Sanmartinite
Chemical formula: Zn<sup>2+</sup>W<sup>6+</sup>O<sub>4</sub>
Sanmartinite is a rare zinc tungstate, forming small, tabular crystals of brown color and resinous luster.
Properties
- Mohs hardness
- 4
- Luster
- Resinous to submetallic
- Streak
- Reddish-brown
- Density
- 7.95-8.12
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Tetragonal
Diagnostic features
## Identification Identifying sanmartinite under amateur conditions is very difficult due to its rarity and small crystal size. Key features include high density, brown color, resinous luster, and co-occurrence with other tungstates in a specific geological environment. Definitive identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Distinguishing from Similar Minerals Sanmartinite can be confused with other tungstates, such as wolframite or scheelite. It differs from wolframite by its lighter, reddish-brown streak (wolframite has an almost black streak) and tetragonal crystal system. Scheelite is usually lighter, often exhibits fluorescence under UV light, and has a lower density. ## Crystal Forms Sanmartinite crystals are very small, usually tabular or platy in habit, often arranged in rosette-like aggregates. It also occurs as granular aggregates and masses filling small fractures in the rock.
Geological environment
## Genesis Sanmartinite is a secondary mineral, forming in the oxidation zones of tungsten deposits. It forms as a result of the weathering of primary tungsten minerals, such as scheelite and wolframite, in the presence of zinc-rich solutions derived from the oxidation of sphalerite. ## Mineral Associations It most often co-occurs with minerals from which it forms or which accompany it in the oxidation zone. These are primarily scheelite, wolframite, sphalerite, as well as quartz, mica, and tungstite. ## Localities The most important and classic occurrence is the type locality – the Los Cerros mine in San Martín, San Luis Province, Argentina. In addition, its presence has been reported in several other locations worldwide, including the Kara mine in Tasmania (Australia) and the Hemerdon mine (Devon, England).
Rarity
Very rare
For collectors
## Quality Criteria The quality of sanmartinite specimens is assessed primarily based on the presence of well-formed, even microscopic, crystals. The most valued specimens are those with distinct, sharp crystals of intense, brown color, forming aesthetic groups on a contrasting rock matrix. Due to the mineral's rarity, any specimen with confirmed identity is valuable to specialized collectors. ## Popular Localities By far the most sought-after specimens come from the type locality in Argentina, which has yielded the best known crystals of this mineral. Specimens from other localities, such as Tasmania, are also valued for their rarity.
Care and storage
## Cleaning Specimens should be cleaned only very carefully, using a soft brush to remove dust. Due to their brittleness and small crystal size, washing with water, and especially ultrasonic cleaners, should be avoided. ## What to Avoid Avoid contact with acids and other chemicals that can damage the mineral's surface. Protect from impacts and scratching, as it is relatively soft and brittle. ## Storage It is recommended to store specimens in sealed display boxes to protect them from dust and mechanical damage. The best way to display them is to place the specimen in a box with a magnifying glass (a "micromount box") due to the small size of the crystals.