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Braunite, Mn²⁺Mn₆³⁺O₈SiO₄, crystallizes in space group I 4₁/a c d and is stable over a wide range of temperatures, pressures and oxygen fugacity. It has a layered structure consisting of octahedral layers, containing Mn³⁺ (M2 and M3 sites), and layers containing eight-fold coordinated Mn²⁺ (M1 site), six-fold coordinated Mn³⁺ (M4 site) and tetrahedrally coordinated Si. Previous X-ray diffraction studies on Cu-substituted braunites showed that copper occupies predominantly the M1 site. Substitution of Cu²⁺ for Mn³⁺ on the M2, M3 and M4 sites was also considered but has not been confirmed beyond doubt.

In order to study further the mechanism of Cu substitution in braunite, the cation distribution in two Cu-substituted braunite samples synthesized by solid-state reaction has been determined by combined neutron and X-ray Rietveld refinements. The samples with starting bulk composition CuMn₆SiO₁₂ (Sample I) and Cu_0.5Mn_6.5SiO₁₂ (Sample II) were annealed at 1050^oC for about 12 days. The X-ray powder diffraction patterns were measured on a Siemens D500 diffractometer. The neutron powder diffraction pattern of Sample I was measured on the powder diffractometer MAN1 at the research reactor FRM/Garching. The neutron powder pattern of Sample II was measured on the time-of-flight diffractometer ROTAX at the spallation source ISIS, UK. The samples contained minor quantities of (Cu,Mn)₃O₄ spinel, Mn₃O₄, Mn₂O₃, -quartz and -cristobalite. The Rietveld refinements were carried out with the GSAS package.

Figure 3.3-5 shows the observed and calculated powder diffraction patterns for Sample I. The multiphase refinements gave reliability factors R_p = 6.2% for Sample I (a = 9.4200(2) Å, c = 18.6181(4) Å) and R_p = 12.4% for Sample II (a = 9.4247(2)Å, c = 18.6688(4) Å). The change of both the c parameter and the c/a ratio are in agreement with previous studies according to which c and c/a decrease with increasing copper content. The combined refinements show that in addition to the eight-fold coordinated M1 site, a significant fraction of Cu enters the M3 site. The selective substitution of copper onto the M3 site can be easily understood as the result of the Mn³⁺ in the M3 site having the strongest Jahn-Teller octahedral distortion.
 

Fig. 3.3-5: Part of the X-ray (upper diagram) and neutron (lower diagram) diffraction patterns of sample I: (+) observed intensities; (-) Rietveld refinement. The tick marks indicate the position of the allowed reflections (the reflections of braunite are indicated as the bottom line of ticks).