Norrishite
Chemical formula: KLiMn³⁺₂Si₄O₁₀O₂
Norrishite is an extremely rare silicate with a characteristic dark brown to black color and a platy appearance, found in metamorphic rocks.
Properties
- Mohs hardness
- 2.5
- Color
- Black to brownish black
- Luster
- Submetallic
- Streak
- Light brown
- Density
- 3.264
- Cleavage
- {001} perfect; {100} and {010} poor.
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Norrishite can be identified by its dark brown to black color, platy habit, and strong, almost metallic luster on cleavage surfaces. Its occurrence as small, hexagonal plates in a specific geological environment is a key indicator. ## Distinguishing from Similar Minerals It can be confused with other dark micas, such as biotite. Distinguishing it requires advanced analytical methods, such as chemical analysis (EDS) to confirm the presence of lithium and manganese, and X-ray diffraction (XRD) to determine the crystal structure. Field identification is practically impossible. ## Crystal Forms Crystals are tabular or platy, with a pseudohexagonal outline. They typically form small, rosetted aggregates or occur as individual flakes embedded in the rock.
Geological environment
## Genesis Norrishite forms under conditions of high-pressure and relatively low-temperature metamorphism. It develops in manganese- and potassium-rich sedimentary rocks (e.g., cherts) that have undergone transformations in the blueschist facies. ## Mineral Associations This mineral co-occurs with minerals such as quartz, aegirine, albite, sugilite, richterite, brannockite, and taneyamalite. The presence of these minerals is an important clue for prospecting. ## Localities The only confirmed and type locality for this mineral in the world is the Woods mine, located in New South Wales, Australia. This is the only place where norrishite has been identified to date.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of norrishite specimens is primarily assessed based on the size and development of the crystals. The most desirable specimens are those with distinct, sharply defined, hexagonal crystals forming aesthetic, rosetted clusters. Contrast with a light host rock (e.g., quartz) also enhances visual appeal. Due to its rarity, every authentic specimen is valuable. ## Popular Localities The only source of norrishite specimens is its type locality – the Woods mine in Australia. All specimens available on the collector's market originate from this single location.
Care and storage
## Cleaning Norrishite specimens should be cleaned with utmost care, using only compressed air to remove dust. Due to its softness and perfect cleavage, contact with water and any tools, such as brushes, which could cause the lamellae to separate, should be avoided. ## What to Avoid Contact with chemicals, including acids and detergents, should be avoided. The mineral is susceptible to mechanical damage – even slight pressure can cause it to disintegrate. It should not be heated or exposed to sudden temperature changes. ## Storage Norrishite specimens, typically in the form of micromounts, should be stored under stable conditions in sealed "perky boxes" to protect them from dust and mechanical damage. Vibrations and shocks should be avoided.