ALUNITE
Alunite – KAl3(SO4)2(OH)6 – is a relatively common sulfate, mined as a source of alum (KAl(SO4)2-12H2O), as an ore of Al (36.92% Al2O3) and K (11.37% K2O) or as a raw material for potassium fertilizers. It is a traditional and important ore in certain regions, as alunite occurrences are very common; some deposits are huge. The most famous deposit is in the mountains of Tolfa (Italy), where it was recognized in the 15th century in the mines of Allumiere. Alunite has been used for geochronological dating (by the K-Ar method) of weathering processes in ore deposits and also in relation to its deposition in caves. The more compact alunite masses found in Hungary are so hard that they were used as millstones.
It is classified in the Alunite Group and forms a series with natroalunite (of Na). It usually contains appreciable amounts of Na and is the Al analogue of jarosite. Alunite crystals are, morphologically speaking, rhombohedrons, but have angles of 90º 50′ and are very similar to cubes (they are pseudocubic). In reality they are a combination of two trigonal pyramids. They can develop in the form of flattened rhombohedrons. Alunite is strongly pyroelectric, insoluble in water and weak acids, but soluble in H2SO4.
In hand specimen, the white and finely granular masses of alunite are extremely similar to limestone, dolomite, anhydrite and magnesite. But, unlike limestones and dolomites, alunite does not show effervescence with dilute hydrochloric acid, even if pulverized and hot. The conclusive identification of alunite requires other analytical methods than microscopy.
1. Characteristics
Crystal system: Trigonal, piramidal ditrigonal.
Color: Yellow, red to reddish brown. Colorless when pure, it can be white or in pale shades of gray.
Habit: Fibrous to columnar, tabular, porcelain, usually granular to massive dense. Crystals up to 1 cm.
Cleavage: {0001} perfect.
Tenacity: Brittle.
Twinning: No.
Fracture: Conchoidal.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: White.
Lustre: Vitreous, pearly, earthy.
Diaphaneity: Transparent.
Density (g/cm³): 2.6 – 2.9
2. Geology and Deposits
Alunite is formed between 15 – 400ºC by the action of solutions containing sulfuric acids on rocks rich in potassic feldspars, a process called “alunitization”, usually accompanied by kaolinization and silicification.
The sulfuric acids are generated by the oxidation and leaching of sulphides (pyrite) or from volcanic fumaroles. They accompany hydrothermal solutions generally rich in certain metals. In this way, alunite occurs in veins and as replacement masses in trachytes, rhyolites and other aluminous and potassium-rich volcanic rocks, in these cases being a rock-forming mineral. Sometimes very bright small crystals are found loose in cavities (vesicles) of altered rhyolites. There may also be crystals in the walls of rock fissures with alunite.
Alunite can also be found near volcanic fumaroles. It is associated with epithermal deposits of quartz-alunite-gold and in the oxidation zone of hydrothermal sulphide deposits.
3. Mineral Associations
Alunite occurs with other Al minerals (kaolinite, halloysite, diaspore), sulfides (pyrite, stibnite, famatinite, enargite), oxides (hematite), barite, gypsum, jarosite, goethite, native copper, native gold and quartz. Also with some rare minerals like pyracmonite, rhodalkylarite, blatonite and walfordite.
4. Transmitted Light Microscopy
Refraction indices: nω: 1.572 – 1.620 nε: 1.592 – 1.641, increase with increasing Fe content.
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: Distinct (basal) cleavage (0001). There is a very bad rhombohedral cleavage {10-12}.
Habits: Crystals are rare, tabular parallel to (0001) or rhombohedral (pseudo-cubic!) providing square or diamond-shaped sections. It usually forms granular, lamellar or feathery aggregates. Also as disseminated grains, they can present very small sizes, which makes their identification difficult.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.020: colors from gray and orange to first order blue. They can show only 1st order colors, between gray and yellow.
Extinction: Parallel to the cleavage.
Elongation sign: ES(-).
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: U(+)
2V angle: No.
Alterations: generally alunite is formed by the alteration of alkali feldspar.
May be confused with: brucite, for example, but brucite occurs in another paragenesis and often has anomalous interference colors.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of alunite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with alunite.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics: