BORAX
Borax – Na2[B4O5(OH)4].8H2O – is a very rare hydrated borate characteristic of evaporitic environments, mainly ephemeral salt lakes. It is the most important ore of B, an element with dozens of important applications, mainly in glass.
In mining and industry, the term “borax” is used indiscriminately for various minerals that are a source of B, such as borax, kernite, ulexite, ascharite, pandermite, hydroboracite, and colemanite. Another designation used for borax is “tincal.” The best-known borax deposit is Boron (California, USA), and the largest borax reserve is located in Turkey.
It is highly soluble in water and dehydrates rapidly once exposed to air, forming a brittle (crumbly) mass of the mineral tincalconite. It is diamagnetic.
Be very careful! It has a sweet, alkaline taste, but ingestion of borax causes headaches, lethargy, and gastrointestinal disturbances such as nausea, vomiting, abdominal pain, and diarrhea. High doses of borax over several years can cause liver cancer.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Colorless, gray, white, yellowish, rarely bluish or greenish.
Habit: Short prismatic to tabular. Crystals up to 10 cm, typically deformed. Generally massive to granular.
Cleavage: {100} perfect, {110} good, {010} poor.
Tenacity: Brittle.
Twinning: On {100}, rare.
Fracture: Conchoidal.
Mohs Hardness: 2 – 2.5
Parting: No.
Streak: White.
Lustre: Vitreous, resinous, earthy.
Diaphaneity: Transparent.
Density (g/cm³): 1.715 (very low!)
2. Geology and Deposits
Borax is characteristic of evaporitic environments, such as ephemeral salt lakes and playas, where repeated evaporation of water containing B occurs. It forms a rock called “borax-stone”.
It crystallizes as efflorescence in soils of arid regions.
It occurs in solution in hot springs.
3. Mineral Associations
It is associated with minerals typical of evaporitic sequences:
– carbonates such as calcite, dolomite, gaylusite, nahcolite and trona,
– halides such as halite, sylvite and carnallite,
– sulfates such as gypsum, anhydrite, aphthitalite, glauberite, hanksite, thénardite, sulphohalite, polyhalite and kieserite,
– nitrates such as nitratine and
– other borates such as ulexite, tincalconite, inyoite, colemanite, kernite and kurnakovite.
4. Transmitted Light Microscopy
Preparing thin sections of borax requires fresh samples that have not yet dehydrated to tinchalcone and special care is needed because it is not possible to work with water!
Refraction indices: nα: 1,447 nβ: 1,469 nγ: 1,472
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate negative.
Cleavage: {100} perfect, {110} good (in two directions, almost perpendicular to each other!) and {010} poor.
Habits: Generally massive, granular. Crystals are short prismatic to tabular. Basal sections have 8 sides.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.025, corresponding to colors up to the end of the 1st order: gray, yellow, orange, red and blue, can reach the beginning of the 2nd order (green).
Extinction: Oblique in sections parallel to (010), with an angle of 33 to 36º in relation to the cleavage.
Parallel in sections parallel to (100) and in the basal sections.
Symmetrical in the basal sections, in relation to the cleavage {110}.
Sections perpendicular to one of the optical axes do not extinguish completely and show anomalous brown or blue interference colors due to strong dispersion.
Elongation sign: ES(-) in sections parallel to (010) and ES(+) in sections parallel to (100).
Twins: Simples twins, rare.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: 39 – 40º
Alterations: When exposed to air, it rapidly dehydrates to tincalconite.
May be confused with: other evaporitic minerals.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying the mineral. Opaque minerals do not occur in the paragenesis.