Heulandite-Sr
Chemical formula: (Sr,Ca,Na)<sub>5</sub>(Si<sub>27</sub>Al<sub>9</sub>)O<sub>72</sub>·24H<sub>2</sub>O
Heulandite-Sr is a rare mineral from the zeolite group, being the strontium analogue of heulandite-Ca, distinguished by its occurrence in tabular crystals.
Properties
- Mohs hardness
- 3.5-4
- Luster
- Vitreous, Pearly
- Streak
- White
- Density
- 2.29
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Heulandite-Sr can be identified by its characteristic, often rhombic, tabular crystal habit, pearly luster on cleavage planes, and occurrence in geodes and fissures of volcanic rocks. It reacts to heating, melting and swelling easily. ## Distinguishing from Similar Minerals It can be confused with other minerals from the heulandite group (Heulandite-Ca, -K, -Na, -Ba). Definitive differentiation requires advanced chemical analyses (EDS, WDS) to determine the dominant cation. From stilbite, with which it often co-occurs, it is distinguished by its crystal form – stilbite forms characteristic sheaf-like aggregates. ## Crystal Forms Crystals are tabular, often with a rhombic outline, resembling coffin shapes. They occur as single crystals grown on a matrix or form fan-shaped and radial aggregates.
Geological environment
## Genesis Heulandite-Sr is a low-temperature hydrothermal mineral. It forms as a result of hydrothermal solution activity that alters volcanic rocks, such as basalts, andesites, and their tuffs. It crystallizes in gas cavities (amygdules) and fissures of these rocks. ## Mineral Associations It often co-occurs with other zeolites, such as stilbite, chabazite, epistilbite, as well as with calcite, quartz (including chalcedony and agate), and apophyllite. ## Localities The most important occurrences of heulandite-Sr worldwide include the area around Campegli in Liguria (Italy) – the type locality; the Cidanela River valley in Sicily (Italy); the Poona district in Maharashtra (India); the Faroe Islands; and some localities in the USA, e.g., in New Jersey.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with large, sharply terminated, transparent crystals with intense coloration (e.g., pink or orange). Aesthetic compositions of heulandite-Sr with other minerals, such as contrasting stilbite or apophyllite, grown on a carefully prepared rock matrix, are also highly regarded. ## Popular Localities Specimens of the highest collector's value primarily come from classic zeolite localities in India, particularly from quarries in the vicinity of Poonah (Pune) and Nashik in Maharashtra state.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. Brief rinsing in distilled water is permissible, followed by thorough drying. Ultrasonic cleaners should be avoided, as they can damage delicate crystals. ## What to Avoid Heulandite-Sr, like other zeolites, is sensitive to high temperatures, which can cause dehydration, leading to a loss of transparency and dulling. Contact with acids and other chemicals should be avoided. Prolonged exposure to direct sunlight is not recommended. ## Storage Collections should be stored under stable conditions, away from heat sources and sudden temperature changes. Due to its relatively low hardness and perfect cleavage, specimens should be protected from impacts and contact with harder minerals.