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Which Minerals Are Sensitive to Humidity, and at What Level?

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: a handful of collector minerals react to the moisture in room air without ever touching water. Halite deliquesces (pulls in water and dissolves) at about 75% relative humidity, pyrite oxidizes above roughly 60%, marcasite alters at moderate humidity, and anhydrite (sold as angelite) slowly turns to gypsum in damp conditions. Borax and very hydrous opal fail the other way: dry air makes them lose water, crumble or crack.

What does humidity actually do to a mineral?

Relative humidity (RH) is how much water vapor the air holds compared with the most it could hold at that temperature. Most minerals ignore it. The ones that do not fall into four groups, and each group needs a different fix.

  1. Deliquescence. A salt pulls water out of the air until a film of brine forms and the crystal starts to dissolve in its own moisture. Halite is the classic case.
  2. Hydration. A mineral that lacks water in its structure takes some in and becomes a different mineral, usually with a change in volume. Anhydrite turning into gypsum is the example collectors meet.
  3. Dehydration. A mineral that holds water loses it to dry air, changes species and often powders. Borax is the textbook case.
  4. Oxidation. Moisture lets oxygen attack a mineral and form new, often acidic compounds. Iron sulfides such as pyrite and marcasite are the problem here.

Humidity matters in both directions, so the goal is steady, moderate conditions rather than "as dry as possible."

The humidity table

Mineral Process Trouble zone Signs to watch for
Halite (NaCl) Deliquescence About 75% RH and up Wet film, rounded edges, a salt crust on the shelf
Pyrite (FeS2) Oxidation Above about 60% RH; once started, hydrates form as low as 30% Cracks, white or yellowish needles, sulfur smell
Marcasite (FeS2) Oxidation, faster than pyrite Moderate humidity Brown tarnish, crumbling, acid damage to labels and boxes
Anhydrite, including "angelite" Hydration to gypsum Prolonged damp storage Softening, swelling or a dull skin on polished surfaces
Borax Dehydration to tincalconite Dry air White powder skin, turning opaque, crumbling
Chalcanthite Absorbs water, unstable Damp air or contact with water Crumbling, dissolving, blue stains
Hydrous opal (some localities) Water loss Dry air after mining Fine cracks (crazing)

The halite figure comes from conservator Robert Waller writing on the NHCOLL-L collections list: halite dissolves spontaneously at relative humidity of 75% or higher, and that limit holds regardless of temperature. The pyrite figures are from the Natural History Museum's Project Airless report on the NatSCA blog.

Why do halite specimens suddenly start to dissolve?

Halite is soft (Mohs 2.5 according to Geology.com) and lists solubility among its diagnostic properties, so it never tolerates water. What surprises collectors is that a dry halite can start to weep inside a closed case.

A 2021 thread on the NHCOLL-L list shows how. A curator in Istanbul reported that a large halite specimen in a glass-covered display started to dissolve, with the case held at about 70% RH and 20 to 22 °C. Waller's reply pointed to the 75% threshold and named the most likely cause: a washed but not completely dried specimen or material placed into the case, creating a temporary spike in humidity. A sealed case traps that moisture, so one damp object can push the whole case over the line.

Practical consequences:

  • Never put anything freshly washed (a stand, a cloth, another specimen) into a case with halite.
  • Keep halite off bathroom and kitchen shelves, where showers and cooking push RH up in minutes.
  • A small pack of silica gel in the box costs almost nothing and buys a margin.

Our halite guide covers pink and blue salt crystals and how they form.

Why is pyrite decay so destructive?

Pyrite rot starts quietly and is hard to stop. According to the NHM team, pyrite oxidation can occur when the mineral reacts with atmospheric oxygen at relative humidities above 60%, and once it has begun, mineral hydrates will form at as low as 30% RH. The by-products often include sulphuric acid and hydrated ferrous sulphates, which attack the specimen, its label and its box.

The museum's signs list is worth memorizing: expansion cracks, white or yellowish needle-like crystal growths, and a sulfurous smell. The unstable form is usually porous, microcrystalline pyrite, often in or around fossils. Well-crystallized cubes are far more stable, which is why a bright Peruvian cluster on an ordinary shelf usually lasts for decades while a pyritized fossil in a damp basement can fall apart.

At the museum's scale, the fix is serious: affected specimens are heat-sealed in barrier film bags with oxygen-scavenging sachets to create an anoxic microenvironment, and at the time of the report about 14,000 specimens at the Museum were judged in urgent need of that treatment. A home collection needs less, but the principle is the same: cut off moisture and oxygen. The full chemistry and treatment options are in pyrite decay.

