Sarmientite
Chemical formula: Fe³⁺₂(As⁵⁺O₄)(S⁶⁺O₄)(OH)·5H₂O
Sarmientite is a rare, secondary iron arsenate and sulfate, forming aggregates of small, yellow crystals, valued by collectors of rare minerals.
Properties
- Mohs hardness
- 2.5
- Color
- Light yellow-orange
- Luster
- Vitreous, Pearly
- Streak
- Pale yellow
- Density
- 2.58
- Cleavage
- Perfect on {010}
- Fracture
- Uneven
- Transparency
- Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Sarmientite can be identified by its characteristic lemon-yellow color, radial or rosette aggregates, and occurrence in the oxidation zones of arsenic deposits. Its low hardness and pearly luster on cleavage planes are also helpful features. ## Distinguishing from Similar Minerals Sarmientite is sometimes confused with other secondary iron sulfates and arsenates, such as jarosite or scorodite. It differs from jarosite by the presence of arsenic and usually a lighter, more lemon-yellow hue. It differs from scorodite in color (scorodite is more often greenish or bluish) and crystal form. However, definitive differentiation requires advanced chemical (EDS) or X-ray (XRD) analyses. ## Crystal Forms Sarmientite crystals are small, usually tabular or bladed, often flattened. They form characteristic fan-shaped, radial, rosette, and also spherical aggregates and crusts.
Geological environment
## Genesis Sarmientite is a secondary mineral, formed as a result of weathering (oxidation) of sulfidic and arsenical ore minerals, mainly pyrite and arsenopyrite. It forms under low temperature and high acidity conditions in the oxidation zones (iron caps) of polymetallic deposits. ## Mineral Associations This mineral co-occurs with other oxidation products, such as goethite, jarosite, scorodite, pitticite, bukovskýite, zýkaite, as well as native sulfur and gypsum. ## Localities The most important and historical locality for sarmientite is the Santa Elena mine in Calingasta Department (San Juan Province, Argentina), where it was discovered. It is also known from several other places worldwide, including the Goldstrike mine in Eureka County (Nevada, USA), Chidlov in the Czech Republic, and the Clara mine in the Black Forest, Germany.
Rarity
Very rare
For collectors
## Quality Criteria Specimens with well-formed, sharp crystals forming aesthetic, radial or rosette aggregates are most valued by collectors. An intense, lemon-yellow color is a key factor enhancing attractiveness. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to the small size of the crystals, specimens suitable for microscopic viewing (so-called "micromounts") are the most common form in collections. ## Popular Localities Specimens from the type locality in Argentina are historically most important, although rarely available on the market. Currently, small quantities of collector material come from the Goldstrike mine in Nevada (USA) and from European localities, such as the Clara mine in Germany.
Care and storage
## Cleaning Sarmientite specimens should be cleaned with the utmost care, using a soft brush to remove dust. Due to its water solubility, contact with liquids should be avoided. If wet cleaning is absolutely necessary, use a minimal amount of isopropyl alcohol on a cotton swab, avoiding saturating the specimen. ## What to Avoid Contact with water, which can dissolve and destroy the mineral, must be strictly avoided. Sarmientite is unstable in low humidity conditions and can dehydrate, losing its structure and luster. It should be protected from high temperatures, direct sunlight, and all chemicals and acids. ## Storage Sarmientite specimens require storage under stable conditions. It is best to keep them in tightly sealed containers (e.g., "membrane boxes") or in display cases with controlled humidity to prevent dehydration. It should be isolated from other minerals that could mechanically damage it.