Amphibole -  Crystallography, Crystal Chemistry and Provenance Potential

Amphiboles are common rock-forming minerals in the earth’s crust.

• It forms in igneous and metamorphic rocks, but can be a relatively common detrital mineral for clastic sediments that have not undergone extensive diagenesis

• The crystal chemistry of amphiboles is quite complex.

  • clinoamphiboles have seven distinct cation positions that can be occupied by cations of different charge and atomic radius

• This is good because it has a wide stability range and responds to the local chemical environment well

Crystallographically, the amphibole structure is related to that of the pyroxenes, and consists of double chains of tetrahedra along the c axis, a pair of chains with apices facing each other forming a broad I-beam. Between the chains there are five other types of cation positions, designated M1 to M4 and A.

The amphibole structure can be considered as interleaved modules of pyroxene and mica structure. Much of the crystal-chemical behavior of amphiboles can be considered in this light: the M2 and M4 amphiboles sites are equivalent to the M1 and M2 sites, respectively, of the pyroxene structure. The M1 and M3 amphibole sites are like the octahedral sites in micas, and the A site resembles the mica interlayer site.   image source: http://www.gfz-potsdam.de/pb4/pg1/projects/A_PropofGeomat/a11amphi.html

End members and site allocations

The amphibole formula unit has a large number of sites and great flexibility of substitution. The basic formula unit is

A_0-1 M4₂ M13₃ M2₂ T₈ O₂₂(OH)₂

where A is the large vacant or partly-filled site between the bases of chains, M13 represents the two M1 and one M3 (whose properties are very similar), and T are the tetrahedral sites.

The site preferences are as follows

• T (tetrahedra): Si, Al. The limit of Al substitution appears to be about 2 out of 8.
• M2 (small octahedron): Al, Cr, Fe³⁺, Ti, Fe²⁺, Mg.
• M1+M3 (medium octahedra): Fe²⁺, Mg, Mn.
• M4 (larger cation site): Ca, Na, Mn, Fe²⁺, Mg.
• A: Na, K, vacancies.

Classification of the amphiboles

(in accordance with the International Mineralogical Association)

General classification of the amphiboles, exclusive of Mg-Fe-Li amphiboles

The following list contains the most commonly-used end members for monoclinic amphiboles. [V] is used to represent an A-site vacancy, Fe is Fe²⁺ unless expressly stated.

• [V]Ca₂Mg₅Si₈O₂₂(OH)₂ : tremolite
• [V]Ca₂Fe₅Si₈O₂₂(OH)₂ : ferro-actinolite
• [V]Ca₂Mg₃Al₂[Al₂Si₆]O₂₂(OH)₂ : tschermakite
• NaCa₂Mg₅[AlSi₇]O₂₂(OH)₂ : edenite
• [V]Mg₂Mg₅Si₈O₂₂(OH)₂ : cummingtonite
• [V]Na₂Mg₃Al₂Si₈O₂₂(OH)₂ : glaucophane
• [V]Na₂Mg₃Fe³⁺₂Si₈O₂₂(OH)₂ : magnesio-riebeckite

Variation of amphibole composition with grade

Because amphiboles show extensive solid solution, many aspects of their composition are controlled by the host rock chemistry. However, for hornblendes, there are a few useful grade-dependent trends:

• There is a general dependence of Ti content with grade (provided there is enough Ti in the rock to saturate the amphibole). This is also reflected in the color of Ca-amphibole in transmitted light, which varies from blue-green through green and olive green to brown with increasing grade and Ti content.

• At lower grades, the Al and alkali contents (edenite and tschermakite) increase markedly from near zero (tremolite-actinolite) to values more typical of hornblende (around 0.5 atoms Na+K, 2.5 atoms Al).

• High contents of octahedral Al, or of Na in the M4 site, indicate relatively high pressure.

• At high grade (uppermost amphibolite to granulite facies) there tends to be a relative shift, compared to lower grades, of increasing edenite and decreasing tschermakite components.

Many of the substitutions will be controlled by equilibria with other phases in the assemblage, and some of these may be sufficiently T- or P-sensitive to be of practical use in thermobarometry.

• Some examples are the hornblende-plagioclase thermometers of Holland and Blundy (1994), and the Al-in-hornblende barometer in igneous rocks (e.g. Anderson and Smith, 1995) and combined geothermobarometry (Ernst and Liu, 1998; Zenk and Schulz, 2004).

A vector treatment for hornblendes, using tremolite as the additive component, and accounting for Ti, Mn and K substitutions, might use the following set of exchange components:

Note that other commonly-used end members can be expressed as linear combinations of these, e.g. pargasite = edenite + tschermak.

• Fe²⁺Mg_-1
• MnMg_-1
• Al₂Mg_-1Si_-1 : tschermak substitution
• Fe³⁺AlMg_-1Si_-1 : ferri-tschermakite
• TiAl₂Mg_-1Si_-2 : Ti-tschermak substitution
• TiO₂R_-2(OH)_-2: Ti-deprotonation substitution
• NaAl[V]_-1Si_-1 : edenite vector
• KAl[V]_-1Si_-1 : K-edenite vector
• MgCa_-1 : cummingtonite vector
• NaAlCa_-1Mg_-1 : glaucophane vector

Possible crystal chemical signatures for provenance

• Plutonic amphiboles - complex textural and compositional ranges (Pe-Piper, 1988)
   

• PT conditions of amphiboles (use with caution!). Al in hornblende, Al and Ti thermobarometry (Ernst and Liu, 1998; Zenk and Schulz, 2004)
   

• Unusual amphiboles diagnostic of source rocks

  • glaucophane - high P, moderate T

  • riebeckite or arvedsonite - alkaline igneous alteration

  • grunerite - metamorphosed ironstones

  • anthopyllite - metamorphosed ultramafic or unusual mafic

  • cummingtonite - medium grade metamorphosed mafic

  • kaersutite - volcanic phenocrysts

  • and more