Mawbyite
Mawbyite is a lead iron zinc arsenate that was named in honor of Maurice Alan Edgar Mawby. It has been approved by the IMA in 1988, and was published just a year after being described by Pring. Mawbyite is a member of the tsumcorite group, the monoclinic dimorph of carminite. It was first believed to be tsumcorite; however, crystal-structure determination showed iron and zinc occupying the same crystallographic site instead, and through the analysis it turned out mawbyite is isostructural with tsumcorite, meaning the two share a similar formula. More accurately, mawbyite appears to be the ferric analogue of the aforementioned mineral. The relationship between helmutwinklerite – which shares a similar formula with tsumcorite's – and mawbyite had been suggested, but due to lack of data it remains unclear. A full crystal-structure analysis is required in order to understand the relationship between their structures.
| Mawbyite | |
|---|---|
| General | |
| Category | Mineral |
| Formula (repeating unit) | Pb(Fe3+,Zn)2(AsO4)2(OH)2 |
| IMA symbol | Mby |
| Strunz classification | 08.CG.15 |
| Dana classification | 40.02.09.04 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic H-M Symbol: 2/m |
| Space group | B2/m |
| Unit cell | 391.13 |
| Identification | |
| Formula mass | 649.02 |
| Color | Pale brown to orange-brown to bright reddish brown |
| Crystal habit | Spherical or wheat sheaf shaped crystals |
| Twinning | Common, V-shaped at about {100} |
| Cleavage | Good on {001} |
| Fracture | Conchoidal |
| Mohs scale hardness | 4 |
| Luster | Adamantine |
| Streak | Orange-yellow |
| Diaphaneity | Transparent to Translucent |
| Specific gravity | 5.365 |
| Density | 5.5 |
| Optical properties | Biaxial (−) |
| Refractive index | nα = 1.940(2) nβ = 2.000(2) nγ = 2.040(2) |
| Birefringence | 0.100 |
| Pleochroism | Weak Brown to reddish brown |
| 2V angle | Measured: 80° (5) Calculated: 76° |
| Dispersion | Relatively weak |
| Length fast/slow | Length-fast |
| Ultraviolet fluorescence | None |