00079 (9) |
CrM | 0.00477 (9) | 0.00477 (9) | 0.00477 (9) | −0.00037 (2) | −0.00037 (2) | −0.00037 (2) |
AlM | 0.00477 (9) | 0.00477 (9) | 0.00477 (9) | −0.00037 (2) | −0.00037 (2) | −0.00037 (2) |
Fe3M | 0.00477 (9) | 0.00477 (9) | 0.00477 (9) | −0.00037 (2) | −0.00037 (2) | −0.00037 (2) |
Fe2T | 0.0062 (3) | 0.0062 (3) | 0.0062 (3) | 0.000 | 0.000 | 0.000 |
MgT | 0.0062 (3) | 0.0062 (3) | 0.0062 (3) | 0.000 | 0.000 | 0.000 |
AlT | 0.0062 (3) | 0.0062 (3) | 0.0062 (3) | 0.000 | 0.000 | 0.000 |
O—Fe3Mi | 1.9790 (4) | AlM—CrMviii | 2.9382 |
O—Fe3Mii | 1.9790 (4) | AlM—CrMiii | 2.9382 |
O—Fe3Miii | 1.9790 (4) | AlM—CrMii | 2.9382 |
O—AlMi | 1.9790 (4) | AlM—CrMix | 2.9382 |
O—Miii | 1.9790 (4) | AlM—CrMi | 2.9382 |
O—CrMii | 1.9790 (4) | Fe3M—Oiv | 1.9789 (4) |
O—CrMi | 1.9790 (4) | Fe3M—Ov | 1.9789 (4) |
O—AlMiii | 1.9790 (4) | Fe3M—Oii | 1.9790 (4) |
O—AlMii | 1.9790 (4) | Fe3M—Oiii | 1.9790 (4) |
O—T | 1.9797 (8) | Fe3M—Oi | 1.9790 (4) |
O—AlT | 1.9797 (8) | Fe3M—Ovi | 1.9790 (4) |
O—MgT | 1.9797 (8) | Fe3M—Fe3Mvii | 2.9382 |
CrM—Oiv | 1.9789 (4) | Fe3M—CrMviii | 2.9382 |
CrM—Ov | 1.9789 (4) | Fe3M—CrMiii | 2.9382 |
CrM—Oii | 1.9790 (4) | Fe3M—CrMii | 2.9382 |
CrM—Oiii | 1.9790 (4) | Fe3M—CrMix | 2.9382 |
CrM—Oi | 1.9790 (4) | Fe3M—CrMi | 2.9382 |
CrM—Ovi | 1.9790 (4) | Fe2T—Ox | 1.9798 (8) |
CrM—Fe3Mvii | 2.9382 | Fe2T—Oxi | 1.9798 (8) |
CrM—CrMviii | 2.9382 | Fe2T—Oxii | 1.9798 (8) |
CrM—CrMiii | 2.9382 | MgT—Ox | 1.9798 (8) |
CrM—CrMii | 2.9382 | MgT—Oxi | 1.9798 (8) |
CrM—CrMix | 2.9382 | MgT—Oxii | 1.9798 (8) |
CrM—CrMi | 2.9382 | MgT—MgTxiii | 3.5986 |
AlM—Oiv | 1.9789 (4) | MgT—MgTxiv | 3.5986 |
AlM—Ov | 1.9789 (4) | MgT—MgTxv | 3.5986 |
AlM—Oii | 1.9790 (4) | MgT—MgTxvi | 3.5987 |
AlM—Oiii | 1.9790 (4) | AlT—Ox | 1.9798 (8) |
AlM—Oi | 1.9790 (4) | AlT—Oxi | 1.9798 (8) |
AlM—Ovi | 1.9790 (4) | AlT—Oxii | 1.9798 (8) |
AlM—Fe3Mvii | 2.9382 | ||
Fe3Mi—O—Fe3Mii | 95.87 (3) | Ovi—AlM—CrMviii | 42.066 (13) |
Fe3Mi—O—Fe3Miii | 95.87 (3) | Fe3Mvii—AlM—CrMviii | 60.0 |
Fe3Mii—O—Fe3Miii | 95.87 (3) | Oiv—AlM—CrMiii | 137.934 (13) |
Fe3Mii—O—AlMi | 95.87 (3) | Ov—AlM—CrMiii | 94.27 (2) |
Fe3Miii—O—AlMi | 95.87 (3) | Oii—AlM—CrMiii | 42.066 (13) |
Fe3Mi—O—CrMiii | 95.87 (3) | Oiii—AlM—CrMiii | 85.73 (2) |
