Mawbyite
Mawbyite | ||
---|---|---|
2V angle Measured: 80° (5) | Calculated: 76° | |
Dispersion | Relatively weak | |
Length fast/slow | Length-fast | |
Ultraviolet fluorescence | None |
Mawbyite is a lead iron zinc arsenate that was named in honor of Maurice Alan Edgar Mawby. It has been approved by the
Visual properties
Mawbyite occurs in pale brown to orange-brown to bright reddish brown colors. The iron in mawbyite's formula may be replaced with minor zinc.[1] Its color seems to be closely correlated to its composition. Specimens which contain an equal ratio of iron to zinc in their composition tend to be more orange, while the reddish specimens' composition is closer to the pure iron end member. The color of the streak of the mineral doesn't change however despite the range in composition. It has a weak pleochroism, which is an optical phenomenon. Depending on which axis the mineral is inspected on, it appears to be changing colors. Mawbyite appears to go from brown to reddish brown in color. Just like its color, mawbyite's optical properties differ based on its composition. With increased iron content, its refractive indices also increase. For pure iron end members, all of the indices are above 2.
Mawbyite has a number of habits, the most prevalent one being a "dogtooth"-like bladed crystals, which can reach up to 0.15 mm in length, and the dominant forms include {110}, {101} and with minor {001}. It can form cylindrical, hemispherical, and wheat-sheaf aggregates with a platy or spongy appearance. Another habit is in the form of scattered clusters of prismatic crystals as coatings, which can reach up to 0.2 mm in length. It may also occur as tabular crystals with V-shaped twins with composition plane (100). It rarely forms tabular crystals however, prismatic habit is more common. The mineral had also been observed to be able to form thin compact crusts.[2] Mawbyite has a conchoidal fracture, where breaking it results in smooth and curvy surfaces resembling a seashell.[4]
Chemical properties
The pure ferric end-member of the mineral is dimorphous with carminite, meaning they share the same formula but form different crystal structures. Furthermore, the unit cells of the two minerals are related.[2] Mawbyite does not show any radioactive or fluorescent properties. It mainly consists of lead (35.12%), oxygen (24.16%), arsenic (23.09%) and iron (16.35%) by weight, but otherwise has zinc (1.01%) and contains a negligible amount of hydrogen (0.28%) as well.[4] Due to lack of material of any composition, it could not be determined whether the mineral has water content in its composition, hence it is unknown whether the endmember is anhydrous or not. Due to the similarities between mawbyite and tsumcorite in systematic absences in the powder data,[2] and due to laue[clarification needed] symmetry,[3] the space group is believed to be C2/m. It forms drusy crusts in spessartine- and quartz-rich host rocks in small cavities and on fractures, by the oxidation of primary sulfides and arsenites under less acidic pH conditions compared to its dimorph, which forms at around a level of 3 in pH.[2] The type of the principal rocks which host the mineral are almost entirely made of quartz and spessartine. These hosts are friable to compact granular metamorphic rocks. In quartz, they line solution cavities.[2] Due to its appearance, it may be confused with another lead-iron-arsenate, arsenbrackebuschite, although the latter has a much higher lead content.[1]
Occurrences
At
References
- ^ a b c "Mawbyite". www.mindat.org. Retrieved 2023-01-25.
- ^ a b c d e f g h Pring, Allan; McBriar, E. Maud; D. Birch, William (1988). "Mawbyite, a new arsenate of lead and iron related to tsumcorite and carminite, from Broken Hill. New South Wales" (PDF). American Mineralogist. 74: 1377–1381.
- ^ S2CID 53767166.
- ^ a b "Mawbyite Mineral Data". www.webmineral.com. Retrieved 2023-01-25.
- ^ "Mawbyite" (PDF). Handbook of Mineralogy.