Moganite
{{Short description|Silica mineral, rare monoclinic polymorph of quartz}}
{{Distinguish|Morganite (gem)}}
{{Infobox mineral
| name = Moganite
| category = Tectosilicates, quartz group
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| image =Moganite.jpg
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| caption = Moganite. Medio Almud ravine, Mogán, Gran Canaria, Spain. Height 5cm.
| formula = SiO2
| molweight =
| strunz = 4.DA.20
| dana = 75.01.04.02
| system = Monoclinic
| class = Prismatic (2/m)
(same H–M symbol)
| symmetry = I2/a
| colour =
| color = Grey
| habit = Massive
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| mohs = 6
| luster = Earthy, dull
| streak = White
| diaphaneity = Transparent
| gravity = 2.52 {{endash}} 2.58
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| opticalprop = Biaxial
| refractive = nα = 1.524
nγ = 1.531
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| references = {{cite web |url=https://www.mineralienatlas.de/lexikon/index.php/MineralData?mineral=Mogánite |title=Mineralienatlas Lexicon – Mogánite |website=Mineralienatlas |access-date=13 August 2023}}{{cite web|last1=Ralph|first1=Jolyon|first2=Ida|last2=Ralph|title=Moganite: Moganite Mineral Information and Data |website=MinDat|date=2007|access-date=1 August 2007|url=http://www.mindat.org/min-2739.html}}{{cite web|last=Barthelmy|first=David|year=2014|access-date=28 January 2025|url=http://webmineral.com/data/Moganite.shtml|title=Moganite Mineral Data|website=Mineralogy Database}}
}}
Moganite is a tectosilicate mineral with the chemical formula SiO2 (silicon dioxide) that was discovered in 1976. It was initially described as a new form of silica from specimens found in the Barranco de Medio Almud, in the municipality of Mogán on the island of Gran Canaria, in the Canary Islands (Spain),{{cite journal|last=Flörque|first=O.W.|author2= Jones, J.B.|author3= Schmincke, H.U.|date=1976|periodical=Zeitschrift für Kristallographie|volume=143|pages=156–165|title=A new microcrystalline silica from Gran Canaria}} receiving in a later work the name derived from this locality.{{cite journal|last=Flörque|first =O.W.|author2=Flörque, U.|author3=Giese, U.|date=1984|periodical=Neues Jahrbuch für Mineralogie (Abhandlungen)|volume=149|pages=325–336|title=Moganite, a new microcristaline silica-mineral}} In 1994 the International Mineralogical Association decided to disapprove it as a valid mineral, since it was considered indistinguishable from quartz.Origlieri, M., 1994. "Moganite: a New Mineral – Not!" Lithosphere. Aug. 2007 http://geopress.rbnet.net/moganite.htm Subsequent studies allowed the IMA to rectify it in 1999, accepting it as a mineral species.{{cite journal|last1=Grice|first1=Joel D.|first2=Giovanni|last2=Ferraris|title=New minerals approved in 1999 by the commission on new minerals and mineral names, International Mineralogical Association|journal=The Canadian Mineralogist|volume=38|issue=1|date=2000|pages=245–250|doi=10.2113/gscanmin.38.1.245|bibcode=2000CaMin..38..245G }}
It has the same chemical composition as quartz, but a different crystal structure.
This mineral has been mainly found in dry locales such as Gran Canaria and Lake Magadi.{{cite journal|last1=Heaney|first1=Peter J.|first2=Jeffrey E.|last2=Post|title=The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties|journal=Science|volume=255|date=1992|issue=5043|pages=441–443|doi=10.1126/science.255.5043.441|pmid=17842895|bibcode=1992Sci...255..441H|s2cid=32497622}} It has been reported from a variety of locations in Europe, India and the United States. Physically, it has a Mohs hardness of about 6, a dull luster and appears as a semitransparent gray in color.
Structural information
The main infrared spectroscopy (IR) differences between moganite and α-quartz occur in the wavenumber region below 650 cm−1. Above this wavenumber, the frequencies of Si–O stretching vibrations of moganite are almost identical to those of quartz. Additional moganite bands were recorded near 165, 207, 296, 343, 419, 576, and 612 cm−1.{{cite journal|last1=Zhang|first1=Ming|first2=Terry|last2=Moxon|title=Infrared absorption spectroscopy of SiO2-moganite|journal=American Mineralogist |volume=99 |issue=4 |year=2014 |pages=671–680|doi=10.2138/am.2014.4589|bibcode=2014AmMin..99..671Z|s2cid=56255006}}
Structural phase transition
Synchrotron X-ray powder diffraction data for moganite from {{cvt|100|to|1354|K|°C}} has revealed a reversible phase transition from space group I2/a to Imab at approximately {{cvt|570|K|°C}}.{{cite journal|last1=Heaney|first1=Peter J.|first2=Jeffrey E.|last2=Post |title=Evidence for an I2/a to Imab phase transition in the silica polymorph moganite at ~570 K|journal=American Mineralogist|volume=86|issue=11–12 |date=2001 |pages=1358–1366 |doi=10.2138/am-2001-11-1204|bibcode=2001AmMin..86.1358H|s2cid=6093768}}
The in-situ Fourier transform infrared spectroscopy shows that while the thermal responses of H2O and OH in moganite display similarities to agate, the spectra are not completely identical. Absorptions in the O–H stretching region reveal that dehydration and dehydroxylation is a multistage process. Although hydrogen loss starts below {{cvt|400|–|500|K|°C}}, hydrous species may well remain in moganite even at {{cvt|1060|K|°C}}.{{cite journal|last1=Zhang |first1=Ming |first2=Terry |last2=Moxon |title=In situ infrared spectroscopic studies of OH, H2O and CO2 in moganite at high temperatures. |journal=European Journal of Mineralogy |volume=24|issue=1|date=2012|pages=123–131 |doi=10.1127/0935-1221/2011/0023-2165|bibcode=2012EJMin..24..123Z }}
References
{{Reflist}}
{{commons category|Mogánite}}
{{Silica minerals}}
Category:Minerals in space group 15
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