Fe3Mii—O—CrMiii | 95.87 (3) | Oi—AlM—CrMiii | 42.066 (13) |
AlMi—O—CrMiii | 95.87 (3) | Ovi—AlM—CrMiii | 137.934 (13) |
Fe3Mi—O—CrMii | 95.87 (3) | Fe3Mvii—AlM—CrMiii | 120.0 |
Fe3Miii—O—CrMii | 95.87 (3) | CrMviii—AlM—CrMiii | 180.0 |
AlMi—O—CrMii | 95.87 (3) | Oiv—AlM—CrMii | 137.934 (13) |
CrMiii—O—CrMii | 95.87 (3) | Ov—AlM—CrMii | 137.934 (13) |
Fe3Mii—O—CrMi | 95.87 (3) | Oii—AlM—CrMii | 85.73 (2) |
Fe3Miii—O—CrMi | 95.87 (3) | Oiii—AlM—CrMii | 42.066 (13) |
CrMiii—O—CrMi | 95.87 (3) | Oi—AlM—CrMii | 42.066 (13) |
CrMii—O—CrMi | 95.87 (3) | Ovi—AlM—CrMii | 94.27 (2) |
Fe3Mi—O—AlMiii | 95.87 (3) | Fe3Mvii—AlM—CrMii | 120.0 |
Fe3Mii—O—AlMiii | 95.87 (3) | CrMviii—AlM—CrMii | 120.0 |
AlMi—O—AlMiii | 95.87 (3) | CrMiii—AlM—CrMii | 60.0 |
CrMii—O—AlMiii | 95.87 (3) | Oiv—AlM—CrMix | 42.066 (13) |
CrMi—O—AlMiii | 95.87 (3) | Ov—AlM—CrMix | 42.066 (13) |
Fe3Mi—O—AlMii | 95.87 (3) | Oii—AlM—CrMix | 94.27 (2) |
Fe3Miii—O—AlMii | 95.87 (3) | Oiii—AlM—CrMix | 137.934 (13) |
AlMi—O—AlMii | 95.87 (3) | Oi—AlM—CrMix | 137.934 (13) |
CrMiii—O—AlMii | 95.87 (3) | Ovi—AlM—CrMix | 85.73 (2) |
CrMi—O—AlMii | 95.87 (3) | Fe3Mvii—AlM—CrMix | 60.0 |
AlMiii—O—AlMii | 95.87 (3) | CrMviii—AlM—CrMix | 60.0 |
Fe3Mi—O—Fe2T | 120.99 (2) | CrMiii—AlM—CrMix | 120.0 |
Fe3Mii—O—Fe2T | 120.99 (2) | CrMii—AlM—CrMix | 180.0 |
Fe3Miii—O—Fe2T | 120.99 (2) | Oiv—AlM—CrMi | 94.27 (2) |
AlMi—O—Fe2T | 120.99 (2) | Ov—AlM—CrMi | 137.934 (13) |
CrMiii—O—Fe2T | 120.99 (2) | Oii—AlM—CrMi | 42.066 (13) |
CrMii—O—Fe2T | 120.99 (2) | Oiii—AlM—CrMi | 42.066 (13) |
CrMi—O—Fe2T | 120.99 (2) | Oi—AlM—CrMi | 85.73 (2) |
AlMiii—O—Fe2T | 120.99 (2) | Ovi—AlM—CrMi | 137.934 (13) |
AlMii—O—Fe2T | 120.99 (2) | Fe3Mvii—AlM—CrMi | 180.0 |
Fe3Mi—O—AlT | 120.99 (2) | CrMviii—AlM—CrMi | 120.0 |
Fe3Mii—O—AlT | 120.99 (2) | CrMiii—AlM—CrMi | 60.0 |
Fe3Miii—O—AlT | 120.99 (2) | CrMii—AlM—CrMi | 60.0 |
AlMi—O—AlT | 120.99 (2) | CrMix—AlM—CrMi | 120.0 |
CrMiii—O—AlT | 120.99 (2) | Oiv—Fe3M—Ov | 83.81 (3) |
CrMii—O—AlT | 120.99 (2) | Oiv—Fe3M—Oii | 96.19 (3) |
CrMi—O—AlT | 120.99 (2) | Ov—Fe3M—Oii | 96.19 (3) |
AlMiii—O—AlT | 120.99 (2) | Oiv—Fe3M—Oiii | 96.19 (3) |
AlMii—O—AlT | 120.99 (2) | Ov—Fe3M—Oiii | 180.0 |
Fe3Mi—O—MgT | 120.99 (2) | Oii—Fe3M—Oiii | 83.81 (3) |
Fe3Mii—O—MgT | 120.99 (2) | Oiv—Fe3M—Oi | 180.0 |
