Meionite

Meionite (scapolite) crystals (cross-shaped) in a quartz matrix, 7.2 × 4.0 × 3.0 cm. Worcester County, Massachusetts.

General
Category| Tectosilicate
Scapolite group
Formula
(repeating unit)| Ca4Al6Si6O24CO3
Strunz classification| 9.FB.15
Crystal system| Tetragonal
Crystal class| Dipyramidal (4/m)
(same H-M symbol)
Space group| I4/m
Unit cell| a = 12.179(1) Å,
c = 7.571(1) Å, Z = 2
Identification
Color| Colorless, white, grey, pink, violet, blue, yellow, orange-brown, brown
Cleavage| Distinct/good on {100}{110}
Fracture| Irregular/uneven, conchoidal
Tenacity| Brittle
Mohs scale hardness| 5–6
|re|er}}| Vitreous, resinous, pearly
Streak| White
Diaphaneity| Transparent, opaque
Specific gravity| 2.74–2.78
Optical properties| Uniaxial (−)
Refractive index| 1.556 to 1.600
Birefringence| 0.024 to 0.037
References| [1][2][3][4]

Meionite is a tectosilicate belonging to the scapolite group with the formula Ca4Al6Si6O24CO3.[6] Some samples may also contain a sulfate group. It was first discovered in 1801 on Mt Somma, Vesuvius, Italy. It was named by Rene Just Haüy after μειωυ, the Greek word for less, in reference to the less acute pyramidal form as compared to vesuvianite.

As an end-member of the scapolite solid solution, meionite has the largest cell dimension [7] and very high thermal stability at high pressures. This indicates that meionite is one of the primary minerals in deep seated basic or intermediate magmatic processes. Meionite also breaks down to grossularite + kyanite \+ quartz \+ calcite at high pressure, similar to the upper pressure limits of anorthite. This sets its occurrence to the crustal rocks.[8]

## References

1. ↑ Mineralienatlas
2. ↑ Mindat.org
3. ↑ Webmineral.com
4. ↑ Handbook of Mineralogy
5. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine 85 (3): 291–320. doi:10.1180/mgm.2021.43. Bibcode: 2021MinM...85..291W. https://www.cambridge.org/core/journals/mineralogical-magazine/article/imacnmnc-approved-mineral-symbols/62311F45ED37831D78603C6E6B25EE0A.
6. ↑ Teertstra, David K.; Sherriff, Barbara L. (1997-04-25). "Substitutional mechanisms, compositional trends and the end-member formulae of scapolite" (in en). Chemical Geology 136 (3): 233–260. doi:10.1016/S0009-2541(96)00146-5. ISSN 0009-2541. Bibcode: 1997ChGeo.136..233T. http://www.sciencedirect.com/science/article/pii/S0009254196001465.
7. ↑ Eugster, Hans P.; Prostka, Harold J.; Appleman, Daniel E. (14 September 1962). "Unit-Cell Dimensions of Natural and Synthetic Scapolites". Science 137 (3533): 853–854. doi:10.1126/science.137.3533.853. PMID 17787337.
8. ↑ Newton, Robert C.; Goldsmith, Julian R. (1975). "Stability of the scapolite meionite (3CaAl2Si2O2 · CaCO3) at high pressures and storage of CO2 in the deep crust". Contributions to Mineralogy and Petrology 49 (1): 49–62. doi:10.1007/BF00371078.

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