Petermegawite
A valid IMA mineral species
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About Petermegawite
Dr Peter Megaw
Formula:
Al6(Se4+O3)3[SiO3(OH)](OH)9 · 10H2O
Colour:
colorless
Lustre:
Vitreous
Hardness:
2 - 2½
Specific Gravity:
2.27
Crystal System:
Orthorhombic
Name:
Named after Dr Peter Megaw, of Tucson, Arizona, an exploration geologist who has worked and collected minerals all over Mexico for over 35 years, and who is the Mindat expert and reviewer for Mexican minerals. He has published extensively on Mexican mines and minerals, and also promotes appreciation for mineral recognition among exploration geologists as the basis for "visual geochemistry" in their projects. Recently he has also been engaged as an exploration geologist in Potosí Department, Bolivia, and he puts in lots of volunteer hours helping the Tucson Gem and Mineral Society organize the world's most important mineral show.
New structure type. Unique combination of elements (at the upload time). The first selenate (selenite) - silicate mineral to date. Third Al-Se mineral after alfredopetrovite and llantenesite.
A polyoxometallate in terms of its anion - [Al6(SiO3OH)O6(OH)9(H2O)6]6− - topologically identical to the arsenic-bearing ones in bettertonite and penberthycroftite.
The clusters comprise 6-membered rings of edge-sharing AlO6 octahedra and SiO3(OH) tetrahedra residing at centres of each ring. Three SeO3 triangular pyramids are outside the particular ring. The clusters are merged via hydrogen bonds to form heteropolyhedral layers || (001). The water molecules - not bonded to any non-hydrogen cations - are located between the layers.
A polyoxometallate in terms of its anion - [Al6(SiO3OH)O6(OH)9(H2O)6]6− - topologically identical to the arsenic-bearing ones in bettertonite and penberthycroftite.
The clusters comprise 6-membered rings of edge-sharing AlO6 octahedra and SiO3(OH) tetrahedra residing at centres of each ring. Three SeO3 triangular pyramids are outside the particular ring. The clusters are merged via hydrogen bonds to form heteropolyhedral layers || (001). The water molecules - not bonded to any non-hydrogen cations - are located between the layers.
Unique Identifiers
Mindat ID:
55696
Long-form identifier:
mindat:1:1:55696:7
IMA Classification of Petermegawite
Approved
IMA Formula:
Al6(Se4+O3)3[SiO3(OH)](OH)9(H2O)6·4H2O
Type description reference:
Yang, Hexiong; Gu, Xiangping; Scott, Michael M.; Jenkins, Robert A.; Gibbs, Ronald B.; Mcglasson, James A.; Downs, Robert T. (2023) Petermegawite, Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, a New Al-Bearing Selenite Mineral, from the El Dragón Mine, Potosí, Bolivia. The Canadian Journal of Mineralogy and Petrology, 61 (5). 987-998 doi:10.3749/2300004
Classification of Petermegawite
4.JJ.
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
J : Arsenites, antimonites, bismuthites, sulfites, selenites, tellurites; iodates
J : Selenites with additional anions, with H2O
4 : OXIDES (Hydroxides, V[5,6] vanadates, arsenites, antimonites, bismuthites, sulfites, selenites, tellurites, iodates)
J : Arsenites, antimonites, bismuthites, sulfites, selenites, tellurites; iodates
J : Selenites with additional anions, with H2O
Mineral Symbols
As of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams.
| Symbol | Source | Reference for Standard |
|---|---|---|
| Pmw | IMA–CNMNC | Warr, L.N. (2021). IMA–CNMNC approved mineral symbols. Mineralogical Magazine, 85(3), 291-320. doi:10.1180/mgm.2021.43 |
Physical Properties of Petermegawite
Vitreous
Transparency:
Transparent
Colour:
Colorless
Streak:
White
Hardness:
2 - 2½ on Mohs scale
Tenacity:
Brittle
Cleavage:
Perfect
{001}
{001}
Density:
2.27(5) g/cm3 (Measured) 2.32 g/cm3 (Calculated)
Optical Data of Petermegawite
Type:
Biaxial (+)
RI values:
nα = 1.545(5) nβ = 1.554(5) nγ = 1.567(5)
Max. Birefringence:
δ = 0.022
Based on recorded range of RI values above.