Fe3Miii—O—MgT | 120.99 (2) | Ov—Fe3M—Oi | 96.19 (3) |
AlMi—O—MgT | 120.99 (2) | Oii—Fe3M—Oi | 83.81 (3) |
CrMiii—O—MgT | 120.99 (2) | Oiii—Fe3M—Oi | 83.81 (3) |
CrMii—O—MgT | 120.99 (2) | Oiv—Fe3M—Ovi | 83.81 (3) |
CrMi—O—MgT | 120.99 (2) | Ov—Fe3M—Ovi | 83.81 (3) |
AlMiii—O—MgT | 120.99 (2) | Oii—Fe3M—Ovi | 180.0 |
AlMii—O—MgT | 120.99 (2) | Oiii—Fe3M—Ovi | 96.19 (3) |
Oiv—M—Ov | 83.81 (3) | Oi—Fe3M—Ovi | 96.19 (3) |
Oiv—M—Oii | 96.19 (3) | Oiv—Fe3M—Fe3Mvii | 85.73 (2) |
Ov—CrM—Oii | 96.19 (3) | Ov—Fe3M—Fe3Mvii | 42.066 (13) |
Oiv—CrM—Oiii | 96.19 (3) | Oii—Fe3M—Fe3Mvii | 137.934 (13) |
Ov—CrM—Oiii | 180.0 | Oiii—Fe3M—Fe3Mvii | 137.934 (13) |
Oii—CrM—Oiii | 83.81 (3) | Oi—Fe3M—Fe3Mvii | 94.27 (2) |
Oiv—M—Oi | 180.0 | Ovi—Fe3M—Fe3Mvii | 42.066 (13) |
Ov—CrM—Oi | 96.19 (3) | Oiv—Fe3M—CrMviii | 42.066 (13) |
Oii—CrM—Oi | 83.81 (3) | Ov—Fe3M—CrMviii | 85.73 (2) |
Oiii—CrM—Oi | 83.81 (3) | Oii—Fe3M—CrMviii | 137.934 (13) |
Oiv—CrM—Ovi | 83.81 (3) | Oiii—Fe3M—CrMviii | 94.27 (2) |
Ov—CrM—Ovi | 83.81 (3) | Oi—Fe3M—CrMviii | 137.934 (13) |
Oii—CrM—Ovi | 180.0 | Ovi—Fe3M—CrMviii | 42.066 (13) |
Oiii—CrM—Ovi | 96.19 (3) | Fe3Mvii—Fe3M—CrMviii | 60.0 |
Oi—CrM—Ovi | 96.19 (3) | Oiv—Fe3M—CrMiii | 137.934 (13) |
Oiv—CrM—Fe3Mvii | 85.73 (2) | Ov—Fe3M—CrMiii | 94.27 (2) |
Ov—CrM—Fe3Mvii | 42.066 (13) | Oii—Fe3M—CrMiii | 42.066 (13) |
Oii—CrM—Fe3Mvii | 137.934 (13) | Oiii—Fe3M—CrMiii | 85.73 (2) |
Oiii—CrM—Fe3Mvii | 137.934 (13) | Oi—Fe3M—CrMiii | 42.066 (13) |
Oi—CrM—Fe3Mvii | 94.27 (2) | Ovi—Fe3M—CrMiii | 137.934 (13) |
Ovi—CrM—Fe3Mvii | 42.066 (13) | Fe3Mvii—Fe3M—CrMiii | 120.0 |
Oiv—CrM—CrMviii | 42.066 (13) | CrMviii—Fe3M—CrMiii | 180.0 |
Ov—CrM—CrMviii | 85.73 (2) | Oiv—Fe3M—CrMii | 137.934 (13) |
Oii—CrM—CrMviii | 137.934 (13) | Ov—Fe3M—CrMii | 137.934 (13) |
Oiii—CrM—CrMviii | 94.27 (2) | Oii—Fe3M—CrMii | 85.73 (2) |
Oi—CrM—CrMviii | 137.934 (13) | Oiii—Fe3M—CrMii | 42.066 (13) |
Ovi—CrM—CrMviii | 42.066 (13) | Oi—Fe3M—CrMii | 42.066 (13) |
Fe3Mvii—CrM—CrMviii | 60.0 | Ovi—Fe3M—CrMii | 94.27 (2) |
Oiv—CrM—CrMiii | 137.934 (13) | Fe3Mvii—Fe3M—CrMii | 120.0 |
Ov—CrM—CrMiii | 94.27 (2) | CrMviii—Fe3M—CrMii | 120.0 |
Oii—CrM—CrMiii | 42.066 (13) | CrMiii—Fe3M—CrMii | 60.0 |
Oiii—CrM—CrMiii | 85.73 (2) | Oiv—Fe3M—CrMix | 42.066 (13) |
