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geology · classification

Rock sample dichotomous key

This dichotomous key separates four rock samples using observable texture and one simple test. A glassy surface points to obsidian. For the other rocks, a dilute-acid reaction distinguishes limestone. Visible sand-sized grains indicate sandstone, leaving granite as the crystalline, non-fizzing sample. The questions are arranged so each one narrows the remaining options. In a classroom, explain safety before using acid and substitute a different property if testing is not appropriate. Real rock identification can require additional observations, but a short key is useful for learning how classification decisions work. It gives readers a concise reference they can reuse when discussing the result with colleagues.

Type Arbre de décisionNorme Howard-Raiffa / CART-sklearn / taxonomyMoteur schematex-decisiontreeMis à jour 24/09/2026
Identify a rock sample
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La demande

“A dichotomous key to identify obsidian, limestone, sandstone, and granite samples using glassy surface, acid reaction, and visible sand-sized grains.”

Essayez ensuiteAdd a shale sample outcomeReplace the acid test with a hardness test
Ce que contient ce dessin

Comprenez les décisions qui le sous-tendent.

01Surface

Identify glassy obsidian first.

02Reaction

Use an acid test for limestone.

03Texture

Separate sandstone from crystalline granite.

Use for a supervised classroom rock-identification exercise.

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FAQ

Questions fréquentes

Why is acid used here?

Limestone commonly reacts with dilute acid because it contains calcite.

Is obsidian a mineral?

No. It is volcanic glass, classified as an igneous rock.

Can a key use tests and observations?

Yes, provided each step remains a clear two-way choice.