Which minerals can't be washed in water
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Halite, borax, ulexite, chalcanthite, gypsum (including selenite), goslarite, kainite, sulfur, mica, kaolinite and other clays, alum and several other minerals should never be washed or soaked in water. The Illawarra Lapidary Club's cleaning notes put it plainly: "A great number of minerals are soluble in water and would be ruined by washing" [1]. Some dissolve, some swell or crumble, and some decay faster when damp. Clean them dry, with a soft brush and a blower. The lookup table below gives each mineral, what water does to it, the source, and how to clean it instead.
Three ways water ruins a specimen
- It dissolves. Soluble minerals lose their crystal faces, edges and luster in seconds and can disappear entirely in a soak. Halite is the familiar case.
- It is absorbed and the specimen falls apart. Porous and clay-rich minerals take up water, swell and then crack or crumble as they dry. The Illawarra notes say of these, "Do not try to dampen them as they are water absorbent and may disintegrate" [1].
- It sets off decay. Some minerals react with moisture over days or months rather than seconds. National Park Service guidance links relative humidity above 60 percent to pyrite decay, and says deliquescent minerals (those that pull water from the air) absorb moisture at high humidity and dissolve [2]; the bounds are set out on humidity, high and low.
Lookup table
"Illawarra" is the Illawarra Lapidary Club's Cleaning Mineral Specimens (2022) [1]. Where the source gives a dry method for a species, it is shown; where it does not, the general dry method below applies.
| Mineral | What water does | Source | Clean it instead |
|---|---|---|---|
| Halite (rock salt) | Dissolves | Illawarra, water-soluble list | The club lists alcohol for halite; otherwise dry brush |
| Borax | Dissolves | Illawarra, two lists | Dry brush; do not immerse in any water-based solution |
| Ulexite | Cannot be immersed in water-based solutions; also water-absorbent and may disintegrate | Illawarra, two lists | Soft dry brush only |
| Chalcanthite | Cannot be immersed in water-based solutions | Illawarra | Dry brush |
| Gypsum, including selenite | Listed as water-soluble | Illawarra, two lists | Dry brush; never soak |
| Goslarite | Dissolves | Illawarra, water-soluble list | Dry brush |
| Kainite | Dissolves | Illawarra, water-soluble list | Dry brush |
| Autunite | Listed as water-soluble | Illawarra, water-soluble list | Dry only; autunite is also radioactive, see radioactive minerals at home |
| Sulfur | Listed as water-soluble; "Never washed, just gently dusted" | Illawarra, list and table | Gentle dusting |
| Cryolite | Cannot be immersed in water-based solutions | Illawarra | Dry brush |
| Orpiment | Cannot be immersed in water-based solutions | Illawarra | Dry brush with gloves; orpiment is an arsenic mineral, see which minerals are dangerous |
| Kaolinite and other clays | Absorbs water and may disintegrate | Illawarra; NPS says clay minerals can crack when humidity changes [2] | Soft dry brush only |
| Potassium alum | Absorbs water and may disintegrate | Illawarra | Soft dry brush only |
| Ammonium salts | Absorb water and may disintegrate | Illawarra | Soft dry brush only |
| Sideronatrite | Absorbs water and may disintegrate | Illawarra | Soft dry brush only |
| Mica, muscovite | "Never washed, just gently dusted"; the club adds that most micaceous minerals should not be immersed in water-based solutions | Illawarra, table and text | Gentle dusting |
| Magnesite | "Do not clean with liquids" | Illawarra, table | Dry brush |
| Marcasite | "Do not clean" | Illawarra, table | Leave it; store dry |
| Pyrite showing decay | "Pyrite decay can be triggered by RH over 60 %" | NPS COG 11/2 [2] | Do not wash a decaying piece; NPS says such specimens "need to be isolated and stored in a very dry environment" [2] |
| Shale and clay-rich rock | Clay minerals crack and split with rapid humidity swings; "Shale specimens will crack and delaminate if RH is low" | NPS COG 11/2 [2] | Dry brush; keep humidity steady |
The club's notes include one inconsistency: gypsum appears on the water-soluble lists and also against dilute oxalic acid, a water-based solution, for iron stains [1]. This page follows the water-soluble warning.
Two species collectors often ask about, sylvite and kernite, are not named in the sources used for this page, so they are not in the table. Keep any mineral from a salt or borate deposit dry until you have checked its species data or asked a curator.
Cleaning a specimen dry
- Work on a tray lined with a clean paper towel, so fragments and loose crystals can be recovered.
- Blow off loose dust with a rubber bulb blower.
- Brush gently with a soft brush. The Illawarra notes suggest "a soft camel hairbrush" and warn against metal and nylon bristles [1].
- Lift stubborn grains with a wooden toothpick or fine tweezers under a lens.
- Stop if the surface starts to come away. NPS guidance says friable specimens should never be dusted at all [2]. Some specimens should simply be left as they are.
The Illawarra notes say that minerals which cannot be immersed in water-based solutions "can be washed or cleaned to some degree with dry cleaning fluid," but they do not name a fluid, a strength or a test [1]. This page gives no solvent method for that reason. Solvents also bring their own fire and fume hazards.
Keeping them dry afterward
Water-soluble and deliquescent minerals can be damaged by the air in a humid room, not only by a sink. National Park Service guidance (Conserve O Gram 11/2, April 1998) sets the bounds: above 60 percent relative humidity, pyrite decay can start and deliquescent minerals absorb water and dissolve; below 35 percent, some hydrated minerals lose water and crack [2]. How to measure and control humidity is on humidity, high and low, and how to build a small dry box for the worst offenders is on setting up a sealed low-humidity box.
In the field, wrap these minerals in dry paper, seal them in a bag and pack them away from wet specimens.
Building a dry box for soluble specimens
A handful of halite, borax and ulexite pieces will outlast a humid summer only in a closed, dry container. Setting up a sealed low-humidity box walks through the container, the desiccant and how to check that it is working.
References
- Minerals Group, Illawarra Lapidary Club Inc., *Cleaning Mineral Specimens*, compiled by Roly Smith, 2022. https://www.illawarralapidaryclub.com.au/wp-content/uploads/2024/02/Cleaning-Minerals.pdf
- Hall, Kathy, *Storage Concerns for Geological Collections*, Conserve O Gram 11/2, National Park Service, April 1998. https://www.nps.gov/subjects/museums/upload/11-02_508.pdf