Marcasite, pyrite's orthorhombic twin, is worse. Geology.com's marcasite page says it will tarnish even in classroom specimen drawers and, at moderate humidity, alters to ferrous sulfate minerals that produce small amounts of sulfuric acid, enough to discolor wooden drawers or rust metal ones. It recommends storage where humidity can be controlled.

Is angelite really at risk from humidity?

Angelite is a trade name for blue anhydrite, calcium sulfate without water. Geology.com's anhydrite page notes that anhydrite readily converts to gypsum under humid conditions or in contact with groundwater, a change that absorbs water and involves a significant change in volume. Minerals.net puts it in collection terms: anhydrite specimens may alter to gypsum if kept in moist conditions over a prolonged period.

That is a slow process, measured in long storage rather than days. A polished angelite tumble in a living room is not in danger. One kept in a bathroom, a damp cellar or a sealed bag with a wet cloth is a different matter. The angelite guide covers the stone itself and its Peruvian source.

Which minerals suffer when the air is too dry?

The opposite problem is real too. Minerals.net's borax entry explains that when borax loses water it alters into tincalconite, a different mineral with half the water. Fresh borax is translucent; dehydrated borax turns opaque, grows a white powder on its surface and will eventually crumble. Collectors often keep borax in a sealed container to slow this.

Chalcanthite, the bright blue copper sulfate, is unstable in damp air: Minerals.net says it must be kept away from water and moist conditions because it will eventually crumble or dissolve, and that some collectors coat specimens with mineral oil and lacquer to seal them.

Some opal also reacts to dry air. The IGS opal entry describes Virgin Valley, Nevada opal as very hydrous, with a strong tendency to crack from loss of water when exposed to the air. That is why some sellers keep fresh rough in water until it has been stabilized.

How do you control humidity around a collection?

  1. Measure first. A cheap digital hygrometer inside the cabinet tells you more than any guess. Check it in summer and after rain.
  2. Keep the case below the trouble zones. For halite, stay well under 75%; for pyrite and marcasite, well under 60%.
  3. Use silica gel, and refresh it. Gel that has absorbed its limit does nothing. Indicating gel changes color when it needs drying.
  4. Separate the extremes. Borax wants a sealed box that holds its moisture; pyrite and halite want dry boxes. Do not store them together.
  5. Avoid problem rooms. Bathrooms, kitchens, basements, garages and exterior walls all swing in humidity.
  6. Never box anything damp. This is the single most common cause of sudden halite loss.
  7. Inspect sulfides twice a year. Look for white fuzz, fresh cracks and a sulfur smell, and isolate any piece that shows them.

For which stones can be washed at all, see water safe crystals.

What to look for when you buy humidity-sensitive minerals

Ask how the piece has been stored, especially for pyrite and marcasite: a specimen that already shows white crust or a sulfur smell is actively decaying. Look closely at halite photos for rounded edges or a frosted, wet look, which suggest it has been through humid storage. For angelite, a crisp polish is a good sign that it has stayed dry.

  • Pyrite: Peruvian pyrite at an entry price, a good piece to learn what healthy, bright pyrite looks like.
  • Cubic Pyrite Cluster (C Grade) Peru: well-formed cubes are the stable form of pyrite; still keep it in a dry cabinet.
  • Halite - Pink: pink rock salt that needs a closed box and a dry room to keep its sharp edges.
  • Angelite Tumbles: polished blue anhydrite; fine in normal rooms, kept out of bathrooms.
  • Desert Rose: a sandy gypsum rosette, already hydrated, which is why it holds up better than anhydrite in ordinary air.

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Frequently asked questions

What humidity makes halite dissolve?

About 75% relative humidity. Conservator Robert Waller notes that halite dissolves spontaneously at 75% RH or higher, and that the limit holds regardless of temperature. A damp object placed in a closed case is the usual way a dry room crosses that line.

At what humidity does pyrite start to decay?

Natural History Museum conservators report that pyrite oxidation can occur above about 60% relative humidity, and that once decay has started, hydrated sulfate minerals can form at as low as 30%. Stable, well-crystallized cubes resist it far better than porous pyrite.

Is angelite damaged by humidity?

Slowly. Angelite is anhydrite, which converts to gypsum under humid conditions, with a change in volume. Ordinary room air is not a problem, but long storage in a bathroom or damp basement can dull and alter it over time.

Why is my borax specimen turning white?

It is losing water. Borax dehydrates into tincalconite, which shows up as an opaque white powder on the surface, and the specimen will eventually crumble. A sealed container slows the change.

Which minerals are hygroscopic?

In a home collection, the ones to know are halite, which deliquesces at high humidity, and chalcanthite, which crumbles or dissolves in damp air. Treat any other water-soluble evaporite salt the same way until you know better. Keep them in closed boxes with silica gel and away from bathrooms and kitchens.

Sources