alum shale

noun

variants or alum slate or less commonly alum schist

: a shale or clay slate originally containing pyrite and after weathering containing aluminum sulfate formed by the action of sulfuric acid from the decomposition of the pyrite on the aluminous materials of the rock

Word History

First Known Use

1737, in the meaning defined above

Love words?

You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Start your free trial today and get unlimited access to America's largest dictionary, with:

• More than 250,000 words that aren't in our free dictionary
• Expanded definitions, etymologies, and usage notes
• Advanced search features
• Ad free!