Mckinstryite
Chemical formula: Ag<sub>5</sub>Cu<sub>3</sub>S<sub>4</sub>
Mckinstryite is a rare silver-copper sulfide, forming steel-gray, metallic aggregates, often in paragenesis with other sulfides.
Properties
- Mohs hardness
- 1.5-2.5
- Luster
- Metallic
- Streak
- Black
- Density
- 6.62
- Cleavage
- Indistinct
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Characteristic features of mckinstryite include its steel-gray color, metallic luster, very low hardness (1.5-2.5), and high density. The mineral is malleable and ductile. It often occurs as massive aggregates or inclusions in other sulfides. It reacts with nitric acid. ## Distinguishing from similar minerals It can be confused with other dark silver and copper sulfides, such as acanthite, chalcocite, or stromeyerite. Acanthite (Ag₂S) is usually darker and has similar hardness but a different chemical composition. Chalcocite (Cu₂S) is harder (2.5-3). Stromeyerite (AgCuS) has very similar properties but crystallizes in the trigonal system, while mckinstryite crystallizes in the orthorhombic system. Certain differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal forms Well-formed crystals are extremely rare. This mineral mainly forms massive, granular, fibrous, or earthy aggregates. It also occurs as dendritic aggregates or as inclusions in other ore minerals.
Geological environment
## Genesis Mckinstryite is a mineral of hydrothermal origin. It forms in the final, low-temperature stages of crystallization in ore veins rich in silver and copper. It is a secondary mineral, forming at the expense of other, earlier-formed silver and copper sulfides. ## Mineral associations It most often co-occurs with other silver and copper sulfides and sulfosalts, such as stromeyerite, chalcocite, bornite, djurleite, tennantite, galena, sphalerite, pyrite, and also with native silver. ## Localities The most important localities worldwide include the Foster mine in the Cobalt-Gowganda area in Ontario (Canada), where it was discovered. It is also known from the McArthur River (HYC) mine in the Northern Territory, Australia; from the Dzhezkazgan mine in Kazakhstan; from the Creede mine in Colorado (USA); and from the Schlema-Alberoda area in Saxony (Germany).
Rarity
Rare
For collectors
## Quality criteria The quality of mckinstryite specimens is primarily assessed based on the abundance of its occurrence in the sample and its paragenesis with other well-formed minerals. Since it rarely forms its own crystals, abundant, pure aggregates with a distinct metallic luster are most valued. Specimens from a well-documented, classic locality (e.g., from the Foster mine in Canada) are more highly prized. ## Popular localities The most classic and sought-after specimens by collectors come from the type locality, the Cobalt area in Ontario, Canada. High-quality material also comes from the Dzhezkazgan mine in Kazakhstan, where it forms interesting parageneses with other copper sulfides.
Care and storage
## Cleaning Mckinstryite specimens should only be cleaned mechanically, using a soft brush or compressed air to remove dust. Due to its softness, all friction and hard tools should be avoided. The use of water, and especially chemical cleaning agents, is absolutely inadvisable. ## What to avoid Avoid contact with water and chemicals, which can cause chemical reactions and destroy the mineral. Mckinstryite is sensitive to air and moisture, which cause its oxidation and surface tarnishing. It should be protected from high temperatures and sudden changes in temperature. It should not be exposed to direct sunlight. ## Storage It is recommended to store specimens in airtight, acid-free containers or "membrane box" type boxes to limit contact with air and moisture. Store in a dry and temperature-stable place. Due to its softness, it should not come into contact with harder minerals.