Oi—CrM—CrMiii | 42.066 (13) | Ov—Fe3M—CrMix | 42.066 (13) |
Ovi—CrM—CrMiii | 137.934 (13) | Oii—Fe3M—CrMix | 94.27 (2) |
Fe3Mvii—CrM—CrMiii | 120.0 | Oiii—Fe3M—CrMix | 137.934 (13) |
CrMviii—CrM—CrMiii | 180.0 | Oi—Fe3M—CrMix | 137.934 (13) |
Oiv—CrM—CrMii | 137.934 (13) | Ovi—Fe3M—CrMix | 85.73 (2) |
Ov—CrM—CrMii | 137.934 (13) | Fe3Mvii—Fe3M—CrMix | 60.0 |
Oii—CrM—CrMii | 85.73 (2) | CrMviii—Fe3M—CrMix | 60.0 |
Oiii—CrM—CrMii | 42.066 (13) | CrMiii—Fe3M—CrMix | 120.0 |
Oi—CrM—CrMii | 42.066 (13) | CrMii—Fe3M—CrMix | 180.0 |
Ovi—CrM—CrMii | 94.27 (2) | Oiv—Fe3M—CrMi | 94.27 (2) |
Fe3Mvii—CrM—CrMii | 120.0 | Ov—Fe3M—CrMi | 137.934 (13) |
CrMviii—CrM—CrMii | 120.0 | Oii—Fe3M—CrMi | 42.066 (13) |
CrMiii—CrM—CrMii | 60.0 | Oiii—Fe3M—CrMi | 42.066 (13) |
Oiv—CrM—CrMix | 42.066 (13) | Oi—Fe3M—CrMi | 85.73 (2) |
Ov—CrM—CrMix | 42.066 (13) | Ovi—Fe3M—CrMi | 137.934 (13) |
Oii—CrM—CrMix | 94.27 (2) | Fe3Mvii—Fe3M—CrMi | 180.0 |
Oiii—CrM—CrMix | 137.934 (13) | CrMviii—Fe3M—CrMi | 120.0 |
Oi—CrM—CrMix | 137.934 (13) | CrMiii—Fe3M—CrMi | 60.0 |
Ovi—CrM—CrMix | 85.73 (2) | CrMii—Fe3M—CrMi | 60.0 |
Fe3Mvii—CrM—CrMix | 60.0 | CrMix—Fe3M—CrMi | 120.0 |
CrMviii—CrM—CrMix | 60.0 | O—Fe2T—Ox | 109.5 |
CrMiii—CrM—CrMix | 120.0 | O—T—Oxi | 109.5 |
CrMii—CrM—CrMix | 180.0 | Ox—Fe2T—Oxi | 109.5 |
Oiv—CrM—CrMi | 94.27 (2) | O—Fe2T—Oxii | 109.5 |
Ov—CrM—CrMi | 137.934 (13) | Ox—Fe2T—Oxii | 109.5 |
Oii—CrM—CrMi | 42.066 (13) | Oxi—Fe2T—Oxii | 109.5 |
Oiii—CrM—CrMi | 42.066 (13) | O—MgT—Ox | 109.5 |
Oi—CrM—CrMi | 85.73 (2) | O—MgT—Oxi | 109.5 |
Ovi—CrM—CrMi | 137.934 (13) | Ox—MgT—Oxi | 109.5 |
Fe3Mvii—CrM—CrMi | 180.0 | O—MgT—Oxii | 109.5 |
CrMviii—CrM—CrMi | 120.0 | Ox—MgT—Oxii | 109.5 |
CrMiii—CrM—CrMi | 60.0 | Oxi—MgT—Oxii | 109.5 |
CrMii—CrM—CrMi | 60.0 | O—MgT—MgTxiii | 70.5 |
CrMix—CrM—CrMi | 120.0 | Ox—MgT—MgTxiii | 70.5 |
Oiv—AlM—Ov | 83.81 (3) | Oxi—MgT—MgTxiii | 70.5 |
Oiv—AlM—Oii | 96.19 (3) | Oxii—MgT—MgTxiii | 179.998 (10) |
Ov—AlM—Oii | 96.19 (3) | O—MgT—MgTxiv | 70.5 |
Oiv—AlM—Oiii | 96.19 (3) | Ox—MgT—MgTxiv | 179.998 (10) |
Ov—AlM—Oiii | 180.0 | Oxi—MgT—MgTxiv | 70.5 |
Oii—AlM—Oiii | 83.81 (3) | Oxii—MgT—MgTxiv | 70.5 |
Oiv—AlM—Oi | 180.0 | MgTxiii—MgT—MgTxiv | 109.5 |
Ov—AlM—Oi | 96.19 (3) | O—MgT—MgTxv | 70.5 |
