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| Formula | (Na,K)Na2(Mg,Fe2+)4Fe3+Si8O22(OH)2 - NaNa2(Mg,Fe2+)4AlSi8O22(OH,F)2 |
| | Optic class & sign | Biaxial negative |
| | Optical orientation | |
| | Optical plane | |
| | Relief | Moderate to high |
| | Refractive indices | nx = 1.605 -1.697
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ny = 1.630 -1.700
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nz = 1.634 -1.709
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| | Birefringence (max.) | 0.005 - 0.029 |
| | | |
| | Optic Angle
| 2Vx
= 0 - 87° |
| | 2Vz
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| | Sign of elongation | |
| | Interference figure | |
| | Colour / pleochroism | |
| | Zoning | |
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| Form | Habit | |
| | | Surface | |
| | Cleavage | |
| | Twinning | |
| | Extinction | |
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| Reaction textures | |
| | Alteration / decomposition | |
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| Occurence | Ign | |
| | | Met | |
| | | Sed | |
| | | Hyd | |
| | | Other | |
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| Distinctive properties | The birefringence of Arf-MgArf is commonly in the range 0.005 - 0.016; eckermannite would typically have a higher Δn, but lower refractive indices. |
| | Additional comments | n - Δn diagram: The plotted data are from Miyashiro (1957), J. Fac. Sci. Univ. Tokyo, Section II 11, 57-83, Deer, Howie & Zussman, Vol. 2B (1997), and Oberti et al. (2015), Am. Mineralogist 100, 909-914.
Data on 2V are scarce and not well-constrained. The low-2V range where the change of orientation of the optic axial plane occurs is not covered at all in the literature.
Optical data on eckermannite are rare in general. The n - Δn diagram insert shows the restricted compositional range of published arfvedsonite-eckermannite analyses that have optical data attached.
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