Is lepidolite a mica?
Yes. Lepidolite is a lithium mica, but a series name, not a species: the IMA mica report puts it on the trilithionite–polylithionite join. Hardness 2.5 to 4.
Yes. Lepidolite is a mica: the IMA mica report (Rieder et al. 1999) defines it as trioctahedral micas on or near the trilithionite–polylithionite join, light micas with substantial lithium. It is a series name, not a species; the IMA list has no lepidolite row. The Handbook sheet gives hardness 2.5 to 4, density 2.8 to 2.9.
In short
- Yes, and the report says which micas. The IMA mica nomenclature report (Rieder et al., Mineralogical Magazine 63, 267–279, 1999) lists lepidolite in its Table 4 of series names: trioctahedral micas on, or close to, the trilithionite–polylithionite join; light micas with substantial lithium. A series name is for incompletely investigated micas; the species are trilithionite and polylithionite.
- The IMA list agrees. A script over the September 2026 IMA-CNMNC Master List finds no line with the word lepidolite, zinnwaldite or biotite (hydrobiotite aside), and rows for polylithionite, KLi2AlSi4O10F2 (A, 1998 s.p.), and trilithionite, KLi1.5Al1.5(Si3Al)O10F2 (Rd, 1998 s.p., Sweden). Muscovite is a row too, KAl2(Si3Al)O10(OH)2.
- The 2.5 line, worked. The report calls a mica dioctahedral below 2.5 octahedral cations per formula unit and trioctahedral at 2.5 or more. From the IMA formulas: polylithionite and trilithionite 3.0, muscovite 2.0. The Handbook of Mineralogy lepidolite sheet’s two analyses give 2.54 (Pala, California) and 2.87 (Radkovice, Czech Republic): both trioctahedral.
- Documented failure: the Pala analysis. It clears the 2.5 line by only 0.04 and carries 0.99 lithium per formula unit, 0.51 short of trilithionite’s 1.5, at 3.70% Li2O against trilithionite’s worked 5.61%. The report’s “close to” has no number. It also warns that lithium cannot be measured by electron microprobe and that failing to establish it has caused a number of erroneous identifications.
- Weight does not help. Worked as 20 − 20/D on the sheets, a 20.00 g piece reads 12.86 to 13.10 g in water as lepidolite and 12.78 to 13.06 g as muscovite. Colour, light against dark, is the report’s own field division between lepidolite and zinnwaldite.
| Name | IMA list | Formula used | Octahedral cations per formula unit | Dioctahedral or trioctahedral (2.5 line) | Li2O by mass | Hardness | Density | 20.00 g piece in water | Report’s field description |
|---|---|---|---|---|---|---|---|---|---|
| Lepidolite (series) | No row | K(Li,Al)3(Si,Al)4O10(F,OH)2 (Handbook) | 2.54 (Pala); 2.87 (Radkovice), from the sheet’s analyses | Trioctahedral, both | 3.70%; 5.42% (sheet analyses) | 2.5 to 4 | 2.8 to 2.9; calc. 2.693 | 12.86 to 13.10 g | Light micas with substantial lithium |
| Polylithionite | A, 1998 s.p. | KLi2AlSi4O10F2 | 3.0 | Trioctahedral | 7.66% | 2 to 3 | 2.58 to 2.82 | 12.25 to 12.91 g | End-member of the lepidolite join |
| Trilithionite | Rd, 1998 s.p., Sweden | KLi1.5Al1.5(Si3Al)O10F2 | 3.0 | Trioctahedral | 5.61% | No sheet | No sheet | - | End-member of the lepidolite join |
| Zinnwaldite (series) | No row | KLiFe2+Al(AlSi3)O10(F,OH)2 (Handbook) | 3.0 on that formula | Trioctahedral | Not worked | 2.5 to 4 | 2.90 to 3.02 | 13.10 to 13.38 g | Dark micas containing lithium |
| Muscovite | A, 1998 s.p. | KAl2(Si3Al)O10(OH)2 | 2.0 | Dioctahedral | 0 | 2.5 ∥ [001], 4 ⊥ [001] | 2.77 to 2.88 | 12.78 to 13.06 g | A species, not a series |
| Biotite (series) | No row | - | - | Trioctahedral (report) | - | - | - | - | Dark micas without lithium |
Is lepidolite a mica or a mineral? What the IMA mica report says
Both answers are partly right: lepidolite is a mica, but the name is a series name, not a mineral species. The IMA mica report, prepared by the Mica Subcommittee appointed by the Commission on New Minerals and Mineral Names and approved by it (Rieder et al., Mineralogical Magazine 63, 267–279, 1999), lists lepidolite in Table 4, Series names: trioctahedral micas on, or close to, the trilithionite–polylithionite join; light micas with substantial lithium. The report says series names are intended for incompletely investigated micas, to be used by field geologists or petrographers, and that once a mica has been studied in detail the end-member names should be preferred.
