Mineral Guide · Mineral

Harmotome

Harmotome is a barium-rich zeolite that forms attractive white to pale crystals, commonly as twinned or radiating aggregates in cavities.

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Crystal structure

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Mineral properties

Formula: (Ba,Na)₂CaAl₄Si₁₂O₃₂·12H₂O
Class: Silicates
Crystal system: Monoclinic
Typical colours: yellow, colourless, white, pink
Lustre: Pearly, Vitreous
Hardness: 4.50-5.00
Specific gravity: 2.30-2.40
Crystal habit: Prismatic, Tabular, Twinned
Origin: basalt cavities, hydrothermal veins and low-temperature zeolite assemblages

What is Harmotome?

Harmotome is a barium-rich zeolite that forms attractive white to pale crystals, commonly as twinned or radiating aggregates in cavities.

Mineralogy and structure

Reference formula: (Ba,Na)₂CaAl₄Si₁₂O₃₂·₁₂H₂O. Harmotome has an open hydrated zeolite framework. Exchangeable cations and water are part of the structure, which explains its sensitivity to heat and dehydration. Harmotome belongs to the silicates and has a monoclinic crystal system. Typical hardness is 4.5–5 on the Mohs scale and specific gravity is approximately 2.3–2.4.

Colour and appearance

Typical colours include white, colourless, pink, yellow. The usual prismatic, tabular, twinned habit and vitreous, pearly lustre are useful descriptive features, but colour alone is not diagnostic.

Formation and localities

Harmotome forms in or is associated with basalt cavities, hydrothermal veins and low-temperature zeolite assemblages. Important documented localities include Germany, Scotland, Italy, Japan, India and the United States.

Collecting and care

Keep dry and away from heat; support delicate twins and avoid ultrasonic or chemical cleaning.

Sources and editorial review

Reviewed by NotOnlyRocks Editorial Team

Last reviewed: 31 August 2026

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