Aphthitalite is a potassium sulfate

Potassium sulfate

Potassium sulfate is a non-flammable white crystalline salt which is soluble in water...

 mineral with the chemical formula: (K,Na)₃Na(SO₄)₂.

It was first described in 1835 for an occurrence on Mt. Vesuvius, Italy

Italy

Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

. The name is from the Greek άφθητος, "unalterable", and άλας, "salt", for its stability in air. It occurs as fumarolic

Fumarole

A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...

 incrustations in volcanic environments, as small crystals and masses in evaporite

Evaporite

Evaporite is a name for a water-soluble mineral sediment that result from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporate deposits, marine which can also be described as ocean deposits, and non-marine which are found in standing bodies of...

 deposits and in guano

Guano

Guano is the excrement of seabirds, cave dwelling bats, and seals. Guano manure is an effective fertilizer due to its high levels of phosphorus and nitrogen and also its lack of odor. It was an important source of nitrates for gunpowder...

 deposits. It occurs associated with thenardite

Thenardite

Thenardite is an anhydrous sodium sulfate mineral, Na2SO4 which occurs in arid evaporite environments. It also occurs in dry caves and old mine workings as an efflorescence and as a crusty deposit around fumaroles. It occurs in volcanic caves on Mt...

, jarosite

Jarosite

Jarosite is a basic hydrous sulfate of potassium and iron with a chemical formula of KFe3+362. This sulfate mineral is formed in ore deposits by the oxidation of iron sulfides...

, sylvite

Sylvite

Sylvite is potassium chloride in natural mineral form. It forms crystals in the isometric system very similar to normal rock salt, halite . The two are, in fact, isomorphous. Sylvite is colorless to white with shades of yellow and red due to inclusions. It has a Mohs hardness of 2.5 and a specific...

 and hematite

Hematite

Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

 in fumaroles; with blodite

Blödite

Blödite is a hydrated sodium magnesium sulfate mineral with formula: Na2Mg2·4H2O.The mineral is clear to yellow in color and forms monoclinic crystals. A synonym for the mineral is bloedite ....

, syngenite, mirabilite

Mirabilite

Mirabilite, also known as Glauber's salt, is a hydrous sodium sulfate mineral with the chemical formula Na2SO4·10H2O. It is a vitreous, colorless to white monoclinic mineral which forms as an evaporite from sodium sulfate bearing brines. It is found around saline springs and along saline playa lakes...

, picromerite, borax

Borax

Borax, also known as sodium borate, sodium tetraborate, or disodium tetraborate, is an important boron compound, a mineral, and a salt of boric acid. It is usually a white powder consisting of soft colorless crystals that dissolve easily in water.Borax has a wide variety of uses...

 and halite

Halite

Halite , commonly known as rock salt, is the mineral form of sodium chloride . Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, pink, red, orange, yellow or gray depending on the amount and type of impurities...

 in evaporites; and with syngenite, whitlockite

Whitlockite

Whitlockite is a mineral, an unusual form of calcium phosphate. Its formula is Ca96PO3OH. It is a relatively rare mineral but is found in granitic pegmatites, phosphate rock deposits, guano caves and in chondrite meteorites...

, monetite, niter

Niter

Niter or nitre is the mineral form of potassium nitrate, KNO3, also known as saltpeter or saltpetre . Historically, the term "niter" – cognate with "natrium", a Latin word for sodium – has been very vaguely defined, and it has been applied to a variety of other minerals and chemical compounds,...

 and gypsum

Gypsum

Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...

in guano deposits.