The IMA list. A script over the September 2026 list (pdftotext -layout, whole-word search) finds no line with the word lepidolite, zinnwaldite, biotite, lithionite or lithia mica; the one biotite substring is hydrobiotite, a separate row. It finds polylithionite, KLi2AlSi4O10F2, A, 1998 s.p., with Denmark (Greenland) as country, and trilithionite, KLi1.5Al1.5(Si3Al)O10F2, Rd, 1998 s.p., Sweden. The Handbook lepidolite sheet files lepidolite in the mica group and ends its Polymorphism & Series line: a group name. The species page lepidolite carries the hardness-bracket check and the two-source check; neither is repeated here.
Is lepidolite a true mica? The interlayer and the 2.5 line
Yes on both of the report’s tests. The report divides micas by the interlayer cation: true micas if more than 50% of the interlayer cations are univalent, brittle micas if more than 50% are divalent. Both lepidolite analyses on the Handbook sheet have potassium, sodium, rubidium and caesium in that position: 100% univalent at Pala and 98.9% at Radkovice, where 0.01 calcium is the only divalent ion. Both are true micas.
The 2.5 line. The report calls a mica dioctahedral if it contains fewer than 2.5 octahedral cations per formula unit and trioctahedral at 2.5 or more. Worked from the IMA formulas: polylithionite Li2 + Al1 = 3.0; trilithionite Li1.5 + Al1.5 = 3.0; muscovite Al2 = 2.0. From the sheet’s analyses: Pala (Al1.50 Li0.99 Mn0.05) = 2.54; Radkovice (Li1.48 Al1.30 Mg0.05 Mn0.03 Fe0.01) = 2.87. The polylithionite sheet’s Mt. Karnasurt analysis, as the sheet groups it, gives 3.06. Lepidolite sits on the trioctahedral side; muscovite on the other.
Lithium, worked. On standard atomic weights, polylithionite is 3.56% lithium (7.66% Li2O) and trilithionite 2.61% (5.61% Li2O). The Radkovice analysis prints 5.42% Li2O and 1.48 lithium per formula unit, beside trilithionite’s 1.5. Lithium-bearing companions such as spodumene are covered on their own pages; petalite’s lithium figures are on is petalite a feldspar.
Where the check fails: the Pala analysis and the missing lithium
The Handbook’s own first lepidolite analysis is a marginal case. The Stewart mine, Pala, California, analysis clears the 2.5 line by 0.04 and carries 0.99 lithium per formula unit, 0.51 short of trilithionite’s end of the join, with 3.70% Li2O. The report’s definition says on, or close to, the join and gives no number for close. This page does not decide whether the Pala material is close enough; it notes that the sheet’s two analyses lie 0.49 lithium apart.
Lithium is easy to miss. The report says lithium, whose concentrations cannot be determined with electron microprobe techniques, is commonly overlooked in wet-chemical analyses because of its low molecular weight, and that failure to establish the concentration of lithium has caused a number of erroneous identifications. A lithium mica without a lithium determination is, in the report’s terms, an incompletely investigated one, and a series name is the honest label.
How do you tell lepidolite from muscovite and zinnwaldite?
Colour is the report’s own field division, and the sheets agree with it. Table 4 calls lepidolite light micas with substantial lithium and zinnwaldite dark micas containing lithium, on or near the siderophyllite–polylithionite join; biotite is dark micas without lithium. The lepidolite sheet prints pink, purple, rose-red, violet-grey, yellowish, white and colourless; the zinnwaldite sheet grey-brown, yellow-brown and pale violet among others. muscovite is a species and dioctahedral.
Weight does not separate them. Worked as 20 − 20/D, a 20.00 g piece reads 12.86 to 13.10 g in water as lepidolite, 12.78 to 13.06 g as muscovite, 12.25 to 12.91 g as polylithionite and 13.10 to 13.38 g as zinnwaldite. The two overlap from 12.86 to 13.06 g. Hardness overlaps too: 2.5 to 4 against muscovite’s 2.5 parallel to [001] and 4 perpendicular to it. Chromium in muscovite, the other common label question, is on is fuchsite mica. The method for weights is on how to measure specific gravity at home.
What should a lepidolite label say?
Lepidolite is a correct field name; trilithionite or polylithionite needs an analysis that includes lithium. The report prefers end-member names once a mica has been studied in detail, and allows the series name before that. A label can carry both, the series name and, where an analysis exists, the species; how to label and catalogue a mineral collection covers adding to an old card without overwriting it. The Handbook lepidolite sheet’s associates include spodumene, elbaite, amblygonite, columbite, cassiterite, topaz and beryl; tourmaline covers the elbaite side. The rest of this section is at the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.