Oii—AlM—Oi | 83.81 (3) | Ox—MgT—MgTxv | 70.5 |
Oiii—AlM—Oi | 83.81 (3) | Oxi—MgT—MgTxv | 179.998 (10) |
Oiv—AlM—Ovi | 83.81 (3) | Oxii—MgT—MgTxv | 70.5 |
Ov—AlM—Ovi | 83.81 (3) | MgTxiii—MgT—MgTxv | 109.5 |
Oii—AlM—Ovi | 180.0 | MgTxiv—MgT—MgTxv | 109.5 |
Oiii—AlM—Ovi | 96.19 (3) | O—MgT—MgTxvi | 180.000 (11) |
Oi—AlM—Ovi | 96.19 (3) | Ox—MgT—MgTxvi | 70.5 |
Oiv—AlM—Fe3Mvii | 85.73 (2) | Oxi—MgT—MgTxvi | 70.5 |
Ov—AlM—Fe3Mvii | 42.066 (13) | Oxii—MgT—MgTxvi | 70.5 |
Oii—AlM—Fe3Mvii | 137.934 (13) | MgTxiii—MgT—MgTxvi | 109.5 |
Oiii—AlM—Fe3Mvii | 137.934 (13) | MgTxiv—MgT—MgTxvi | 109.5 |
Oi—AlM—Fe3Mvii | 94.27 (2) | MgTxv—MgT—MgTxvi | 109.5 |
Ovi—AlM—Fe3Mvii | 42.066 (13) | O—AlT—Ox | 109.5 |
Oiv—AlM—CrMviii | 42.066 (13) | O—AlT—Oxi | 109.5 |
Ov—AlM—CrMviii | 85.73 (2) | Ox—AlT—Oxi | 109.5 |
Oii—AlM—CrMviii | 137.934 (13) | O—AlT—Oxii | 109.5 |
Oiii—AlM—CrMviii | 94.27 (2) | Ox—AlT—Oxii | 109.5 |
Oi—AlM—CrMviii | 137.934 (13) | Oxi—AlT—Oxii | 109.5 |
Symmetry codes: (i) −x+3/4, y, −z+3/4; (ii) −x+3/4, −y+3/4, z; (iii) x, −y+3/4, −z+3/4; (iv) x+1/4, −y+1, z+1/4; (v) −x+1, y+1/4, z+1/4; (vi) x+1/4, y+1/4, −z+1; (vii) −x+5/4, y, −z+5/4; (viii) x, −y+5/4, −z+5/4; (ix) −x+5/4, −y+5/4, z; (x) −x+1/4, y, −z+1/4; (xi) x, −y+1/4, −z+1/4; (xii) −x+1/4, −y+1/4, z; (xiii) −x+1/2, −y+1/2, −z; (xiv) −x+1/2, −y, −z+1/2; (xv) −x, −y+1/2, −z+1/2; (xvi) −x, −y, −z. |
Experimental details
Crystal data | |
Chemical formula | Al0.41Cr1.42Fe0.65Mg0.40O4.00 |
Mr | 194.94 |
Crystal system, space group | Cubic, Fd3m |
Temperature (K) | 298 |
a (Å) | 8.31058 (5) |
V (Å3) | 573.98 (1) |
Z | 8 |
Radiation type | Mo Kα |
µ (mm−1) | 8.67 |
Crystal size (mm) | 0.07 × 0.07 × 0.07 |
Data collection | |
Diffractometer | Siemens P4 diffractometer |
Absorption correction | Analytical (Lundgren, 1982) |
Tmin, Tmax | 0.385, 0.445 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 5083, 220, 219 |
Rint | 0.049 |
(sin θ/λ)max (Å−1) | 1.171 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.015, 0.041, 1.40 |
No. of reflections | 220 |
No. of parameters | 14 |
No. of restraints | 2 |
Δρmax, Δρmin (e Å−3) | 0.96, −0.69 |
Computer programs: XSCANS (Siemens, 1991), XSCANS, SHELXTL-PC (Sheldrick, 1990), SHELXL97 (Sheldrick, 1997), ORTEP-3 (Farrugia, 1997), PLATON (Spek, 2003).
