Harmotome

Chemical formula: Ba<sub>2</sub>(Si<sub>12</sub>Al<sub>4</sub>)O<sub>32</sub>·12H<sub>2</sub>O

Harmotome is a mineral from the zeolite group, forming characteristic, cross-penetrating crystals with a vitreous luster.

## Characteristics Harmotome is a hydrated barium aluminosilicate, belonging to the zeolite group. Its most characteristic feature is its complex crystals, which often form penetration twins resembling a cross or a star. Individual crystals typically have a tabular or prismatic habit. It also occurs in radial, spherulitic forms or forms spherical aggregates. ## Physical Properties Harmotome crystals are brittle and exhibit a vitreous, sometimes pearly luster. The mineral is usually translucent, less often transparent or opaque. Its Mohs hardness is 4.5-5, and its density ranges from 2.41 to 2.5 g/cm³. ## Colors and Varieties It is most often colorless, white, or gray. It can also take on yellowish, pinkish, reddish, or brownish hues, which is associated with the presence of impurities. No distinct commercial or gemological varieties are distinguished. ## History and Name The name harmotome, given in 1801 by René Just Haüy, comes from the Greek words *harmos* (to join) and *temnein* (to cut, to divide), which alludes to the way crystals divide when forming characteristic twins. The mineral was known earlier, but Haüy was the first to formally describe and name it. ## Applications Due to its zeolitic properties, harmotome has potential applications as a molecular sieve, catalyst, or in ion-exchange processes. However, its rarity compared to other zeolites means it is primarily of scientific and collector's interest.

Properties

Mohs hardness
4.5-5
Luster
Vitreous
Streak
White
Density
2.41-2.5
Cleavage
Good on {010}, poor on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of harmotome is its characteristic, cross-penetrating twin crystals. They often form shapes resembling a Maltese cross. Occurrence in geodes and gas vesicles in volcanic rocks is also typical. It reacts with hydrochloric acid, forming silica gel. ## Distinguishing from Similar Minerals Harmotome is often confused with other zeolites, especially phillipsite, which forms similar intergrowths but has a different chemical composition (it is rich in potassium and calcium, not barium). Differentiation often requires chemical analysis (e.g., EDS). It is distinguished from stilbite by its crystal habit – stilbite forms sheaf-like aggregates. ## Crystal Forms Harmotome crystals are monoclinic, but due to repeated twinning, they often mimic orthorhombic or tetragonal symmetry. Penetration twins, forming complex, cross-shaped or star-shaped aggregates, are typical. It also occurs in radial or spherulitic aggregates.

Geological environment

## Genesis Harmotome is a low-temperature mineral, forming in hydrothermal processes. It most often crystallizes in gas vesicles and fissures in basic volcanic rocks, such as basalts and phonolites. It can also form in hydrothermal veins cutting gneisses and schists, as well as an authigenic mineral in marine sediments. ## Mineral Associations It often co-occurs with other zeolites (phillipsite, chabazite, analcime, natrolite), calcite, barite, strontianite, quartz, and minerals from the smectite group. ## Localities Significant harmotome specimens come from many places around the world. In Europe, it is known from Strzegom in Poland, Andreasberg in the Harz Mountains (Germany), Kongsberg (Norway), and Scotland (Strontian). Beautiful crystals have also been found in silver mines in the Santa Fe area of New Mexico (USA) and at Mont Saint-Hilaire in Quebec (Canada).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and distinct cross twins. Large, undamaged crystals or groups of crystals, set on a contrasting rock matrix, are highly prized. The transparency and clarity of the crystals also increase the value of the specimen. Color is usually less important, although unusual coloration (e.g., pink) can be an additional asset. ## Popular Localities Classic specimens, often of historical significance, come from Andreasberg in Germany and Strontian in Scotland. Large and well-formed crystals have also been found at Mont Saint-Hilaire in Canada. Specimens from Strzegom in Poland are also valued in collections.

Care and storage

## Cleaning Harmotome specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, it can be quickly rinsed in distilled water and immediately thoroughly dried. Ultrasonic cleaners should be avoided, as they can damage the brittle crystals. ## What to Avoid As a zeolite, harmotome is sensitive to heat, which can cause water loss from its structure and crystal damage. Contact with acids and other aggressive chemicals should be avoided. It should also not be soaked in water for extended periods. It should be protected from direct sunlight to prevent possible color changes. ## Storage Specimens should be stored in stable conditions, away from heat sources and dust. It is best to keep them in closed boxes or display cases. Due to its brittleness, it should be avoided from being placed in contact with harder minerals that could scratch it.

External references

Sources

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