TRONA
Trona – Na3H(CO3)2.2H2O – is a relatively rare carbonate that occurs in arid regions. It is an important ore of sodium carbonate (“soda ash”) mainly in the USA, where the Green River Formation deposit is the largest known deposit of its type in the world. In the rest of the world, Na carbonate is obtained by the “Solvay Process”.
It is effervescent with acids and may fluoresce yellow or light greenish yellow or green under ultraviolet light, both longwave and shortwave. Under long waves the color is more intense. It has an alkaline flavor.
It is soluble in water and must be stored in a dry environment. Over the years the sample may dehydrate and crumble.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Colorless, white-gray, light yellow to yellow.
It can be pink to red due to the incorporation of algae.
Habit: Usually fibrous or massive columnar. Aggregated in rosettes. Crystals up to 10 cm elongated by [010] or flattened by {001}.
Cleavage: {100} perfect, {-111} bad, {001} bad.
(literature diverges!) Striations on {hol} faces, parallel to [010].
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular, sub-conchoidal.
Mohs Hardness: 2.5
Parting: No.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 2.11 – 2.17
2. Geology and Deposits
Trona is a non-marine evaporite typical of saline lakes and evaporites formed in this environment. It composes the sedimentary rock called “sodic carbonate rock”.
It occurs as efflorescence in soils and along riverbanks in arid regions.
Rarely occurs in fumaroles.
Trona can also be found in cavities of some pegmatites, associated with secondary Na minerals. In these cases, it was formed by autometasomatic reactions of late-margmatic fluids with rocks from the plutonic complex itself or by large-scale unmixing of volatiles from the last stages of magmatism.
3. Mineral Associations
Trona associates with minerals characteristic of evaporites, such as gypsum and halite.
It occurs with other Na carbonates (natron, thermonatrite, nahcolite, sanrománite, juangodoyite, burkeite) and with Na sulfates (mirabilita, glauberite, sulphohalite, thenardite).
Also with Pb minerals such as phosgenite and wulfenite.
In the Green River Formation (Colorado, Wyoming, Utah – USA), trona is associated with other Na carbonates such as shortite, northupite, bradleyite and pirssonite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.416 nβ: 1.494 nγ: 1.542
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: {100} perfect, {-111} bad, {001} bad.
Habits: Usually massive columnar or fibrous. Crystals are elongated or flattened (tabular).
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.128, extremely high, resulting in high-order interference colors (>6th order), in cream tones, typical of carbonates.
Extinction: Oblique.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(-)
2V angle: 72 – 75º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected Light microscopy is not the recommended analytical method for trona identification and opaque minerals associated with it do not occur.