Rivadavite
Chemical formula: Na₆Mg[B₆O₇(OH)₆]₄·10H₂O
Rivadavite is a rare, hydrated sodium and magnesium borate, forming colorless, prismatic crystals with a vitreous luster.
Properties
- Mohs hardness
- 3.5
- Color
- Colourless, white
- Luster
- Vitreous
- Streak
- White
- Density
- 1.905
- Cleavage
- Perfect on {100} <mi>{_102}</mi> poor on {010}
- Fracture
- Splintery
- Transparency
- Transparent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Characteristic features of rivadavite include its colorless, elongated crystals with a vitreous luster, aggregated into radial masses. The key is the locality – borate deposits in Argentina. Water solubility is an important diagnostic feature, though destructive to the sample. ## Distinguishing from similar minerals Rivadavite can be confused with other colorless borates, such as ulexite or colemanite, which often co-occur in the same localities. However, ulexite forms characteristic fibrous aggregates (the so-called "television stone"), and colemanite has different, more complex crystal forms and higher hardness (4.5). Final distinction requires advanced analytical methods (XRD, EDS). ## Crystal forms Crystals are prismatic, elongated along the [001] axis, and often terminated by flat faces. They form radial, stellate, or tangled aggregates.
Geological environment
## Genesis Rivadavite is an evaporite mineral. It forms as a result of the evaporation of saline lakes (salt pans) in an arid, desert climate. It crystallizes directly from solutions rich in boron, sodium, and magnesium. ## Mineral associations This mineral co-occurs with other borates, such as ulexite, kernite, borax, halite, and also gypsum. ## Localities The most important and type locality for rivadavite is the Tincalayu mine in Salar del Hombre Muerto, Salta Province, Argentina. This is the only confirmed locality from which well-formed specimens of this mineral originate.
Rarity
Very rare
For collectors
## Quality criteria The most prized rivadavite specimens are those with sharply terminated, transparent, and lustrous crystals, forming aesthetic, radial aggregates. The absence of mechanical damage and the clarity of the crystals are important. Specimens on a rock matrix are rarer and more desirable than single aggregates. ## Popular localities The only source of collector specimens is the Tincalayu mine in Argentina. All valuable samples on the market come from this single locality.
Care and storage
## Cleaning Rivadavite specimens should be cleaned very carefully, using only a soft brush to remove dust. The mineral is water-soluble, so any contact with liquids should be avoided. ## What to avoid Absolutely avoid contact with water, acids, and other chemicals that could damage or dissolve it. Store away from moisture and heat sources, which can lead to dehydration and destruction of the crystals. ## Storage It is recommended to store specimens in tightly sealed containers (such as "membrane boxes") or in dry display cases away from sources of moisture. Due to its brittleness, avoid vibrations and contact with harder minerals.