X-Ray Diffraction and Vibrational Spectroscopic Characteristics of Hydroxylclinohumite from Ruby-Bearing Marbles (Luc Yen District, Vietnam)

V Hurai,R Thomas,J Luptáková,M Pentrák,M Wierzbicka-Wieczorek,A Swierczewska,M Huraiová

doi:10.1155/2014/648530

Abstract

A honey-yellow hydroxylclinohumite from ruby-bearing marbles of the Luc Yen district in northern Vietnam was characterized by electron microprobe (EPMA), single-crystal X-ray diffraction (XRD), micro-Raman, and Fourier-transform infrared (FTIR) spectroscopy. The studied crystals correspond to nearly ideal clinohumite with the structural formula 4[Mg2SiO4]·[(Mg,Fe,Ti)(OH,F)2] and roughly equal F and OH proportions. Crystal structure analysis showed Ti substitution for Mg only at the Mg3 site. A Fourier-difference map revealed one hydrogen site associated with ninth oxygen atom. The calculated O–H bond distance was shorter than that in other natural clinohumites. FTIR revealed bands corresponding to combination of OH-stretching with Mg–OH and/or Fe–OH bending modes, combinations of OH−and Fe–OH vibrations, combination of fundamental bands of the Si–OH bonding, combination of OH−and Si–OH vibrations, and first (2νOH) and the second (3νOH) overtones of the OH-stretching vibration mode. Two groups of OH-stretching vibration and FTIR absorption bands at 3390–3420 cm−1and 3560–3580 cm−1show reversible temperature-dependent shift. The low-frequency bands absent in pure synthetic hydroxylclinohumites are assigned to OH-planar defects caused by Ti-for-Mg substitution.

Highlights

Humite-group minerals have the general formula n(M2SiO4) M1−xTix(OH,F)2−2xO2x, where M stands for Mg, Fe, Mn, Ca, Zn in decreasing order of abundance, x < 0.5 and n numbers from 1 to 4 define norbergite, chondrodite, humite and clinohumite, respectively [1]
The surplus titanium (0.038– 0.039 a.p.f.u.) over the negligible tetrahedrally coordinated Ti together with the Mg-excess over tetrahedrally coordinated Mg (0.907–0.917 a.p.f.u.), plus minor amounts of Fe (0.040–0.043 a.p.f.u.) and Mn (0.002 a.p.f.u.) sum to 1 atom per formula unit (a.p.f.u.) in the octahedrally coordinated M-site, resulting in the nearly ideal chemical formula 4[Mg2SiO4]⋅[(Mg,Fe,Ti)(OH,F)2] with n = 4 that is diagnostic of clinohumite
Predominance of OH(±O) over F points to hydroxylclinohumite; the exact F/OH ratio remains uncertain due to the unknown content of oxygen anions, which may reach up to ∼0.08 a.p.f.u., assuming their bonding with octahedrally coordinated Ti [1], and due to the matrix effect on the F concentrations analysed in the silicate mineral using the LiF standard [35]

Summary

Introduction

Humite-group minerals have the general formula n(M2SiO4) M1−xTix(OH,F)2−2xO2x, where M stands for Mg, Fe, Mn, Ca, Zn in decreasing order of abundance, x < 0.5 and n numbers from 1 to 4 define norbergite, chondrodite, humite and clinohumite, respectively [1]. The humite-group minerals have attracted attention owing to the suspicion of being volatile carriers in subduction zones and the overlying mantle wedge. Humite-like defects (OH-bearing monolayers) were discovered in nominally anhydrous mantle olivine [16, 17] and the assignment of the specific IR absorption bands in olivines to the clinohumite-like point and planar defects has been a matter of extensive ongoing research [18,19,20] In spite of their importance for estimating the amount of water in the upper mantle, interpretation of the vibrational and absorption spectra of the humite-group minerals remains inconclusive, partly due to their complexity and partly due to International Journal of Mineralogy

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Conclusion