Photon Factory Activity Report 2005 #23 Part B (2006)
Crystallography

10A/2003G208

The crystal structures of natural norbergite at 4.7 and 6.3 GPa conditions
Takahiro KURIBAYASHI1*, Masahiko TANAKA2, Yasuhiro KUDOH1
Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan
2
WEBRAM/SPring8, National Institute for Materials Science, 1-1-1 Kouto, Sayo-cho, Hyogo,679-5148, Japan

1

Introduction
Norbergite (Mg2SiO4⋅Mg(OH,F)2), which is a hydrous
magnesium silicate mineral, is known as one of thehumite minerals described as nMg2SiO4⋅Mg(OH,F)2 (n=14). This mineral has the similar structure to forsterite(Mg2SiO4), which is recognized as the most importantmineral in the mantle of the earth, and is given muchattention as the carriers and reservoirs of water in thesubduction zone. The stability of humite minerals has
been studied some researchers (ex. [1], [2]). Forsterite andthe humite minerals constitute the polysomatic series.
We collected the X-ray reflection intensity data sets ofa natural norbergite at ambient, 4.7 and 6.3 GPa in order
to investigate its crystal structure. In this report, we
described the crystal structures at 4.7 and 6.3 GPa.
Experimental Procedure
The sample used for this study is from Sterling Hill
Mine, New Jersey, U.S.A. A single crystal of norbergite
3(60×50×40 µm in size) was mounted in a modified
Merrille-Bassett type diamond anvil cell with a small
piece of a ruby crystal, which used for the pressure
calibration. The 4:1 fluid mixture of methanol andethanol was used for the pressure medium and a SUS301
stainless plate used for a gasket. Pressure was determinedby the ruby fluorescence method [3]. The wavelength of
X-ray was calibrated by the unit cell volume of the rubystandard crystal at ambient temperature (λ = 0.7002 Å).
The X-ray diffraction intensities were measured using an
automated four-circle X-ray diffractometer installed at thebeam line BL-10A, Photon Factory, High Energy
Accelerator Research Organization. The X-ray reflection
intensity data sets at 4.7 and 6.3 GPa were collected up to-1
sinθ/λ < 0.82 Å (maximum 2θ is 70.0°). The 3/8 of areciprocal sphere was measured. A total of 1346 (for 4.7GPa) and 1259 (for 6.3 GPa) reflections were obtained.
After background and Lorentz corrections were applied to
all measured reflections, the symmetrically equivalent
reflections were averaged by the Laue symmetry of mmm.
The number of 441 and 399 reflections (Fo > 2.0σ(Fo))
were used for the structure refinement, respectively. The
previous model at ambient conditions [4] was used for the
initial parameters of the norbergite structure (Space groupPbnm (#62)). The sigma weights were applied to the all
obtained reflection data. Final agreement factors were R= 5.7 and 6.7 % and Rw = 5.4 and 5.9 %, respectively. All

calculations were performed using the teXsan
crystallographic software. The unit cell parameters ofnorbergite were reported by our previous report [5].
Results
Polyhedral bulk moduli of each polyhedron werecalculated using the Birch-Murnaghan equation of statewith a pressure derivative K′ = 4. KM2 = 106 (5) GPa, KM3= 113(2) GPa and KTet = 293(15) GPa. In norbergite
structure, octahedral bulk moduli were smaller than thecorresponded those of humite minerals and forsterite (KM= 130 - 150 GPa, [5], [6]). In contrast, KTet of tetrahedronis smaller than (KTet = 190 GPa, [6]) that of forsterite,
which has the same environment for tetrahedron (Fig. 1).
The octahedral-tetrahedral shared edges (O2-O3 and O3-3O3; βO2-O3 and βO3-O3 were 1.25(19) and 0.97(19) × 10 GPa
1, respectively) were stiffer than the other O-O distances-3-1(averaged the other βO-O was 2.89 × 10 GPa ).

Figure 1. Octahedral - tetrahedral column in norbergite
structure. Same column structure exists in forsterite
structure.
References[1] B. Wunder, Contrib. Mineral. Petrol., 132, 111-120,(1998). [2] Bunley and Navrotsky, Am. Miner., 81, 317326, 1996. [3] G. J. Piermarini et al. J. Appl. Phys., 46,2774-2780, (1975). [4] Gibbs and Ribbe, Am Miner., 54,376-390 (1969). [5] Kuribayashi et al., KEK activity
report, 2004-22, part B, 200, (2005). [6] Kuribayashi etal., J. Mineral. Petrol. Sci., 99, 118-129, (2004).
* t-kuri@mail.tains.tohoku.ac.jp

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