CARNALLITE
Carnallite – KMgCl3.6H2O – is a less frequent halide that occurs in evaporitic sequences and is very important as a K ore (for fertilizers!); only sylvite (KCl) is more important. Locally, carnallite is mined as a source of Mg.
It is inconspicuous, rarely forming crystals. The mixture of carnallite with halite is called “almeraite”. There is a variety with Br and another with iodine. It may contain Br, Rb, Cs, Tl, and Fe. It has a bitter taste. It is soluble in water, and the solution, once evaporated, leaves a residue composed of sylvite and bischofite.
It undergoes deliquescence (absorbs moisture from the air and liquefies), which makes the preparation of thin sections complicated (it is necessary to work with oil). In addition, moisture enters the thin section from the sides, under the coverslip, dissolving the carnallite and rendering the thin section unusable.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Colorless, white, blue, yellow, red (hematite inclusions).
Habit: Massive to fibrous. Rare, pyramidal or pseudo-hexagonal crystals.
Cleavage: No.
Tenacity: No information available.
Twinning: Polysynthetic due to pressure.
Fracture: Conchoidal.
Mohs Hardness: 2.5
Parting: No.
Streak: White.
Lustre: Greasy, vitreous, resinous, or dull.
Diaphaneity: Transparent.
Density (g/cm³): 1.6
2. Geology and Deposits
Carnallite is formed as a result of soluble salts deposited in an evaporitic environment (evaporation of seawater), when the salt concentration in the water reaches 98.8%, generating fluids with a high potassium content.
It requires very specific conditions: closed basins containing saltwater in arid climates, with freshwater inflow. It forms massive layers up to several meters thick.
3. Mineral Associations
It occurs associated with common minerals in evaporitic sequences, mainly halides (halite, sylvite), carbonates (calcite, dolomite), and sulfates (gypsum, anhydrite).
Less commonly associated minerals include kaninite, picromerite, rokühnite, tachyhydrite, rinneite, polyhalite, bischofite, and kieserite.
4. Transmitted Light Microscopy
Refraction indices: nα = 1.465 – 1.466 nβ = 1.474 – 1.475 nγ = 1.494 – 1.496
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism:
Relief: Moderate.
Cleavage: No.
Habits: Usually massive.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.029 – 0.030
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: Polysynthetic are possible.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 70º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is certainly not the appropriate analytical technique for identifying carnallite, nor are there any opaque minerals in its specific paragenesis.