Is fuchsite a mica?
Fuchsite is mica: the IMA mica report files it under chromian muscovite, and the IMA list has no fuchsite row. The 10-to-50 rule says when chromian applies.
Yes, fuchsite is a mica. The 1999 IMA mica report (Rieder et al.) says the name should be abandoned in favour of chromian muscovite, and the September 2026 IMA list has no fuchsite row. Muscovite's Handbook sheet gives hardness 2.5 along the cleavage and 4 across it, density 2.77 to 2.88, and perfect {001} cleavage.
In short
- Fuchsite is muscovite with chromium. The Rieder et al. 1999, Nomenclature of the micas (Mineralogical Magazine 63, 267–279) lists, in its Table 5 of synonyms and varieties, fuchsite against chromian muscovite, and heads the table: names in the left column should be abandoned in favour of those in the right. Muscovite is a true mica; its species page is muscovite.
- No IMA row. A script over the September 2026 IMA-CNMNC Master List (pdftotext -layout; a row is a line whose first word is the name) finds no fuchsite, mariposite, verdite or chromochre. A plain substring search for “fuchs” finds one line, Louisfuchsite, an unrelated Ca-Mg-Ti-Al silicate approved in 2022. Muscovite and chromphyllite are rows.
- The 10-to-50 rule. The report allows a modifier such as chromian only if the element exceeds 10% but not 50% of the real occupancy of its position, and only on actual determinations. Muscovite, KAl2(Si3Al)O10(OH)2, has two octahedral positions, so chromian muscovite means 0.2 to 1.0 chromium per formula unit. Below 0.2 the report’s form is “chromium-containing”; at 2 chromium the formula is chromphyllite’s, KCr2(AlSi3O10)(OH)2.
- Seven chromian names, one answer. Counted by script over the report’s Table 5, 7 entries point to a chromian mica. All 7 name chromian muscovite; 4 name it alone (chromochre, fuchsite, gaebhardite, verdite), and 3 add chromian phengite (chrome mica, Chromglimmer, mariposite). Fuchsite carries no (s) or (v) mark, which the report says means it could not decide whether the name is a synonym or a variety.
- Green is not a determination. The report requires modifiers to rest on analysis. A green mica labelled fuchsite with no analysis is, in the report’s terms, muscovite; the colour suggests chromium but does not measure it. No chromium figure for fuchsite is given here, because no analysis was fetched.
| Name on a label | Rieder et al. 1999 replacement | Mark in Table 5 | IMA row, September 2026 | Chromium share of muscovite’s 2 octahedral sites |
|---|---|---|---|---|
| Fuchsite | Chromian muscovite | None: synonym or variety undecided | No row | 10% to 50%: 0.2 to 1.0 Cr, if analysed |
| Verdite | Chromian muscovite | None | No row | As chromian muscovite |
| Chromochre | Chromian muscovite | None | No row | As chromian muscovite |
| Gaebhardite | Chromian muscovite | None (the report footnotes gebhardite, a different mineral) | No row | As chromian muscovite |
| Mariposite | Chromian phengite, chromian muscovite | (s), synonym | No row | As chromian muscovite, or a phengite |
| Chrome mica; Chromglimmer | Chromian muscovite, chromian phengite | (s), synonym | No row | As chromian muscovite, or a phengite |
| Chromium-containing muscovite | The report’s form below the modifier threshold, or for an incomplete analysis | — | Muscovite: KAl2(Si3Al)O10(OH)2, A, 1998 s.p. | Under 10%: under 0.2 Cr |
| Chromphyllite | A valid species in the report’s Table 1: KCr2[]AlSi3O10(OH,F)2 | — | KCr2(AlSi3O10)(OH)2, A, 1995-052, Russia | Chromium in both sites: 2 Cr |
Is fuchsite a mica? The definition in one sentence
Fuchsite is chromian muscovite: a muscovite mica in which chromium takes part of the aluminium’s place. The IMA mica nomenclature report (Rieder et al. 1999), by the Mica Subcommittee the IMA’s Commission on New Minerals and Mineral Names appointed, puts fuchsite in its Table 5, Synonyms (s) and varieties (v), opposite chromian muscovite. The table’s heading reads: names in the left column should be abandoned in favour of those in the right.
