Pharmacosiderite

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What Wikipedia says

Pharmacosiderite is a hydrated basic ferric arsenate, with the chemical formula KFe4(AsO4)3(OH)4·(6-7)H2O and a molecular weight of 873.38 g/mol. It consists of the elements arsenic, iron, hydrogen, potassium, sodium and oxygen. It has a Mohs hardness of 2 to 3, about that of a finger nail.

Excerpt from the Wikipedia article “Pharmacosiderite”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: Christian Rewitzer, CC BY-SA 3.0, via Wikimedia Commons.