Hardystonite
Chemical formula: Ca<sub>2</sub>Zn<sup>2+</sup>Si<sub>2</sub>O<sub>7</sub>
Hardystonite is a rare calcium zinc silicate of the melilite group, known for its intense, blue fluorescence under ultraviolet light.
Properties
- Mohs hardness
- 3-4
- Luster
- Vitreous to greasy
- Streak
- White
- Density
- 3.4
- Cleavage
- Perfect on {001}, good on {100} and {110}
- Fracture
- Uneven
- Transparency
- Transparent to translucent
- Crystal system
- Tetragonal
Diagnostic features
## Identification The most important diagnostic feature of hardystonite is its intense blue or violet-blue fluorescence under shortwave UV light. In daylight, it is usually an inconspicuous, whitish or gray mineral with a massive texture. It reacts with hydrochloric acid, gelatinizing. ## Distinguishing from Similar Minerals It is distinguished from other white, massive minerals found in the Franklin deposit (e.g., calcite, willemite) primarily by its characteristic fluorescence. Calcite usually fluoresces red or orange, and willemite fluoresces green. In daylight, it can be difficult to distinguish without chemical tests or fluorescence examination. ## Crystal Forms Hardystonite crystals are very rare. If they occur, they take the form of poorly developed, tabular crystals with a square cross-section, typical of minerals crystallizing in the tetragonal system. It usually forms granular or massive aggregates.
Geological environment
## Genesis Hardystonite is a product of contact metamorphism. It forms as a result of magma interacting with zinc deposits embedded in carbonate rocks (limestones and dolomites). It is a characteristic mineral of metamorphosed zinc deposits, such as Franklin in New Jersey. ## Mineral Associations This mineral co-occurs with other minerals typical of the Franklin deposit, such as willemite, franklinite, zincite, calcite, rhodochrosite, diopside, and andradite. ## Localities The most important and classic locality for hardystonite is the zinc ore deposit in Franklin, Sussex County, New Jersey, USA. This is its type locality, from which the best-known specimens originate. It is also known from several other, much less significant localities worldwide, including Italy and Japan.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those exhibiting the strongest and purest blue fluorescence. Rich associations with other fluorescent minerals from the Franklin deposit, such as calcite (red fluorescence) and willemite (green fluorescence), forming multicolored compositions under UV light, also enhance their attractiveness. Rarely found, well-formed crystals are highly sought after and command significantly higher prices. ## Popular Localities By far the most important and practically the only commercial source of collector specimens is the historic Franklin deposit, New Jersey, USA. Specimens from this locality are the standard for this mineral.
Care and storage
## Cleaning Hardystonite specimens should be cleaned very carefully, using a soft, dry brush to remove dust. Due to its low hardness and possible reactivity, avoid washing it in water, and especially avoid using detergents or acids. ## What to Avoid Avoid contact with chemicals, including acids, which can damage it. The mineral is brittle and sensitive to impact and scratches. It should not be subjected to sudden temperature changes. ## Storage Hardystonite specimens are best stored in separate, padded boxes to prevent abrasions and mechanical damage. Protect from moisture and chemical contaminants.