ASTROPHYLLITE
Astrophyllite – K2NaFe2+7Ti2Si8O26(OH)4F – is a fairly rare inosilicate, typical of some lithotypes of extreme composition. It is not an ore, but collector’s pieces fetch high prices.
It belongs to the Astrophyllite Group and forms a series with kupletskite. It may contain various impurities such as Nb, Ta, Zr, Al, Mg, Mn and Ca.
1. Characteristics
Crystal system: Triclinic pinacoidal.
Color: Bronze yellow, golden yellow, brown to reddish-brown.
Habit: Tabular, lamellar, elongated, acicular crystals form perfect radial aggregates.
Cleavage: {001} perfect, like mica. {100} poor. Striations occur.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 3
Parting: No.
Streak: Golden.
Lustre: Resinous, pearly, vitreous, submetallic.
Diaphaneity: Transparente.
Density (g/cm³): 3.2 – 3.4
2. Geology and Deposits
Astrophyllite is an accessory mineral characteristic of basic intrusive igneous rocks such as nepheline syenites and associated gneisses, alkaline granites, and correlatable pegmatites.
It also occurs in fenites and other types of metasomatic rocks.
3. Mineral Associations
It occurs associated with quartz, feldspars (microcline, abite), feldspathoids (nepheline, cancrinite, sodalite), pyroxenes (augite, aegirine), amphiboles (arfvedsonite, riebeckite and others), micas (biotite, annite), zeolites (natrolite, analcime), garnet (melanite variety) and zircon.
Evidently it occurs with the typical minerals of these more exclusive paragenesis: catapleiite, leucophanite, djerfisherite, rasvumite, nenadkevichite, delhayelite, hainite-(Y), kupletskite, eudyalite, lavenite, aenigmatite, thorite, tritomite-(Ce) and pectolite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1,680 nβ: 1,700 nγ: 1,730
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Strong pleochroism between:
X = deep orange-red,
Y = yellow-orange,
Z = lemon-yellow
Relief: High.
Cleavage: {001} perfect, as micas. {100} poor.
Habits: Tabular, lamellar, elongated, acicular crystals form perfect radial aggregates.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.050, quite high, resulting in colorful and intense colors of up to the 3rd order: yellows, reds, blues and greens. But the mineral’s strong natural color masks these colors.
Extinction: Paralell.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 70 – 90º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying astrophyllite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with astrophyllite, such as magnetite.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx