Montdorite

Where it forms, where it's found

Geological setting: Peralkaline rhyolite (comendite).
Type locality: Charlannes
La Bourboule
Issoire
Puy-de-Dôme
Auvergne-Rhône-Alpes
France
45.5791°, 2.7345°

3recorded occurrences

Source · OpenStreetMap

Physical

Colour: Green to brownish-green
Density: 3.15 g/cm³

Optical

Optical type: Biaxial (-)
Refractive index: 1.58 – 1.605
Surface relief: Moderate
Principal indices: n_α 1.580 · n_β 1.605 · n_γ 1.605
Dispersion: none

Michel-Lévy diagramhighlighted lineδ = 0.0250

Attainable Michel-Lévy rangeΔ ∈ [0, t·δ_max]250 nm1st order

Δ = 0Δ_max

Section thickness10 μmStage rotation0°

Thin-section mosaic70 grains · random 3D orientations

PPL

pleochroism per grain

XPL

independent extinctions · rotate the stage

Interference simulatorsingle grain · PPL ↔ XPL

PPL

pleochroism only · colour blends on rotation

XPL

interference colour · extinct every 90°

Retardation250 nm

Order1st order

XPL colour

Crystallography

Crystal system: Monoclinic
Space group: C2/m
Cell parameters: a = 5.310 Å · b = 9.20 Å · c = 10.175 Å
Cell angles: β = 99.9 °
Ratio a:b:c: 1 : 1.733 : 1.916
Type-locality form: Grains, 5-10 microns (max. 25 microns).
Comment: polytype 1M or 3T

Chemical composition

Constituent elements: Mass composition breakdown
Element Atoms At. mass g/mol Mass g/mol Mass share
8OOxygen Oxygen 12 15.999 191.988
37.88%
14SiSilicon Silicon 4 28.085 112.340
22.16%
26FeIron Iron 1.5 55.845 83.767
16.53%
19KPotassium Potassium 1 39.098 39.098
7.71%
9FFluorine Fluorine 2 18.998 37.996
7.50%
25MnManganese Manganese 0.5 54.938 27.469
5.42%
12MgMagnesium Magnesium 0.5 24.305 12.152
2.40%
1HHydrogen Hydrogen 2 1.008 2.016
0.40%
Total 506.827 100.00%
Mass share = atoms × atomic mass ÷ molar mass × 100
From IMA formula

Mass composition breakdown
Element	Atoms	At. mass g/mol	Mass g/mol	Mass share
8OOxygen	Oxygen	12	15.999	191.988	37.88%
14SiSilicon	Silicon	4	28.085	112.340	22.16%
26FeIron	Iron	1.5	55.845	83.767	16.53%
19KPotassium	Potassium	1	39.098	39.098	7.71%
9FFluorine	Fluorine	2	18.998	37.996	7.50%
25MnManganese	Manganese	0.5	54.938	27.469	5.42%
12MgMagnesium	Magnesium	0.5	24.305	12.152	2.40%
1HHydrogen	Hydrogen	2	1.008	2.016	0.40%
	Total	506.827	100.00%

Synonyms

Montdoriet

In other languages

German: Montdorit
Italian: Montdorite

Classification

Strunz

10th ed.

9.EC.15

9SilicatesClass
9.EPhyllosilicatesDivision
9.ECPhyllosilicates with mica sheets, composed of tetrahedral and octahedral netsGroup
9.EC.15MontdoriteSpecies

Dana

8th ed.

71.02.2a.10

71Phyllosilicates Sheets of Six-membered RingsClass
71.02Sheets of 6-membered rings with 2:1 layersType
71.02.2a— unnamed intermediate level —Group
71.02.2a.10MontdoriteSpecies

CIM

—

14.20.9

14Silicates not Containing AluminumClass
14.20Silicates of Fe and alkali metalsGroup
14.20.9MontdoriteSpecies

Group, growth & confusion

2 members

Literature, links & citation

Citations

1979Robert, J. -L., Maury, R. C. (1979) Natural occurrence of a (Fe, Mn, Mg) tetrasilicic potassium mica. Contributions to Mineralogy and Petrology, 68 (2) 117-123 doi:10.1007/bf00371894DOI: 10.1007/bf00371894
1979Fleischer, Michael, Burns, Roger G., Cabri, Louis J., Chao, George Y., Hogarth, D. D., Pabst, Adolf (1979) New Mineral Names. American Mineralogist, 64 (11-12) 1329-1334
1998Rieder, M., Cavazzini, G., D’Yakonov, Y.S., Frank-Kamenetskii, V.A., Gottardt, G., Guggenheim, S., Koval, P.V., Muller, G., Neiva, A.M.R., Radoslovich, E.W., Robert, J.L., Sassi, F.P., Takeda, H., Weiss, Z., Wones, D.R. (1998) Nomenclature of the micas. The Canadian Mineralogist, 36 (3) 905-912

Cite this entry

@misc{mineral2026,
  author    = {Mineral Index editorial board},
  title     = {Montdorite — Mineral Index},
  year      = {2026},
  url       = {https://mineralindex.org/minerals/montdorite-2762},
  note      = {Accessed 2026-05-11}
}