Based on recorded range of RI values above.
Interference Colours:
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
The colours simulate birefringence patterns seen in thin section under crossed polars. They do not take into account mineral colouration or opacity.
Michel-Levy Bar The default colours simulate the birefringence range for a 30 µm thin-section thickness. Adjust the slider to simulate a different thickness.
Grain Simulation You can rotate the grain simulation to show how this range might look as you rotated a sample under crossed polars. Each grain retains its interference colour (retardation) while its brightness falls to black at extinction and reaches a maximum between extinction positions.
Surface Relief:
Low (positive)
Relative to Canada balsam mounting medium (n ≈ 1.537).
Relative to Canada balsam mounting medium (n ≈ 1.537).
This shows the grain boundary and Becke line effect under plane-polarised
light, based on the contrast between this mineral's average refractive
index and the mounting medium. It does not take into account mineral
colouration.
In focus
Interference Figure:
This shows the idealized biaxial acute bisectrix (Bxa) interference figure
- the conoscopic view for a grain cut perpendicular to the acute bisectrix, using
this mineral's 2V. The two small white dots mark the melatopes - the points
where the two optic axes emerge - and are shown only when they fall within the
field of view. The coloured bands are isochromatics, and the dark bands are
isogyres.
Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
No measured or calculated 2V is on file for this mineral, so the value used here (80°) is estimated from its recorded refractive indices and optic sign, not from a direct 2V measurement.
Rotate the stage: at 0°/90° the isogyres form a cross through the melatopes; at 45° they pull apart into curved hyperbolas. That splitting on rotation - absent in a uniaxial figure - is the standard diagnostic test for telling biaxial minerals from uniaxial ones. If 2V is large, the melatopes may fall outside the field of view, as they often do at the microscope too.
No measured or calculated 2V is on file for this mineral, so the value used here (80°) is estimated from its recorded refractive indices and optic sign, not from a direct 2V measurement.
Chemistry of Petermegawite
Mindat Formula:
Al6(Se4+O3)3[SiO3(OH)](OH)9 · 10H2O
Element Weights:
Crystallography of Petermegawite
Crystal System:
Orthorhombic
Class (H-M):
mm2 - Pyramidal
Space Group:
Cmc21
Cell Parameters:
a = 16.2392(2) Å, b = 10.9637(1) Å, c = 15.3367(2) Å
Ratio:
a:b:c = 1.481 : 1 : 1.399
Unit Cell V:
2,730.57 ų (Calculated from Unit Cell)
Z:
4
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 7.824 Å | (100) |
| 7.642 Å | (88) |
| 5.839 Å | (53) |
| 5.580 Å | (83) |
| 5.514 Å | (88) |
| 4.860 Å | (67) |
| 4.496 Å | (49) |
| 3.017 Å | (81) |
Reference:
Yang, Hexiong; Gu, Xiangping; Scott, Michael M.; Jenkins, Robert A.; Gibbs, Ronald B.; Mcglasson, James A.; Downs, Robert T. (2023) Petermegawite, Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, a New Al-Bearing Selenite Mineral, from the El Dragón Mine, Potosí, Bolivia. The Canadian Journal of Mineralogy and Petrology, 61 (5). 987-998 doi:10.3749/2300004
Type Occurrence of Petermegawite
General Appearance of Type Material:
aggregates of bladed or tabular crystals
Place of Conservation of Type Material:
