AENIGMATITE
Aenigmatite – Na4[Fe2+10Ti2]O4[Si12O36] – is a very rare inosilicate similar to amphiboles (same habits). It has no economic importance.
It is classified in the Sapphirine Supergroup, Aenigmatite Group, forming a series with wilkinsonite. May contain Mn, Mg, Ca, K, Cl and Al as impurities. Aenigmatite is related to malakhovite.
1. Characteristics
Crystal system: Triclinic pinacoidal, pseudo-monoclinic.
Color: Black velvet.
Habit: Prismatic, columnar, acicular, up to 10 cm.
Pseudo-monoclinic. Also as irregular segregations.
Cleavage: {010} good, {100} good, they intersect at 66º, which differentiates them from amphiboles!
Tenacity: Brittle.
Twinning: Common, polysynthetic.
Fracture: Irregular, uneven.
Mohs Hardness: 5.5 – 6
Parting: No.
Streak: Reddish brown.
Lustre: Greasy.
Diaphaneity: Transparent.
Density (g/cm³): 3.81
2. Geology and Deposits
Aenigmatite is a primary mineral of alkaline leucocratic (Na-rich) magmatic rocks such as nepheline syenites (foyaites), sodic trachytes and peralkaline phonolites. It occurs in pegmatites related to nepheline syenites.
It also occurs in other silica-poor igneous rocks such as pantellerites and comendadites.
3. Mineral Associations
Aenigmatite occurs associated with the other typical minerals of these low-silica and high-sodium paragenesis:
– feldspars such as orthoclase, microcline, sanidine and albite (including anorthoclase variety),
– feldspathoids such as nepheline,
– amphiboles such as arfvedsonite and riebeckite,
– clinopyroxenes such as augite, hedenbergite, aegirine, katophorite and katophorite-(Fe),
– oxides such as hematite and ilmenite,
– olivine (fayalite), eudyalite, sodalite, astrophyllite, ferrokentbrooksite, titanite, allanite, lomonosovite, fluorapatite, biotite and opal-AN.
It can be associated with quartz.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.780 – 1.800 nβ: 1.800 – 1.820 nγ: 1.870 – 1.900
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Reddish brown to black, almost opaque (black). Pleochroism between:
X : pale red brown
Y: dark walnut brown
Z: deep dark brown
Relief: Very high
Cleavage: {010} good and {100} good, which intersect at approximately 66º, which differentiates them from amphiboles!
Habits: Prismatic, acicular, xenomorphic (anhedral) grains.
Intergrowths with arfvedsonite are common.
Allanite inclusions have a black to brown halo around them.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.09 to 0.1, extremely high, corresponding to colors of up to 5th order, pale, masked by the intense color of the mineral.
Extinction: Always oblique, with an angle of 40-45º parallel to (010) and an angle of 4º in (100).
Elongation sign: No information available.
Twins: Polysynthetic twins are common. The twin plane bisects the obtuse angle that the cleavages form with each other.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 27 to 55o
Alterations: No information available.
May be confused with: rhönite, which occurs in other paragenesis. In relation to kaersutite (brown hornblende) and arfevdsonite, their twin plane divides the angle formed by the cleavages between them in half.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of aenigmatite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with aenigmatite, like magnetite and ilmenite.
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: