Mackayite
Chemical formula: Fe³⁺Te⁴⁺₂O₅(OH)
Mackayite is a rare iron tellurite, forming small, black or dark green crystals with a strong, almost metallic luster.
Properties
- Mohs hardness
- 4.5
- Color
- Pale bottle-green, olive green, brownish green, greenish black; greenish yellow to yellowish green in transmitted light.
- Luster
- Vitreous to metallic
- Streak
- Pale green
- Density
- 4.86
- Cleavage
- Distinct on {001}
- Fracture
- Sub-Conchoidal
- Transparency
- Transparent
- Crystal system
- Tetragonal
Diagnostic features
## Identification Characteristic features of mackayite include its tetragonal, often dipyramidal crystals, very dark greenish-black color, and exceptionally strong, almost metallic luster. High density can also be helpful in identification. It occurs in the oxidation zones of tellurium deposits. ## Distinguishing from Similar Minerals Mackayite can be confused with other rare tellurites such as emmonsite or poughite, which often co-occur with it. Distinguishing it from these minerals without advanced studies (e.g., XRD) is very difficult. It is distinguished from more common minerals by its crystal form, specific luster, and occurrence environment. ## Crystal Forms Mackayite crystallizes in the tetragonal system. It most often forms simple, pyramidal (dipyramidal) crystals with a square cross-section. It also occurs as short prismatic crystals terminated by pyramids. Crystals rarely exceed a few millimeters in size.
Geological environment
## Genesis Mackayite is a secondary mineral formed in the oxidation (weathering) zones of ore deposits rich in tellurides, especially in arid or semi-arid climates. It forms as a result of the alteration of primary gold, silver, and other metal tellurides under the influence of surface waters. ## Mineral Associations This mineral often co-occurs with other secondary tellurium minerals, such as tellurite, emmonsite, poughite, as well as with quartz, barite, hematite, jarosite, and native gold. ## Localities The most important and well-known occurrences of mackayite are in the USA, particularly at the type locality – the Mohawk Mine in Goldfield (Nevada) and in the mines of the Tombstone district (Arizona). It is also known from the El Plomo Mine in Honduras and several localities in Mexico (Bambolla Mine, Moctezuma in Sonora state).
Rarity
Very rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with sharp, well-formed, and undamaged crystals with a strong luster. Contrast with the host rock matrix, on which the crystals are embedded, is important – most often this is light quartz or rhyolite. Crystal size is crucial, and specimens with crystals exceeding 2-3 mm are considered exceptional. Co-occurrence with other rare tellurites or visible native gold enhances attractiveness. ## Popular Localities Classic and most sought-after mackayite specimens come from historical finds in Goldfield, Nevada, and Tombstone, Arizona (USA). In recent years, high-quality specimens from the Bambolla Mine in Mexico have also appeared on the market, often characterized by excellent crystal development.
Care and storage
## Cleaning Mackayite specimens should be cleaned very carefully. It is best to use compressed air to remove dust. If wet cleaning is necessary, a soft brush moistened with distilled water can be used, followed by immediate and thorough drying of the specimen. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral is sensitive to acids and strong chemicals, which can damage it. Avoid sudden temperature changes and prolonged exposure to moisture. Due to its brittleness, it should be protected from impacts and scratching. ## Storage Mackayite specimens, being typically small and fragile, are best stored in sealed "micromount" boxes or display cases, away from dust and direct contact with harder minerals. It is not sensitive to sunlight.