collections of (1) the University of Arizona Mineral Museum, 1040 E. 4th Street, Tucson, AZ 85721-0077, USA, catalogue no. 22711 (holotype), and (2) the RRUFF Project, deposition no. R210008 (cotype)
Geological Setting of Type Material:
Oxidation zone of a Se-rich deposit
Associated Minerals at Type Locality:
Reference:
Yang, Hexiong; Gu, Xiangping; Scott, Michael M.; Jenkins, Robert A.; Gibbs, Ronald B.; Mcglasson, James A.; Downs, Robert T. (2023) Petermegawite, Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, a New Al-Bearing Selenite Mineral, from the El Dragón Mine, Potosí, Bolivia. The Canadian Journal of Mineralogy and Petrology, 61 (5). 987-998 doi:10.3749/2300004
Other Language Names for Petermegawite
Dutch:Petermegawiet
German:Petermegawit
Common Associates
Associations Based on Photo Data:
| 1 photo of Petermegawite associated with Chalcomenite | CuSeO3 · 2H2O |
Related Minerals - Strunz-mindat Grouping
| 4.JJ. | Kristekite | Cu2(H2O)4(UO2)(SeO3)3 · 4H2O |
| 4.JJ.X | Larisaite | Na(H3O)(UO2)3(SeO3)O2 · 4H2O |
| 4.JJ. | Borzęckiite | Pb(UO2)3(SeO3)2O2 · 3H2O |
| 4.JJ. | Amurselite | (NH4)2(UO2)5(SeO3)3O2(OH)2(H2O) · 8H2O |
| 4.JJ.05 | Marthozite | Cu2+(UO2)3(SeO3)2O2 · 8H2O |
| 4.JJ.10 | Guilleminite | Ba(UO2)3(SeO3)2O2 · 3H2O |
| 4.JJ.15 | Piretite | Ca(UO2)3(SeO3)2(OH)4 · 4H2O |
| 4.JJ.20 | Demesmaekerite | Pb2Cu5(UO2)2(SeO3)6(OH)6 · 2H2O |
| 4.JJ.25 | Haynesite | (UO2)3(Se4+O3)2(OH)2 · 5H2O |
| 4.JJ.30 | Favreauite | PbBiCu6O4(SeO3)4(OH) · H2O |
Other Information
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Internet Links for Petermegawite
mindat.org URL:
https://www.mindat.org/min-55696.html
Please feel free to link to this page.
Please feel free to link to this page.
Search Engines:
External Links:
Mineral Dealers:
References for Petermegawite
Reference List:
Miyawaki, Ritsuro, Hatert, Frédéric, Pasero, Marco, Mills, Stuart J. (2021) CNMNC Newsletter 64. IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) Mineralogical Magazine, 86 (1) 178-182 doi:10.1180/mgm.2021.93
Yang, Hexiong; Gu, Xiangping; Scott, Michael M.; Jenkins, Robert A.; Gibbs, Ronald B.; Mcglasson, James A.; Downs, Robert T. (2023) Petermegawite, Al6(Se4+O3)3[SiO3(OH)](OH)9⋅10H2O, a New Al-Bearing Selenite Mineral, from the El Dragón Mine, Potosí, Bolivia. The Canadian Journal of Mineralogy and Petrology, 61 (5). 987-998 doi:10.3749/2300004
Localities for Petermegawite
Showing 1 localities.
symbol to view information about a locality.
The Locality List
- This locality has map coordinates listed.
- This locality has estimated coordinates.
ⓘ - Click for references and further information on this occurrence.
? - Indicates mineral may be doubtful at this locality.
- Good crystals or important locality for species.
- World class for species or very significant.
(TL) - Type Locality for a valid mineral species.
(FRL) - First Recorded Locality for everything else (eg varieties).
All localities listed without proper references should be considered as questionable.
Bolivia (TL) | |
| Miyawaki et al. (2021) |
Quick NavTopAbout PetermegawiteUnique IdentifiersIMA Classification Classification Mineral SymbolsPhysical Properties Optical Data Chemistry Crystallography X-Ray Powder DiffractionType Occurrence Other LanguagesCommon AssociatesStrunz-MindatOther InformationInternet Links References Localities Locality List
El Dragón mine, Porco Municipality, Antonio Quijarro Province, Potosí, Bolivia