Offretite
Chemical formula: KCaMg(Si<sub>13</sub>Al<sub>5</sub>)O<sub>36</sub>·15H<sub>2</sub>O
Offretite is a rare mineral from the zeolite group, forming small, hexagonal crystals in cavities of volcanic rocks.
Properties
- Mohs hardness
- 4-4.5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.13
- Cleavage
- Distinct/Good on {1010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Offretite is difficult to identify without specialized equipment. It is distinguished by its form of small, hexagonal, prismatic crystals, often gathered in radial aggregates. It occurs in vesicles and cavities in volcanic rocks, such as basalts. Certain identification requires chemical analysis (EDS) or X-ray diffraction (XRD). ## Distinguishing from Similar Minerals It can be confused with other zeolites of similar habit, such as erionite, with which it often forms intergrowths. Distinguishing these two minerals macroscopically is impossible and requires advanced analytical methods. Erionite often forms more fibrous aggregates compared to the prismatic crystals of offretite. ## Crystal Forms It forms simple, hexagonal prisms, terminated by flat basal faces. Crystals are typically elongated along the crystallographic c-axis. It occurs as single, microscopic crystals or in radial, acicular, and fibrous aggregates.
Geological environment
## Genesis Offretite is a low-temperature hydrothermal mineral. It forms as a result of post-magmatic waters filling vesicles and fissures in volcanic rocks, mainly basalts and andesites. It crystallizes from solutions rich in silica, aluminum, potassium, and calcium. ## Mineral Associations It most commonly co-occurs with other zeolites, such as chabazite, phillipsite, erionite (with which it forms epitaxial intergrowths), as well as calcite and opal. ## Localities The most important offretite occurrences worldwide include: Mont Semiol near Châtelneuf (type locality) and Montbrison in France; Bramberg in Austria; the vicinity of Wechselsburg and Vogelsberg in Germany; some localities in Iceland; in the Czech Republic (near Ústí nad Labem); and in the USA in Oregon (e.g., Beech Creek).
Rarity
Rare
For collectors
## Quality Criteria The quality of collectible specimens depends on the size and development of the crystals, although these rarely reach sizes visible to the naked eye. The most prized specimens are those where offretite forms well-defined, hexagonal prisms on a contrasting rock matrix. Due to their microscopic nature, most specimens are sold as micromounts. ## Popular Localities Classic and most valued specimens come from the type locality in Mont Semiol, France. Good quality microcrystals are also found in Bramberg, Austria, and in some basalts in the Czech Republic and Germany.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using compressed air to remove dust. Due to the brittleness and delicacy of the crystals, contact with water and brushes is not recommended. If liquid is necessary, a brief bath in distilled water followed by thorough drying is advised. ## What to Avoid Avoid contact with acids and other chemicals that can damage the zeolite structure. The mineral is sensitive to high temperatures, which can cause the loss of water from its structure (dehydration) and destruction of the crystals. It should be protected from mechanical damage. ## Storage Offretite specimens, especially those in the form of micromounts, are best stored in sealed, transparent boxes that protect them from dust, moisture, and mechanical damage. Avoid exposure to direct sunlight and sudden temperature changes.