Moganite

Moganite is an oxide mineral with the chemical formula SiO₂ (silicon dioxide) that was discovered in 1984.[2] It crystallises in the monoclinic crystal system. Moganite is considered a polymorph of quartz: it has the same chemical composition as quartz, but a different crystal structure.[3]

Moganite
General
Category	Tectosilicate, quartz group
Formula (repeating unit)	SiO2
Strunz classification	4.DA.20
Dana classification	75.01.04.02
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H–M symbol)
Space group	I2/a
Identification
Color	Grey
Crystal habit	Massive
Mohs scale hardness	6
Luster	Earthy, dull
Streak	White
Diaphaneity	Transparent
Optical properties	Biaxial
Refractive index	nα = 1.524 nγ = 1.531
References	[1][2][3]

In 1994, the International Mineralogical Association (IMA) had disapproved it as being a separate mineral because it was not clearly distinguishable from quartz.[4] It has only recently (1999) been approved as a valid mineral by the CNMNC, the Commission on New Minerals, Nomenclature and Classification (part of the IMA). Its IMA number is (CNMMN, No. 99-035).[5]

This mineral has been mainly found in dry locales such as Gran Canaria and Lake Magadi.[6] It has been reported from a variety of locations in Europe, India and the United States.[2] It was named for the municipality of Mogán on Gran Canaria.[3] 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.[7]

Structural phase transition

Synchrotron X-ray powder diffraction data for moganite from 100 to 1,354 K (−173 to 1,081 °C) has revealed a reversible phase transition from space group I2/a to Imab at approximately 570 K (297 °C).[8] 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 400–500 K (127–227 °C), hydrous species may well remain in moganite even at 1,060 K (790 °C).[9]

Further reading

• Zhu, Tianfei; Xin, Lipeng; Fang, Aiping; Li, Wei; Wang, Xiangjian; Lou, Xiaojie; Zhang, Ming (2017). "A Review of Research on a New Silica Polymorph—Moganite". Acta Mineralogica Sinica. 37 (1–2): 159–167. doi:10.16461/J.CNKI.1000-4734.2017.01.020.

References

1. "Mineralienatlas".
2. Ralph, Jolyon; Ralph, Ida (2007). "Moganite: Moganite Mineral Information and Data". MinDat. Retrieved 1 August 2007.
3. "Webmineral data".
4. Origlieri, Marcus (1994). "Moganite: a New Mineral – Not!". Lithosphere. Retrieved 1 August 2007.
5. Grice, Joel D.; Ferraris, Giovanni (2000). "New minerals approved in 1999 by the commission on new minerals and mineral names, International Mineralogical Association". The Canadian Mineralogist. 38 (1): 245–250. doi:10.2113/gscanmin.38.1.245.
6. Heaney, Peter J.; Post, Jeffrey E. (1992). "The Widespread Distribution of a Novel Silica Polymorph in Microcrystalline Quartz Varieties". Science. 255 (5043): 441–443. Bibcode:1992Sci...255..441H. doi:10.1126/science.255.5043.441. S2CID 32497622.
7. Zhang, Ming; Moxon, Terry (2014). "Infrared absorption spectroscopy of SiO₂-moganite". American Mineralogist. 99 (4): 671–680. Bibcode:2014AmMin..99..671Z. doi:10.2138/am.2014.4589. S2CID 56255006.
8. Heaney, Peter J.; Post, Jeffrey E. (2001). "Evidence for an I2/a to Imab phase transition in the silica polymorph moganite at ~570 K". American Mineralogist. 86 (11–12): 1358–1366. Bibcode:2001AmMin..86.1358H. doi:10.2138/am-2001-11-1204. S2CID 6093768.
9. Zhang, Ming; Moxon, Terry (2012). "In situ infrared spectroscopic studies of OH, H₂O and CO₂ in moganite at high temperatures". European Journal of Mineralogy. 24 (1): 123–131. Bibcode:2012EJMin..24..123Z. doi:10.1127/0935-1221/2011/0023-2165.