The mica itself. The September 2026 IMA list gives muscovite as KAl2(Si3Al)O10(OH)2, approved, 1998 s.p. The Handbook muscovite sheet gives KAl2(Si3Al)O10(OH,F)2, perfect {001} cleavage, laminae flexible and elastic, hardness 2.5 parallel to [001] and 4 perpendicular to it, density 2.77 to 2.88, and green among its colours. Those numbers, and the bend test that separates a mica from other sheet minerals, are on the muscovite page and are not repeated here.
Where the name turns up. The International Gem Society quartz article says aventurine is quartzite with small reflecting crystals, usually a green, chrome-rich mica called fuchsite. A piece of aventurine is a quartz rock that contains fuchsite, not fuchsite. The live phantom crystal page records fuchsite among the minerals GIA names as drawing phantoms in quartz, and finds no IMA row for it either.
When is a mica chromian? The 10-to-50 rule, worked
A muscovite is chromian when chromium fills more than 10% and not more than 50% of its octahedral positions, which is 0.2 to 1.0 chromium per formula unit. The 1999 report says chemical deviations from end-member compositions may be expressed by adjectival modifiers, which must be based on actual determinations, and that a modifier like rubidian should be used only if the element exceeds 10%, but not 50%, of the real occupancy of the respective position. Its own example: a rubidian muscovite may contain between 0.1 and 0.5 Rb atoms per formula unit.
Worked for chromium. Muscovite’s formula, KAl2(Si3Al)O10(OH)2, has two octahedral aluminium positions. 10% of 2 is 0.2; 50% of 2 is 1.0. So chromian muscovite, and therefore fuchsite in the report’s reading, is a muscovite with 0.2 to 1.0 chromium in those two positions. Below 0.2 the report’s wording is chromium-containing muscovite, which it also prescribes when an analysis is incomplete. At the far end, the formula the IMA list gives chromphyllite, KCr2(AlSi3O10)(OH)2, has chromium in both positions.
What the rule does not let a label do. It does not turn colour into chemistry. The report says modifiers are not mandatory and must rest on determinations, so a green mica with no analysis is properly muscovite, and “fuchsite” on an old label records an observation, not a measurement. No chromium percentage for fuchsite is given on this page: none of the sources fetched for it carries a fuchsite analysis. Recording an old name beside a current one is covered on how to label and catalogue a mineral collection.
Fuchsite, mariposite, verdite: seven chromium names, one mica
Every chromium name in the IMA mica report’s synonym table points to chromian muscovite. Method: the report’s text (pdftotext -layout) between the Table 5 heading and the Table 6 heading was parsed by script into left and right columns, and every row whose right-hand entry contains “chromian” was listed. Seven rows do. All seven name chromian muscovite. Four name it alone: chromochre, fuchsite, gaebhardite and verdite. Three name it beside chromian phengite: chrome mica and Chromglimmer, marked (s), and mariposite, marked (s), which the table writes chromian phengite, chromian muscovite.
Synonym or variety. The table marks a synonym (s) and a variety (v), and says no symbol indicates cases where it could not be decided whether it is a synonym or a variety. Fuchsite has no symbol. So the report does not say fuchsite is a variety of muscovite, or a mere synonym for it; it says the name should give way to chromian muscovite.
One near-miss to ignore. The IMA list does contain a name with fuchs in it: Louisfuchsite, Ca2(Mg4Ti2)(Al4Si2)O20, approved 2022, which has no chromium or potassium in its formula. A label search that matches substrings will find it; it is not fuchsite. The mica and other sheet-mineral species are indexed on the species index.
When is fuchsite not chromian muscovite? Where the answer changes
Three cases change the answer. First, if an analysis puts chromium above half of the octahedral positions, the mica is past the chromian range; the end member with chromium in both positions is chromphyllite, a separate IMA species, KCr2(AlSi3O10)(OH)2, approved 1995, Russia. Second, mariposite and chrome mica may be chromian phengite rather than chromian muscovite, per the report’s table; the same report defines phengite as a series name: potassic dioctahedral micas between, or close to, the joins muscovite–aluminoceladonite and muscovite–celadonite. Third, a specimen sold as fuchsite may be a rock: aventurine is quartzite with fuchsite in it, per the IGS, which gives its density as usually 2.64 to 2.69; quartz is hardness 7 on its sheet.
In the hand. A mica splits into thin sheets along its perfect {001} cleavage, and its hardness along those sheets is 2.5 on the muscovite sheet. Quartz is 7. That difference, not the colour, says whether a green piece is the mica or the rock that contains it; the method is on how to test mineral hardness properly. Nothing on this site is offered for sale; the wanted list is the only commercial route.