Ferro-tschermakite (original) (raw)

Formula:

◻{Ca2}{Fe2+3Al2}(Al2Si6O22)(OH)2

Ferro-tschermakite is defined as a tschermakite group member with the following dominant elements:
C position: Fe as the dominant divalent cation and Al as the dominant trivalent cation.
W position: (OH) as the dominant anion

Colour:

Medium green to dark green to green-black to black, also brownish-green.

Crystal System:

Monoclinic

Name:

For its iron-rich relationship to tschermakite, named for Gustav Tschermak.

Ferro-tschermakite is defined as a Tschermakite-Group member with Fe2+ >Mg and Al>Fe3+ in the C position. Ferro-tschermakite has been redefined in the 2012 Amphibole nomenclature in that:

1. The formula is changed from ☐(Ca2)(Fe2+3AlFe3+)(Al2Si6O22)(OH)2 to ☐(Ca2)(Fe2+3Al2)(Al2Si6O22)(OH)2. Consequently all pre-2012 literature references to ferro-tschermakite should be checked to verify that Al>Fe3+ in the C position. If Fe3+>Al in the C position the material will be ferro-ferri-tschermakite.

2. The tschermakite group is in the 2012 nomenclature defined with C(Al+Fe3++2Ti)>1.5 apfu, whereas the pre-2012 definition was defined with 5.5 < Si < 6.5 apfu. This redefinition does not change the formula, but many amphiboles qualifying as tschermakite pre-2012 will now be ferro-hornblende or ferro-ferrihornblende.

It should also be noted that pre-1978 tschermakite was defined with A+B(Na+K+Ca)<2.5. As tschermakite often contains less than 2 apfu Ca, older literature may describe amphiboles with A(Na+K)>0.5 as tschermakites. These are now considered pargasites.

NOTE: Most analyzed samples are close to one or more of the tschermakite/hastingsite/pargasite/sadanagaite joins. Any tschermakite specimen should therefore be analysed with qualitative chemical analysis (EMPA minimum, ideally in combination with Mössbauer spectroscopy).

Compositionally similar to (Na-bearing) ferro-pargasite.
May be confused with Ca-bearing almandine.

Place of Conservation of Type Material:

Type material is deposited in the collections of the Museo di Mineralogia, Sistema Museale di Ateneo, University of Pavia, catalogue number 2016-02

Empirical Formula of Type Material:

A(Na0.29K0.08)?=0.37 B(Ca1.69Fe2+0.11Mn2+0.02Na0.18)?=2.00C(Fe2+1.84Mg1.54Al1.33Fe3+0.24V3+0.01Ti0.04)?=5.00 T(Si6.15Al1.85)?=8.00O22

Reference:

Oberti, R., Boiocchi, M., Hawthorne, F.C., Ciriotti M.E. (2018) Ferro-tschermakite from the Ploumanac'h granitic complex, Brittany, France: mineral description. European Journal of Mineralogy: 30: 171–176.

- This locality has map coordinates listed. - This locality has estimated coordinates. ⓘ - Click for references and further information on this occurrence. ? - Indicates mineral may be doubtful at this locality. - Good crystals or important locality for species. - World class for species or very significant. (TL) - Type Locality for a valid mineral species. (FRL) - First Recorded Locality for everything else (eg varieties). Struck out - Mineral was erroneously reported from this locality. Faded * - Never found at this locality but inferred to have existed at some point in the past (e.g. from pseudomorphs).

All localities listed without proper references should be considered as questionable.