Hastingsite
Chemical formula: NaCa₂(Fe²⁺₄Fe³⁺)(Si₆Al₂)O₂₂(OH)₂
Hastingsite is a dark green or black mineral from the amphibole group, found mainly in iron-rich metamorphic and igneous rocks.
Properties
- Mohs hardness
- 5-6
- Color
- black, dark-green, greenish-brown, yellow
- Luster
- Vitreous
- Streak
- greenish grey
- Density
- 3.35
- Cleavage
- on {110}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features of hastingsite include its dark green to black color, vitreous luster, and prismatic crystal habit. A key diagnostic feature, visible under a microscope or on larger crystals, is the angle between cleavage planes, which is approximately 56° and 124°. ## Distinguishing from Similar Minerals Hastingsite is difficult to distinguish from other dark amphiboles, such as hornblende or arfvedsonite, without advanced chemical (EDS) or optical analyses. It can also be confused with pyroxenes (e.g., augite), which, however, have cleavage at an angle close to 90°. ## Crystal Forms Hastingsite crystallizes as elongated, columnar, or prismatic crystals, often terminated by flat faces. It also occurs in granular, fibrous, or radial aggregates embedded in the host rock.
Geological environment
## Genesis Hastingsite is a characteristic mineral of igneous and metamorphic rocks. It forms during crystallization from alkaline and iron-rich magmas, creating rocks such as syenites, nepheheline syenites, and alkaline granites. It also occurs in metamorphic rocks of the amphibolite facies, particularly in skarns and amphibolites formed from the alteration of carbonate or basic igneous rocks. ## Mineral Associations This mineral often co-occurs with other minerals such as nepheline, biotite, titanite, apatite, magnetite, ilmenite, as well as plagioclase and alkali feldspars. In skarns, it is often accompanied by diopside, garnets (andradite), and epidote. ## Localities The most important occurrences of hastingsite are in Canada (Bancroft, Ontario - type locality), USA (Adirondack Mountains in New York State), Norway (Langesundsfjorden area), Sweden, Finland, and Russia (Kola Peninsula). It is also found in alkaline complexes in Brazil and Greenland.
Rarity
Not very common
For collectors
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharply terminated crystals exhibiting a distinct luster and intense, dark green or black color. Crystals contrastingly set on a light rock matrix, such as white nepheline or calcite, are also highly valued. Crystal size is important, and specimens exceeding several centimeters in length are rare and sought after. ## Popular Localities Classic and most desirable specimens come from historical localities around Bancroft in Hastings County, Ontario (Canada). Good quality crystals are also found in nepheline syenites on the Kola Peninsula in Russia and in the Ilimaussaq igneous complex in Greenland.
Care and storage
## Cleaning Hastingsite specimens can be cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water and a soft brush may be used. Ultrasonic cleaners should be avoided, as they can damage crystals along cleavage planes. ## What to Avoid The mineral is sensitive to strong acids, which can cause slow etching. It should be protected from sudden temperature changes and prolonged exposure to direct sunlight, although the latter is not considered particularly harmful. Avoid contact with household chemicals and cleaning agents. ## Storage Specimens should be stored in stable conditions, away from sources of moisture and chemical contamination. It is best to place them in sealed boxes or display cases to protect them from dust and mechanical damage. Due to its brittleness, it is advisable to avoid placing it in direct contact with harder minerals.