Parvo-mangano-edenite and parvo-manganotremolite, two new Group 5 monoclinic amphiboles from Fowler, New York, and comments on the solid solution between Ca and Mn2+ at the M4 site.

This work reports the crystal-chemical characterization of Mn-rich amphiboles from the Grenville
Marble of the Arnold mine, Fowler, St. Lawrence Co., New York (U.S.A.), which were previously described
by Benimoff et al. (1991) as .manganoan silicic edenite.. According to the new nomenclature
scheme for monoclinic amphiboles (Leake et al. 2004) the ideal composition of reference, ANa B(CaMn)
CMg5
T(Si7Al)O22(OH)2 with Ca > 1 apfu, is an end-member of the newly deÞ ned Group 5, and is named
parvo-mangano-edenite. Re-examination of the original rock specimen showed signiÞ cant inter- and intra-
crystalline compositional variations, which can be expressed by the ANa.1
TAl.1
A ¦
1
TSi1 and BMn2
BCa.2
exchange vectors. The Þ rst vector leads to parvo-manganotremolite, ideally A ¦ B(CaMn)CMg5
TSi8O22(OH)2
with Ca > 1. The second mechanism was never found to reach Mn dominance; however, crystal-chemical
analysis does not provide any evidence of structural limits, and thus the magno-calcic counterparts
of the Group 5 amphiboles of this work may occur in similar but Mn-richer genetic environments. The
presence of Mn at the B site helps to stabilize the charge arrangement of edenite.
The parvo-mangano-edenite crystal with composition closest to the end-member, i.e., A(Na0.74
K0.02)B(Ca1.27Mn0.73)C(Mg4.51Mn2+
0.28Fe2+
0.05Fe3+
0.03Al0.12Ti0.01)T(Si7.07Al0.93)O22(OH)2, has a = 9.8260(5), b
= 18.0487(9), c = 5.2840(4) Å, ß = 104.55(1)º, V = 907.1 Å3 (Z = 2); the calculated density is 3.11
g/cm3. The parvo-mangano tremolite crystal, with composition A(Na0.44K0.01)B(Ca1.13Mn0.83Na0.04)C(Mg
4.69Mn2+
0.21Fe2+
0.03Fe3+
0.01Al0.06)T(Si7.52Al0.48)O22 (OH)2, has a = 9.7807(5), b = 18.0548(9), c = 5.2928(4) Å,
ß = 104.19(1)º, V = 906.1 Å3 (Z = 2); the calculated density is 3.08 g/cm3. The different compositions
are virtually indistinguishable under the optical microscope, but can be identiÞ ed by a measure of their
unit-cell parameters. The single-crystal FTIR spectrum of parvo-mangano-edenite in the OH-stretching
region shows two main absorptions at 3711 and 3671 cm.1, plus shoulders at 3695, 3660, and 3641 cm.1,
respectively. FTIR spectroscopy indicates extensive short-range-order of cations in these amphiboles.