Apatite -  Crystallography, Crystal Chemistry, CL and Provenance Potential

Apatite is the most phosphate mineral in the crust

• It forms in igneous and metamorphic rocks.

• It is surprisingly common detritus in clastic sedimentary rocks.

Apatite species and crystallography

Apatite is generally considered a ternary system among the following end-members:

• Fluor-apatite - Ca₅(PO₄)₃F

• Hydroxy-apatite - Ca₅(PO₄)₃OH

• Chlor-apatite - Ca₅(PO₄)₃Cl

 

Crystallographically, apatite has PO₄ tetrahedra linked with two Ca polyhedra: Ca1O₉ and Ca2O₆X (where X is the F, OH or Cl anion). 

The symmetry of apatite is P6₃/m - consequently, it typically forms hexagonal crystals.

Other important substituent cations include

• For Ca²⁺ we can substitute Na⁺, Sr²⁺, Ba²⁺, Mg²⁺, Eu²⁺, Pb²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Eu³⁺, Y³⁺, LREE³⁺ and others

• For P⁵⁺ (or PO₄^3- group) we can substitute Si⁴⁺, SO₄^2-, CO₃^2-, and others

• A common substitution for REEs is: REE³⁺ + SiO₄^4- = Ca²⁺ + PO₄^3-

 

Possible crystal chemical signatures for provenance

F-OH-Cl relations

	Apatites from igneous layered mafic intrusives tend to have higher Cl than metamorphic apatites, particularly those associated with the sulfide-bearing mafic pegmatites.       Apatites from metapelites are generally rich in OH and F with the apatites becoming progressively F-richer with grade. (from Spear and Pyle, 2002)

Metamorphic apatites (Spear and Pyle, 2002):

• generally <0.1 apfu (per 8 oxygens) Fe, Mn, Mg, Y and LREE

• likely partitioning related to coexisting minerals and bulk composition

• minor or no zoning

Igneous apatites (Piccoli and Candela, 2002):

Significant homovalent Mn2+ substitution.	Very strong Na for Ca substitution - Na+ + S6+ = Ca2+ + P5+

Kempe and Gotze (2002) point out that:

• alkaline igneous apatites are generally enriched in LREEs, Na and Sr

• apatites from Sn-W deposits are rich in Mn and low in REEs

CL in apatite

Apatites most commonly exhibit the following CL

• infrared range (870-900 nm) - Nd³⁺ activator

• bright yellow [most common] - generally attributed to Mn²⁺ (576 nm) and/or Dy³⁺  (480 and 580 nm). The Mn²⁺ CL commonly decays with time of exposure.

• reddish orange (3 lines at 560, 600 and 645 nm) - generally attributed to Sm³⁺

• blue (410-450 nm) - attributed to Eu²⁺

• violet (365 nm) - probably a Ce³⁺ activator

• However, there is no simple relation contents of individual REEs and the emission intensities, but an interplay of the relative concentration of the individual REE, its efficiency in activating CL and the presence or absence of sensitizers or quenchers

Additional influences on CL in apatite

• Quenching effect of Fe²⁺

• Self-quenching by Mn²⁺

• Radiation damage by U and Th

• Crystallographic orientation of the apatite

Amphibolite from the Beartooth Mountains with zoned apatite

Apatite CL in rare-metal deposits (Kempe and Gotze, 2002)