Ferro-tschermakite
Chemical formula: ☐Ca<sub>2</sub>(Fe<sup>2+</sup><sub>3</sub>Al<sub>2</sub>)(Si<sub>6</sub>Al<sub>2</sub>)O<sub>22</sub>(OH)<sub>2</sub>
Ferro-tschermakite is a rare amphibole from the hornblende group, forming dark green to black, prismatic crystals in metamorphic rocks.
Properties
- Mohs hardness
- 5-6
- Luster
- Vitreous
- Streak
- Pale green
- Density
- 3.28
- Cleavage
- Good on {110}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Ferro-tschermakite is most easily identified by its dark green to black color, vitreous luster, and the elongated crystal habit characteristic of amphiboles. A key diagnostic feature is the cleavage angle of approximately 124°, which distinguishes it from pyroxenes (which have an angle close to 90°). The streak is pale green. ## Distinguishing from Similar Minerals It can be confused with other dark amphiboles, such as hornblende, ferro-hornblende, or arfvedsonite, as well as with pyroxenes (e.g., augite). Distinguishing it from other amphiboles is very difficult without advanced chemical analyses (EDS, WDS). It is distinguished from augite and other pyroxenes by the aforementioned cleavage angle and often a more elongated crystal habit. ## Crystal Forms Crystals are usually monoclinic, with a prismatic or columnar habit. They occur as single, well-formed crystals embedded in rock, as well as in the form of granular, fibrous, or radial aggregates.
Geological environment
## Genesis Ferro-tschermakite is a mineral characteristic of medium- to high-grade metamorphic rocks, mainly amphibolites and granulites. It forms under conditions of elevated pressure and temperature. Its presence, especially in equilibrium with other minerals, provides important information about the metamorphic conditions to which the rock was subjected. ## Mineral Associations It most commonly co-occurs with other minerals typical of metamorphic rocks, such as plagioclase (especially anorthite), garnet (almandine), pyroxenes (diopside, hypersthene), epidote, titanite, and quartz. ## Localities Significant, confirmed occurrences of ferro-tschermakite include, among others, the Fiskenæsset complex in Greenland (type locality), where it occurs in anorthosites. It is also found in the Adirondack Highlands in New York (USA), the Bushveld Complex in South Africa, in some regions of Sweden (e.g., Värmland), and in the Rogaland massif in Norway.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, sharply terminated crystals with a distinct luster. Contrast with a light matrix rock (e.g., white anorthosite) is important, as it highlights the dark color of the crystals. Large, single crystals or attractive groups are rare and sought after. Purity is not a key criterion, as this mineral is inherently opaque. ## Popular Localities The localities in Greenland (Fiskenæsset) are considered classic and yield the best specimens, producing well-formed crystals in anorthosite. Specimens from other localities, such as New York State, are also sometimes available on the collector's market, although they often have less aesthetic value.
Care and storage
## Cleaning Ferro-tschermakite specimens can be cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water may be used. Ultrasonic cleaners should be avoided, as they can damage crystals, especially if they show fractures or inclusions. ## What to Avoid Avoid contact with strong acids and bases, which can react with the mineral. It should not be heated or exposed to sudden temperature changes. Although relatively hard, it should be protected from impacts and scratching by harder minerals (e.g., quartz). ## Storage Specimens are best stored in individual boxes or on a soft surface to prevent abrasions and mechanical damage. It does not require special conditions regarding lighting or humidity, but standard collecting conditions (stable temperature and humidity) are recommended.