Sodium Amphibole Subgroup

The sodium amphibole subgroup is one of the subgroups belonging to the W(OH,F,Cl) amphibole group. It is defined with Na as the dominant element in the X position (= IMA B position).
(The IMA-CNMNC definition is ^B(Na + Li)/ΣB ≥ 0.75, ^BNa/ΣB ≥ ^BLi/ΣB)

The sodium amphibole subgroup are further divided into root-name groups that are defined by the charge balance and dominant elements in the A and Z positions (Z=IMA C position) in the generic formula above. The various positions in the formula can have the following content:

A position: n=0 or 1, when 1, it contains Na or K as the dominant elements.

Z position (= IMA C position): For all root name groups except the Nybøite and Leakeite groups, the content in the Z position is Z²⁺_3+nZ³⁺_2-n, and with Si₈ in the T position.

The Nybøite group has n=1 and Z²⁺_3+nZ³⁺₂, balancing the formula with m=1 in the T position.

The leakeite group contains Li in the Z position and it's Z position is defined with Z²⁺₂Z³⁺₂Li.

Mg, Fe²⁺, Al and Fe³⁺ are normally the dominant elements in the Z²⁺ and Z³⁺ positions for all root name groups.

(OH), F or Cl dominance define mineral species within a root name group.

On mindat.org we avoid using variables with the same symbol as elements and deviate from the IMA nomenclaure in that we use X rather than B, Z rather than C and (OH,F,Cl,O) rather than W.

w(OH, F, Cl)-dominant Amphibole Group > Amphibole Supergroup

This amphibole subgroup was named "Alkali-amphiboles" in Leake's "Nomenclature of amphiboles"(1978) and renamed as "Sodic amphiboles" in the 1997 amphibole nomenclature. The group was again renamed in the 2012 amphibole nomenclature to "sodium amphibole subgroup".