Johillerite, an anhydrous arsenate member of the alluaudite group with the idealized crystal chemical formula A(2)NaA(1)CuMMg3(TAsO4)3, is an abundant mineral in the exhalations of the Arsenatnaya fumarole on the Second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Kamchatka, Russia. Chemical variations of johillerite from Tolbachik were studied and the crystal structure was solved for three samples that represent different chemical varieties of the mineral. Johillerite forms a continuous solid-solution system with badalovite, Na2Mg2Fe3+(AsO4)3, and nickenichite, Na(□,Cu)(□,Ca)(Mg,Fe)3(AsO4)3, and a discontinuous solid solution with bradaczekite, NaCu4(AsO4)3. The major substitution schemes are: (1) MMg = MCu2+; (2) ACu2+ + M2+ = ANa+ + M3+; (3) ANa+ + M2+ = A0 + M3+; and (4) ACu2+ + 2M2+ = A0 + 2M3+. The T sites are occupied by As5+ with only minor admixtures of other constituents (P, V, S, Si).

Structurally studied johillerite crystals (monoclinic, C2/c) have the following crystal chemical formulae:

(1) A(2)′Na0.75A(1)′Cu0.85A(1)Na0.13M(1)(Mg0.91Fe0.09)M(2)(Mg1.60Fe0.34Cu0.06)(AsO4)3;

(2) A(2)′Na0.84A(1)′Cu0.68A(1)Na0.33M(1)(Mg0.68Zn0.32)M(2)(Mg0.90Cu0.62 Fe0.16Al0.08)(AsO4)3;

(3) A(2)′Na0.91A(1)′Cu0.98M(1)(Mg0.74Cu0.16Zn0.10)M(2)(Mg1.34Cu0.52Fe0.14)(AsO4)3.

Mg and Zn prefer the M(1)O6 octahedra rather than the M(2)O6, whereas Fe3+ and Al3+ mainly concentrate at the M(2) site.

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