ALLANITE

Allanites – (Ca,Mn,Ce,Y,Th,La)2(Fe,Ti,Al)3O(Si2O7)(SiO4)(OH) – are relatively common sorosilicates that constitute ore of Rare Earth Elements (REEs), as they can contain up to 20% of them.

In reality, “allanite” does not designate a mineral, but is just a generic name for the minerals that make up the Allanite Group, classified in the Epidote Supergroup. Based on the predominant Rare Earth Element, four minerals are recognized in the Allanite Group: allanite-(Ce), allanite-(La), allanite-(Nd) and allanite-(Y). Allanite-(Ce) is the most common; the other three allanites are extremely rare.

Radioactivity from the elements contained in allanites (i) creates pleochroic halos around allanite grains when they occur as inclusions in mafic minerals and (ii) makes allanites metamictic, when the mineral’s structure is destroyed and it tends to become isotropic, which (iii) can form epidote reaction rims around these metamictic allanites.

Allanite-(Ce) often forms a solid solution with epidote and has four varieties, the others have no varieties. May contain appreciable amounts of Be.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Black, can be brown to violet-brown.

Habit: Tabular, prismatic, acicular, granular, massive, usually metamictic.

Cleavage: {001} imperfect, {100} poor, {110} poor.

Tenacity: Brittle.

Twinning: Polysynthetic on {100}, common.

Fracture: Conchoidal, irregular.

Mohs Hardness: 5.5 – 6

Parting: No.

Streak: Gray.

Lustre: Vitreous, greasy, submetallic.

Diaphaneity: Transparent.

Density (g/cm³): 3.5 – 4.2

 

2. Geology and Deposits

Allanite is an accessory mineral of many types of igneous and metamorphic rocks.

In igneous rocks it is present in granites, granodiorites, diorites, syenites and pegmatites and equivalent volcanic rocks. It rarely occurs in gabbroic pegmatites.

In metamorphic rocks it is found more rarely, in schists, amphibolites, gneisses, skarns and marbles.   It also occurs as a clastic component of sediments.

 

3. Mineral Associations

Allanite-(Ce) occurs associated with common silicates such as quartz, feldspars (albite, microcline), micas (biotite, muscovite), epidote, titanite, zircon, amphiboles (green hornblende, riebeckite), pyroxenes (diopside), tourmaline and chlorite (clinochlor).

It also occurs with carbonates (calcite, dolomite), pyrite, hematite, talc, monazite, fluorite, pyrochlore, bastnäsite (Ce), yttrofluorite and fluorbritholite (Ce).

The other allanites occur associated with a series of rare and very rare minerals. See specific bibliography.

 

4. Transmitted Light Microscopy

Refraction indices: nα: 1.690 – 1.813          nβ:  1.700 – 1.857             nγ: 1.706 – 1.891

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: When unaltered (non-metamict), weak gray-yellow to orange-brown, with red-brown pleochroism, also with green colors.

Moderate to strong pleochroism:

X = pale olive green, reddish brown;

Y = dark brown, yellow-brown;

Z = dark reddish brown, greenish brown.

If metamict, pleochroism in brown, brown-red, yellow-brown, brown-green or green. It can form pleochroic halos around it.

Relief: High (low relief in strongly metamict varieties).

Cleavage: {001} good, {100} poor and {110} poor. They are normally not visible.

Habits:

Tends to be subhedral to anhedral granular, similar to epidote. Columnar, tabular, acicular, prismatic, massive. It can form pleochroic halos around it. It may be encased in epidote and clinozoisite.

As it contains Th and U, the crystal is destroyed and absorbs water, causing the grain to swell, creating radial fractures into neighboring minerals.

When allanite occurs as an inclusion in biotite, chlorite and hornblende, dark halos form around the allanite grains.

Altered allanite (metamict) can form a reaction rim of epidote and/or clinozoisite around it; this rim can be partial or complete.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.018 to 0.031, resulting in strong 1st, 2nd and 3rd order final colors, which are usually masked by the intense color of the mineral.

Isotropic if metamict.

Extinction:

Parallel in the longitudinal sections. Oblique with an angle of 26 – 72º in the other sections.

The mineral’s strong colors can make it difficult to determine the extinction type.

Elongation sign: Cannot be identified due to high birefringence.

Twins: Eventual, polysynthetic, according to {100}.

Zoning: Almost always zoned; the core is darker than the edges.

CONVERGENT LIGHT

Character: B(+) and B(-), isotropic if metamictic. Usually B(-), varieties are B(+).

2V angle: 40 – 90º, B(+): 90-57º

Alterations: alters to glass, due to the radiation emitted by its radioactive components (metamictization). It can change the epidote, forming a corona texture. Due to weathering, it is replaced by carbonate and limonite. Can alter to minerals of the Ancylite Group.

May be confused with: some minerals of dark brown color and with pleochroism.

Brown hornblende has good cleavage in two directions in the basal sections (amphibole cleavage!), different habit and occurs in other paragenesis.

Anatase can be very similar.

 

Allanite 01: idiomorphic allanites in PPL, showing one of the pleochroism colors.
Allanite 01: same crystals as in the previous image, rotated 90 degrees, in PPL, with its second pleochroism color.
Allanite 01: now in XPL, the same group of crystals of the previous images. Strong colors are typical for allanite without metamictization.
Metamict allanite in PPL with many radial fractures caused by the increase of the grain volume.
Allanite 02: allanite crystal in PPL with beginning metamictization. showing the radial fractures due to swelling of the grain.
Allanita 02: same crystal of the previous figure, this time in XPL. It is not isotropic because metamictization is only starting.
Allanite 03: Metamict allanite with epidote corona in PPL. Allanite, despite metamictization, still with remnants of the original colors.
Allanite 03: same crystal as in previous figure, this time in XPL. Epidote shows strong and bright colors and allanite is almost isotropic.
Allanite 04: in PPL, allanite crystal with epidote corona. High relief and a very faint green color identifies epidote.
Allanite 04: in XPL, allanite shows its twin and epidote its strong colors.
In XPL, big zoned grain of allanite with very dark colors. In PPL, the colors are almost the same.
In XPL, big zoned grain of allanite. In PPL the colors are brownish, but very similar, with a faint pleochroism.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of allanite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with allanite.

Sample preparation: Allanite polishing is simple and does not require specific care. It looks better than the polishing of the gangue minerals

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, but lighter than feldspars and quartz and much lighter than micas, for example.

Pleochroism: No.

Reflectivity: Low (<<10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.

Internal reflections: Generalized, very dark and uniform, forming a very homogeneous, almost black surface. At some points, such as fractures, boundaries and holes, some dim reflections of dark reddish brown color occur.

May be confused with: many other minerals, such as ferberite, hübnerite, columbite, tantalite and several others that do not present diagnostic characteristics neither in PPL nor in XPL.

General Characteristics:

Grain shape tends to tabular.

Polishing scratches do not occur.

Fluid inclusions may occur, even large and in large quantities.

Allanita 01: in PPL, allanite (right) shows better polishing and a lighter color than quartz (left).
Allanite 02: in XPL, allanite is very dark, almost black. Quartz with clear internal reflections.
In XPL, at some spots where the crystal is thinner, reddish-brown internal reflection can be seen in allanite.
In PPL, sometimes relatively large, single-phase fluid inclusions can be seen in allanite.
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