Is okenite asbestos?
Not by name: UK law lists six fibrous silicates as asbestos, and okenite, Ca10Si18O46(H2O)18, is not one. Its sheet records a fibrous habit. Not safety advice.
Not by name. Regulation 2 of the UK Control of Asbestos Regulations 2012 defines asbestos as six named fibrous silicates; okenite, Ca10Si18O46(H2O)18, is not one, and its formula fits none of the six compositions in HSE’s analysts’ guide, all with magnesium or iron. Its sheet records fibrous balls to 4 cm. This is not safety advice.
In short
- What the law names. The Control of Asbestos Regulations 2012, regulation 2 reads: asbestos means the following fibrous silicates, (a) asbestos actinolite, (b) asbestos grunerite (amosite), (c) asbestos anthophyllite, (d) chrysotile, (e) crocidolite and (f) asbestos tremolite, each with its CAS number. Okenite is not on the list, and a script finds the word 0 times on the regulation’s page.
- The two-family check. HSE’s HSG248, Asbestos: The Analysts’ Guide says chrysotile is a sheet silicate of the serpentine group and the other types of asbestos are chain silicates of the amphibole group. Every nominal composition in its Table A2.1 carries magnesium or iron, from Mg3(Si2O5)(OH)4 to Na2Fe2+3Fe3+2(Si8O22)(OH)2. Okenite, Ca10Si18O46(H2O)18, has neither, so it is none of the six under another name.
- The seven-sheet ledger. The Handbook of Mineralogy sheets for actinolite, grunerite, anthophyllite, clinochrysotile, riebeckite (whose sheet records asbestiform “crocidolite” in iron formations) and tremolite all record a fibrous habit, and so does okenite’s. Five of the six regulated sheets also print asbestiform; tremolite’s and okenite’s do not. Fibrous is a habit word, shared well beyond the list.
- Documented failure: elastic. Okenite’s tenacity is printed as elastic, and the anthophyllite sheet prints brittle; fibers are elastic. The word that might seem to set okenite apart is printed for a regulated mineral’s fibres too, so no line of a sheet answers the legal question.
- No verdict here. Not named is not the same as assessed safe, and this page does not say okenite is or is not a hazard, or anything about its fibres beyond the sheet’s words. Questions go to HSE’s asbestos pages and a clinician. This is not health, safety or legal advice.
| As written in regulation 2 | Species sheet | Group (HSG248) | Nominal composition (HSG248) or formula | Mg or Fe? | Habit words on the sheet | Asbestiform printed? | Tenacity printed |
|---|---|---|---|---|---|---|---|
| asbestos actinolite | Actinolite | Amphibole | Ca2(Fe2+,Mg)5(Si8O22)(OH)2 | Both | Bladed crystals to 15 cm; radiating fibrous to asbestiform | Yes | Brittle, tough in fibrous aggregates |
| asbestos grunerite (amosite) | Grunerite | Amphibole | (Fe2+,Mg)7(Si8O22)(OH)2 | Both | Acicular or fibrous radiating crystals; may be asbestiform | Yes | Brittle |
| asbestos anthophyllite | Anthophyllite | Amphibole | (Mg,Fe2+)7(Si8O22)(OH)2 | Both | Bladed aggregates; commonly lamellar or fibrous, asbestiform | Yes | Brittle; fibers are elastic |
| chrysotile | Clinochrysotile | Serpentine | Mg3(Si2O5)(OH)4 | Mg | Asbestiform, fibrous along [100], curled to cylindrical | Yes | Not printed |
| crocidolite | Riebeckite | Amphibole | Na2Fe2+3Fe3+2(Si8O22)(OH)2 | Fe | Prismatic crystals to 20 cm; commonly fibrous, asbestiform | Yes | Brittle |
| asbestos tremolite | Tremolite | Amphibole | Ca2Mg5(Si8O22)(OH)2 | Mg | Elongated, stout prismatic or flattened bladed crystals to 20 cm; also fibrous, granular or columnar aggregates | No | Brittle |
| Not named | Okenite | Neither; the sheet prints no group | Ca10Si18O46(H2O)18 (IMA) | Neither | Bladed crystals or laths, typically fibrous; compact radial fibrous balls to 4 cm | No | Elastic |
Is okenite asbestos? What regulation 2 names
In UK law, asbestos is a list of six named minerals, and okenite is not on it. Regulation 2 of the Control of Asbestos Regulations 2012, read on legislation.gov.uk on 4 November 2026 in its latest available revised form, says: asbestos means the following fibrous silicates, and lists (a) asbestos actinolite, CAS No 77536-66-4; (b) asbestos grunerite (amosite), CAS No 12172-73-5; (c) asbestos anthophyllite, CAS No 77536-67-5; (d) chrysotile, CAS No 12001-29-5 or CAS No 132207-32-0; (e) crocidolite, CAS No 12001-28-4; and (f) asbestos tremolite, CAS No 77536-68-6.
Okenite, as recorded. The September 2026 IMA list writes Ca10Si18O46(H2O)18, grandfathered from 1828. The Handbook of Mineralogy okenite sheet records bladed crystals or laths, typically fibrous with [010] as the fibre axis, aggregates of curved crystals and compact radial fibrous balls to 4 cm, with an elastic tenacity. The species page is okenite.
Not repeated here. The name test and its fibrous-habit survey are on is scolecite asbestos, which already lists okenite as not named; the tensile-strength and refractive-index points are on is tremolite asbestos; the counting rule and CAS pair on is actinolite asbestos; the IMA name check on is tiger’s eye asbestos.
Could okenite be one of the six under another name? The two-family check
No: its composition fits none of the six. HSE’s analysts’ guide, HSG248 (second edition, 2021), paragraph A2.2, says chrysotile is classified as a sheet silicate and is a member of the serpentine group, and that the other types of asbestos are chain silicates in the amphibole group of minerals. Its Table A2.1 gives each regulated variety a non-asbestos mineral analogue and a nominal composition: chrysotile Mg3(Si2O5)(OH)4; crocidolite, riebeckite, Na2Fe2+3Fe3+2(Si8O22)(OH)2; amosite, grunerite, (Fe2+,Mg)7(Si8O22)(OH)2; anthophyllite (Mg,Fe2+)7(Si8O22)(OH)2; actinolite Ca2(Fe2+,Mg)5(Si8O22)(OH)2; tremolite Ca2Mg5(Si8O22)(OH)2.
The check, by script. All six of those compositions contain magnesium, iron or both. Okenite’s IMA formula contains calcium, silicon, oxygen and hydrogen only, and the analysis on its sheet, from Bombay, prints no MgO or FeO and 0.01% Fe2O3. Its sheet prints no mineral group. Merlino 1983, American Mineralogist 68, 614–622 describes its structure as tetrahedral sheets and double chains made by pairing two wollastonite chains, and calls okenite the first silicate with both. Having a chain and a sheet does not make it an amphibole or a serpentine, and its formula is none of the six in the table. A script finds the word okenite 0 times in HSG248, against 64 for chrysotile.
What the check does not do. It shows that a specimen correctly identified as okenite is not one of the six named minerals. It says nothing about risk, and it cannot identify a specimen: HSG248 describes fibres being positively identified as one of the six regulated types on their detailed optical properties by polarised light microscopy, in a laboratory.
Okenite is fibrous: does fibrous mean asbestos?
No sheet line answers that; the seven-sheet ledger shows why. A script read the Crystal Data and tenacity lines of seven Handbook sheets: the species sheets for the six regulated minerals and okenite’s, matching fibrous through the ligature, which this text layer renders as a stray glyph. All seven record a fibrous habit. Asbestiform is printed on five: actinolite, grunerite, anthophyllite, clinochrysotile and riebeckite. The tremolite sheet says also fibrous aggregates without it, and okenite’s says typically fibrous without it. The tremolite sheet is a regulated mineral’s; the word is not the test.
Documented failure: elastic. Okenite’s tenacity line reads elastic. It would be tempting to read that as the difference from the brittle amphiboles, and five of the six regulated sheets do print brittle. But the anthophyllite sheet’s line reads brittle; fibers are elastic, and the clinochrysotile sheet prints no tenacity at all. The tenacity word does not line up with the list either.
Why okenite looks fuzzy. Its sheet’s habit, typically fibrous laths gathered into compact radial balls, is the whole of the recorded reason; the species page okenite sets it out, and is okenite a zeolite compares it with the fibrous zeolites. This page makes no claim about the size of okenite’s fibres or whether they can become airborne: no source fetched for it says.
What would change the answer?
Three things, none of them a property of okenite. First, the law: regulation 2 is a list, and the answer is only as current as the text, read here on 4 November 2026. Second, the scope of the list: HSG248 paragraph A2.4 says other non-regulated amphibole minerals are also known to occur in the asbestiform habit but their identification is not within the scope of the guide, and A2.16 says laboratories should hold reference samples of naturally occurring mineral ‘fibres’ such as wollastonite, as well as of the six. Not named, in other words, is a statement about wording, not about fibres.
Third, the specimen. A label that says okenite names one mineral. The okenite sheet lists its associates as zeolites, apophyllite, prehnite, calcite, quartz and chalcedony, and its Distribution field includes the Jeffrey mine at Asbestos, Quebec, Canada. A place name classifies nothing, and a label cannot say what else is on a piece; only a laboratory can.
Is okenite dangerous or toxic?
This page does not say. It reports what the regulation names, what HSE’s analysts’ guide classifies, and what the okenite sheet records. It gives no verdict on okenite or any specimen, and no handling, cutting, cleaning or storage advice. Questions about exposure belong with HSE’s asbestos pages and a clinician; other mineral hazards are routed on toxic minerals in a collection. This is not health, safety or legal advice.
Where to go next. The zeolite question is on is okenite a zeolite, the other asbestos questions on is scolecite asbestos and is tremolite asbestos, and more guides in the collecting guides. We hold no catalogue and nothing here is offered for sale; the wanted list is the only commercial route.