O—Mi | 1.9790 (4) | O—T | 1.9797 (8) |
Oii—M—Oiii | 83.81 (3) | O—T—Oiv | 109.5 |
Symmetry codes: (i) x, −y+3/4, −z+3/4; (ii) x+1/4, −y+1, z+1/4; (iii) −x+1, y+1/4, z+1/4; (iv) x, −y+1/4, −z+1/4. |
Chromite, (Fe2+)[Cr3+]2O4, which belongs to the spinel group of minerals, is an important material widely used in the metallurgical and chemical industries (Rollinson, 1973). The first structural studies concerning spinels were performed by Bragg (1915) and Nishikawa (1915). Spinels are mixed-valence compounds, AB2X4, and for most compounds, X is oxygen, A is a divalent cation and B is a trivalent cation. The spinels may be classified as normal spinel, (A)[B2]O4, and inverse spinel, (B)[A,B]O4, where (···) and [···] represent the tetrahedral and octahedral sites, respectively. In general, (A1-i, Bi)[Ai, B2-i]O42− represents a partially inverse spinel, where i is the degree of inversion. Chromite is a normal spinel with cubic Fd3m symmetry. Several overviews of spinels and their properties are available in the literature (O'Neill & Navrotsky, 1983; Valenzuela, 1994, and references therein).
In natural chromites, chromium and iron may be replaced by other metallic cations with similar charges and/or ionic radii. Therefore, because a wide variety of solid solutions may form in natural spinels, it can be helpful to use complementary experimental techniques in order to establish their structures. This strategy can be seen in several studies concerning chromites, e.g. powder and single-crystal X-ray diffraction (Salviulo et al., 2000), and Mössbauer spectroscopy and single-crystal X-ray diffraction (Lenaz et al., 2004; Figueiras & Waerenborgh, 1997).
This work forms part of a project involving the structural studies of natural spinels from Brazil. In particular, attention is focused on the cation distribution in this class of compounds. The structure of a magnesium- and aluminium-rich natural chromite has been analysed by single-crystal X-ray diffraction combined with Mössbauer spectroscopy and electron microprobe analysis. Single crystals were collected in a geological site near Piumhi town (20° 22' 00" S and 45° 56' 00" W), Minas Gerais, Brazil. A typically well shaped chromite single-crystal was mounted on a diffractometer to perform the X-ray data collection. This same crystal was later characterized by electron microprobe analysis as (Mg0.45 (8) Al0.41 (5) Cr1.49 (4) Fe0.65 (6))O4.00 (s.u. values calculated from measurements on four differents points using MgO, Al2O3, Cr2O3 and Fe2O3 as standards. Mössbauer spectra of a powder sample showed that around 70% of the total Fe content is tetrahedral Fe2+ and 30% is octahedral Fe3+. The refined chemical formula and cation distribution is (Mg2+0.40 (11), Al3+0.28 (5), Fe2+0.39 (4))[Al3+0.13 (5), Cr3+1.42 (6), Fe3+0.26 (4), Φ0.19]O4, where Φ represents the vacancy. This composition is in agreement with the microprobe analysis and the Mössbauer data. Moreover, the tetrahedral site occupancy is 1.1 (1) and the crystal electroneutrality is maintained with a total cation charge of +7.9 (3). Fig. 1 shows the structure of the Mg,Al-rich chromite. As can be seen, it has a trigonal distortion in the [111] direction. Table 1 gives selected geometrical parameters.