STROMEYERITE
Stromeyerite – AgCuS – is a rare sulfide found in Cu-Ag-Pb ores.
It may contain Fe, and there is a variety containing up to 12.54% Hg.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Blue-gray, dark blue, dark steel-gray, or steel-gray with a pinkish hue.
Habit: Long prismatic crystals in disordered aggregates, resembling stibnite. Massive, compact.
Cleavage: No.
Tenacity: Brittle.
Twinning: Common, on {101}.
Fracture: Subconchoidal.
Mohs Hardness: 2.3 – 3
Parting: No.
Streak: Steel gray.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 6.2 – 6.3
2. Geology and Deposits
Stromeyerite typically occurs in hydrothermal Cu-Ag-Pb veins within the upper enrichment zone.
At the type locality (Zmeinogorsk Mine, Altai Krai, Russia), it occurs in hydrothermal Cu and Ag veins, where Ag replaces Cu in bornite.
It is a very late-stage mineral (one of the last to form) that fills fractures in brecciated supergene ores or occupies spaces between other ore minerals. It occurs in small quantities at many localities but is rarely found in larger amounts. It forms from Ag-rich tetrahedrite-tennannite, as well as from minerals with low Ag content.
In masses of supergene chalcocite, it forms as the final mineral.
3. Mineral Associations
It occurs associated with common gangue minerals such as quartz, calcite and barite.
Occurs with:
– Cu minerals (bornite, chalcopyrite, djurleite, enargite, covellite and tennantite),
– Pb and/or Cu and/or Fe minerals (galena, bournonite, betekhinite, seligmannite),
– Ag minerals (native silver, acanthite, minerals from the Freibergite Subgroup, sternbergite,
polybasite), renierite-(As) and
– sphalerite and native copper.
4. Transmitted Light Microscopy
Not applicable, as stromeyerite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Stromeyerite takes a very good polish, better than that of chalcocite. Some care is required to avoid polishing scratches. The polishing hardness of stromeyerite is low – higher than that of acanthite, but lower than that of galena and chalcocite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dull grayish-white with a pinkish-violet tint.
Compared to the color of chalcocite, the color of stromeyerite is lavender-gray.
Compared to the colors of jalpaite and mckinstryite, the color of stromeyerite is very similar.
Pleochroism: Faint, ranging from bluish-gray to brownish-gray; perceptible at grain boundaries.
Reflectivity: 27.36 – 29.57%
Bireflectance: Distinct.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong anisotropy showing light violet, purple, vivid blue, brown, and yellow-orange hues.
Variations in anisotropy colors may be due to deformation (smearing) caused during polishing.
Anisotropy in euhedral (acicular) crystals may be less distinct than in stromeyerite aggregates or intergrowths of stromeyerite with other minerals.
Internal reflections: No.
May be confused with: Few other minerals, considering the paragenesis and the strong, characteristic anisotropy colors. The intense, vivid blue – one of the anisotropy colors – is particularly distinctive. If the veins are thin and native silver is present – which is far more conspicuous (reflective) – stromeyerite is easily overlooked.
Jalpaite is very similar to stromeyerite, and its identification under the microscope is difficult.
Mckinstryite is very similar to stromeyerite, and its identification under the microscope is difficult.
General Characteristics:
Grain shape: Stromeyerite generally occurs as granular aggregates or thin veins; it can form long prisms resembling stibnite. It is frequently intergrown with other silver minerals. Acicular crystals are very rare; the largest single crystal found measured 0.5 cm and had a highly irregular external shape. In veins (fissure fillings), stromeyerite may be uniformly oriented over large distances or areas, or the crystalline aggregates may develop a shape roughly perpendicular to the vein walls.
Polishing scratches may occur if polishing is not performed carefully.
Cleavage is not visible.
Twinning does not occur.
Zoning does not occur.
Intergrowths – typically replacement or myrmekitic – occur with tetrahedrite-tennantite.
Alteration of stromeyerite yields covellite and native silver.
Heating stromeyerite to ~93°C transforms its structure from orthorhombic to cubic. Upon cooling, stromeyerite reverts to the orthorhombic form but displays a structure composed of irregular aggregates of spindle-shaped lamellae (“oleander leaf” texture). If the sample was heated – even moderately – during polished section preparation, it will exhibit this structure, which the specimen did not originally possess. If the sample was prepared without heating yet still displays this spindle-shaped lamellar texture, it serves as an important geological thermometer.
Reaction rims of stromeyerite occur between native silver and chalcocite.
Inclusions of stromeyerite occur in galena and chalcocite.
Intergrowths of stromeyerite may occur with chalcocite (via exsolution), native silver, minerals of the freibergite subgroup, tetrahedrite-tennantite, bornite, chalcopyrite, and galena.
Substitutions 1: stromeyerite replaces pyrite, bornite, tetrahedrite-tennantite, galena, chalcocite, chalcopyrite, and pitchblende.
Substitutions 2: stromeyerite can be replaced by covellite through supergene alteration.
