Feldspars - Species, Crystal Chemistry, Textures, CL and Provenance Potential

Feldspars is the most abundant mineral in the crust

• It forms in igneous, sedimentary and metamorphic rocks.

• Consequently, it is a likely source for framework grains in detrital sedimentary rocks.

• The challenge is to recognize the feldspar species, textures, chemistry that may be preserved in sedimentary rocks without alteration during burial and diagenesis

Feldspar species and crystallography

The feldspars are generally considered a ternary system among the following end-members:

• Albite - NaAlSi₃O₈

• Anorthite - CaAl₂Si₂O₈

• Orthoclase - KAlSi₃O₈

Uncommonly, the species Celsian (BaAl₂Si₂O₈) is found

Other important cations, generally at trace element concentrations include: Li⁺, Rb⁺, NH₄⁺, Sr²⁺, Eu²⁺, Pb²⁺, Mg²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Eu³⁺ Ti⁴⁺

 

Crystallographically, the feldspar minerals are aluminosilicates whose structures are composed of corner-sharing AlO₄ and SiO₄ tetrahedra linked in an infinite 3D array with relatively large (1 A) charge-balancing cations in the irregular cavities of the tetrahedral framework.

The tetrahedral sites are unique and upon cooling partial-to-complete ordering of Al and Si in the tetrahedra may develop, depending on the T of crystallization and the cooling rate. For K-feldspar (KAlSi3O8) has several forms depending on tetrahedral ordering Sanidine - completely disordered Orthoclase - equal probability of Al in the T1(0) and T1(m) sites Microcline - complete ordering of Al on the T1(0) site - loss of symmetry and development of grid twinning.

The ternary feldspars are typically considered in terms of two binary subsystems:

Albite - K-feldspar (NaAlSi₃O₈ - KAlSi₃O₈) - alkali feldspar series

Features:

• large solvus that could theoretically be used as a geothermometer.

• Practically, the alkali feldspar continue to exsolve on cooling and develop the common perthite texture (albite in K-feldspar) and anti-perthite texture (K-feldspar in albite)

• change in structural state with temperature

Albite - Anorthite (NaAlSi₃O₈ - CaAl₂Si₂O₈) - plagioclase series

Features:

• continuous solid solution upon crystallization (plagioclase loop).

• Ordering of the plagioclase structure and the development of 3 miscibility gaps - Peristerite, Boggild and Huttonlocher gaps.

• The presence of these intergrowths in detrital feldspars have a provenance potential. E.g. Lee et al. (2003)

Feldspar ternary

The general systematics of the ternary feldspars form the basis for compositional criteria for provenance studies using feldspars

• empirical source rock fields in the ternary system (Trevena and Nash, 1981)

Other textural features in feldspars:

Subsolidus

• myrmekites

Alteration (low temperature and diagenetic)

• sericitization

• saussaritization

• albitization

CL in feldspars

Feldspars of the system orthoclase-albite-anorthite in crystalline rocks show variable luminescence

• bright blue,

• greenish yellow,

• violet

• red

Three broad emission bands can be distinguishedin feldspar CL:

• a blue emission band (420–500 nm) - mostly due to intrinsic defects (O- hole centers, Al-O-Al centers and Si-O-Si centers), Ti⁴⁺, and possibly Eu²⁺ and Cu²⁺

• one emission band in the green to yellow wavelength range (540–570 nm) - due to trace Mn²⁺

• a red band at the edge of the infrared (690–760 nm) - due to Fe³⁺

• several rare, narrow bands related to rare earth elements such as Sm³⁺, Dy³⁺, Eu³⁺, Tb³⁺, Nd³⁺)

Sandstones can have quite variable CL characteristics e.g.  Zinkernagel (1975) found that the Verrucano-Sandstone of the central Alps contains five different CL-types of alkali feldspar:

a) homogeneous light blue,

b) patchy light blue-Prussian blue

c) patchy light blue-light brown

d) homogeneous light brown

e) homogeneous olive-greenish yellow.

Feldspar cements in sandstones and authigenic feldsparsshow no CL according to observations with a cold cathode equipment .

• Using the hot cathode CL-microscope, responses have been observed.

• Richter and Zinkernagel (1975) described microcline cements with grey-olive or middle to dark brown CL and albite-cements with middle to dark brown colours in Lower Triassic siliciclastics of the central Alps. According to a more recent CL-spectroscopic investigation, the dark olive luminescence of authigenic albite in different carbonate rocks with diagenetic to epimetamorphic overprint is caused by Mn²⁺- and Fe³⁺-activation