Photon Factory Activity Report 2016 #34 (2017)

BL-10A/2014G653

Kinetic effect on the cation-ordering of mineral in crystal growth process
Toshiro Nagase1*, Takahiro Kuribayashi2, Yurie Nakamura2
1
The Tohoku University Museum, Tohoku Univ., 6-3 Aoba, Sendai 980-8578 Japan2
Department of Earth Science, Graduate School of Science, Tohoku Univ., Sendai 980-8578, Japan
1 Introduction
Symmetry of crystal structure is related to cations
ordering in the sites. Under thermodynamic behaviour,
the order-disorder of cations at a site is controlled bytemperature and pressure conditions. Usually, ordering
degree of cations is higher with decreasing temperature.
However, in two decades, it has been clarified that thegrowth process on the crystal surface affects the ordering
degree of cation sites. The kinetic effects of crystal
growth are also important factor for symmetry of crystal.
The texture caused by the order-disorder transition issimilar to that produced by growth process. Therefore, thecation ordering is complex and controversial issue. We
propose here a study based on X-ray diffraction analyses
to elucidate the cation ordering controlled by the growth
process. For that purpose, the cation ordering of eachgrowth domain of a crystal were measured by single Xray diffraction method.
2 Experiment
Samples of andradite-grossular garnet obtained fromKose Mine, Tenkawa village, Nara prefecture, Japan andfrom Kayas, Mali were studied. The crystal isapproximately 1 cm in diameter. The crystal form isconsisted of well-developed {110} faces and small {211}faces. Garnet crystals were cut into thin sections
perpendicular to the [001] direction and (110) face acrossthe center of the crystal for the optical observation,
chemical analysis and preparing the sample for the X-rayexperiment. The garnet crystals show birefringence under
the crossed polarizer and monoclinic or triclinic
symmetry is suggested.
The single-crystal X-ray diffraction experiment wasconducted using synchrotron radiation (λ = 0.71215 Å) at
the beam line BL-10A, Photon Factory, KEK, Japan. The
wavelength of synchrotron radiation was calibrated by theunit cell volume of the NIST ruby standard. Unit-cellparameters of the specimen were determined by leastsquares fitting of 24 centered reflections in the 2θ range
from 20˚ to 30˚. Reflection intensity data were collectedby ω−scan method for the half reciprocal space (±h, +k,±l) with maximum 2θ = 60°. The extinction rule for thebody-centered lattice (hkl: h + k + l = 2n) was applied forthe reflection data collection. The space groups of both
based on thesamples were determined as
observations of reflections violating the extinction rule.
Especially, for the {211} specimen, the ψ-scan method
was applied to distinguish a multiple scattering
phenomenon.

3 Results and Discussion[Kose sample]
The unit-cell parameters were obtained as follows; a =12.0540(17) Å, b = 12.0580(13) Å, c = 12.0612(13) Å,
α = 89.986(9)º, β = 90.994(10)º and γ = 89.994(10)º.
Violate reflections indicate that all glide plane is absent
should beand triclinic symmetry with the space group
suitable for the structure refinement. The refinement with3+
model was convergent with R = 3.19 %. The Fe
aoccupancies of the Y11, Y12, Y13, Y14, Y21, Y22, Y23 andY24 were determined as 86.9(6), 96.1(6), 90.9(6), 98.0(6),95.1(6), 91.3(6), 93.7(6) and 99.6(6) %, respectively. The3+Fe occupancy of the Y11 site is relatively lower, and thatof the Y 24 site is higher.
[Mali sample]
Reflections (e.g. 013, 103, 130) violated the extinction
rule for the space group Ia3d, and crystal structure
models for both the {110}c and {211}c sectors should. The
have triclinic symmetry with the space groupunit-cell parameters of both growth sectors wererespectively obtained as follows: a = 11.974(1) Å, b =11.974(1) Å, c = 11.974(1) Å, α = 89.999(9)˚, β =90.098(9)˚ and γ = 89.99(1)˚ for the {110}c sector; a =11.952(6) Å, b = 11.952(6) Å, c ＝ 11.957(6) Å, α =90.01(4)˚, β = 90.01(4)˚ and γ = 89.94(4)˚ for the {211}cmodels of both
sector. Structure refinements forsectors were converged with R = 3.02 % and R = 3.36 %,3+respectively. Fe occupancies of Y11, Y12, Y13, Y14,Y21, Y22, Y23 and Y24 sites in the {110}c sector weredetermined as follows: 66.9(8), 66.0(8), 63.4(8), 60.2(8),49.9(8), 49.5(8), 47.8(8) and 46.9(8) %. The eightoctahedral Y sites of {110}c can be divided into two
3+groups. In view of symmetry, Fe distribution in Y site of3+{110} sector seems to be orthorhombic. In contrast, Fe
distributions in Y sites of the {211}c sector were refined asfollows: 60.0(10), 41.0(10), 62.7(10), 69.1(10), 55.5(10),3+53.2(10), 55.7(10) and 43.5(10) %. The Fe occupancies
in Y sites of the {211} sector are at random. There are3+significant differences of Fe distribution trend inoctahedral Y sites between the {110}c and {211}c sectors.
3+
These observations implied that the differences of Fe
distribution trend between both sectors would be strongly
affected by the growth process in each sector.
* nagase@m.tohoku.ac.jp