AESCHYNITE

Aeschynite is a fairly rare oxide, typical of some granitic pegmatites. It is an ore of Rare Earth Elements (REEs).

“Aeschynite” actually refers to a group of oxides containing REEs, the most common member of the group being that with yttrium.

The Aeschynites Group is made up of:

Aeschynite-(Ce) – (Ce,Ca,Fe)(Ti,Nb)2(O,OH)6

Nioboaeschynite-(Ce) – (Ce,Ca)(Nb,Ti)2(O,OH)6,

Aeschynite-(Y) – (Y,Ca,Fe,Th)(Ti,Nb)2(O,OH)6,

Tantalaeschynite-(Y) – (Y,Ce,Ca)(Ta,Ti,Nb)2O6,

Aeschynite-(Nd) – (Nd,Ce)(Ti,Nb)2(O,OH)6,

Nioboaeschynite-(Nd) – (Nd,Ce)(Nb,Ti)2(O,OH)6 and

Nioboaeschynite-(Y) – [(Y,REE),Ca,Th,Fe](Nb,Ti,Ta)2(O,OH)6 .

Aeschynite, when massive, is indistinguishable from rutile, but is magnetic, unlike rutile. It can occur in well-formed crystals. Altered crystals are often covered by an earthy crust of limonite.

1. Characteristics

The data below refers to aeschynite-(Y), which is the most common:

Crystal system: Orthorhombic bipyramidal.          

Color: Pale yellow, yellow, orange-yellow, pale greenish.     

Habit: Generally tabular, it can be prismatic, granular, with crystals up to 10 cm.       

Cleavage: {100} perfect, {010} perfect, {001} perfect. 

Tenacity: Brittle.        

Twinning: No.       

Fracture: Irregular.       

Mohs Hardness: 5 – 6

Parting: No.         

Streak: White, yellowish to reddish.         

Lustre: The luster on crystalline surfaces is submetallic to pearly, while on fracture surfaces it is resinous to greasy.          

Diaphaneity: Transparent.           

Density (g/cm³): 4.82 – 4.93

          

2. Geology and Deposits

Aeschynites occur in granitic pegmatites rich in REEs.

They can occur in dolomitic carbonatites with ankerite.

Also as a detrital mineral in placers.

 

3. Mineral Associations

They are associated with the common constituent minerals of pegmatites, such as quartz, feldspars (albite, orthoclase), micas (biotite, muscovite), columbite, tantalite, monazite, titanite, zircon, clinochloride, and pyrite.

In addition, they are associated with specific minerals of the paragenesis, such as allanite, bazzite, corundum, fergusonite, gadolinite, synchysite-(Ce), thorite, and various minerals with yttrium (xenotime-(Y), kainosite-(Y), and euxenite-(Y)).

In carbonatites, it is associated with calcite.

 

4. Transmitted Light Microscopy

The information available in the literature is scarce and fragmented. About aeschynite-(Y):

Refraction indices:  nα: 2,190 – 2,280    nβ: 2,210 – 2300      nγ: 2,340 – 2500

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Reddish brown to light brown.

Relief: Very high.           

Cleavage: {100} perfect, {010} perfect, {001} perfect.           

Habits: No information available.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Maximum birefringence of 0.15 – 0.22, very high, 9th order, pearly colors difficult to classify. If metamict, it is isotropic.

Extinction: Probably paralell, as expected by an orthorhombic mineral.          

Elongation sign: No information available.            

Twins: No.        

Zoning: No information available.               

CONVERGENT LIGHT

Character: B(+)         

2V angle: 70-80º         

Alterations: It may undergo metamictization and become isotropic.         

May be confused with: No information available.         

Caption

5. Reflected Light Microscopy

No Reflected Light Database addresses aeschynites, mainly because Reflected Light is not the analytical technique indicated for their identification.

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: 

Caption