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The high-temperature behavior of synthetic potassic-ferro-richterite was addressed to obtain data relevant to understanding petrogenetic processes as well as to testing complementarity and mutual calibration of single-crystal X-ray diffraction (XRD) analysis plus structure refinement (SREF) with single-crystal FTIR spectroscopy. This experimental approach aims at: (1) better quantifying the onset of deprotonation, its development and the amount, if any, of relict OH at the end of the process; (2) verifying whether or not the process is homogeneous within the crystal; and (3) evaluating local changes in cation environments close to the OH dipole. In this first part of a series of two papers, we report on the crystal-chemical characterization of potassic-ferro-richterite and on a single-crystal XRD study at high T . Detailed analysis of the available data allowed us to obtain a full characterization of the bulk and crystal chemistry of the studied crystal, hence improving the unit formula suggested by Redhammer & Roth (2002). In operando H T measurements up to 1073 K showed quite anomalous behavior with respect to pargasite/kaersutites, specifically a much lower T for the onset of the deprotonation process (around 500 K), and strongly anomalous behavior of the β angle, which shows inverse slopes for protonated and deprotonated phases. “Oxo-potassic-ferro-richterite” is formed upon deprotonation and remains stable at least up to 1073 K under the conditions of this study. Structure refinements from data collected at different temperatures allowed us to detect changes in the crystal-structure geometry and in turn to decipher the way in which amphiboles with such a peculiar composition respond to increasing T and deprotonation. The thermal expansivity coefficients α (×10 −5 K −1 ) are: potassic-ferro-richterite: α a = 1.30(6), α b = 0.93(6), α c = 0.12(3), α β = −0.49(5), α asinβ = 1.34(8), α V = 2.59(2); “oxo-potassic-ferro-richterite”: α a = 1.71 (3), α b = 0.97(1), α c = 0.193(8), α β = 0.22(1), α asinβ = 1.59(4), α V = 2.74(2).

The crystal-chemical characterization of an amphibole with an unusual composition, A(Na0.76K0.24)B(Ca1.42Na0.56Mn2+0.02)C(Mg2.64Fe2+1.95Mn2+0.07Mg2.64Zn0.01Fe3+ 0.01Ti4+ 0.32)T(Si7.18Al0.82)O22 W[(OH)0.58O0.27F1.15], found in pegmatitic veins at Kariåsen, Larvik Plutonic Complex, Norway, provides an excellent example of the detection and estimation of the oxo component in amphibole. The use of Ti as a proxy for the oxo component is discussed and a procedure to derive accurate Ti partitioning from the results of structure refinement is described. Because the presence and amount of oxo component in amphiboles are important in order to determine values of fO2 and fH2O, especially in igneous and magmatic systems, this procedure should be applied any time the compositional data or the petrological context indicate the presence of significant Ti, or suggest that the oxo component may be a relevant issue.

In the paper on which we are commenting ([Joy & Evans 2014][1]), the authors (hereafter referred to as J&E) introduced a new mineral belonging to the amphibole supergroup, gave it a name, and discussed its relations with coexisting amphiboles and amphiboles reported in previous literature. First

The crystal structures of four amphiboles from the Bear Lake diggings, in the Bancroft area of Ontario, fluorotremolite, fluororichterite and two samples of potassian fluor-magnesiokatophorite, have been refined to R indices in the range 1.5–2.2% using ~1380 unique reflections measured with Mo K α X-radiation. The crystals used in the collection of the X-ray intensity data were subsequently analyzed by electron and ion microprobes, and formulae were calculated using the Fe3+ and Fe2+ contents established by crystal-structure refinement. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. Difference-Fourier maps through the A (2/ m ) site show that the A ( m ) site is occupied and the A (2) site is vacant in all crystals. Occupancy of the A ( m ) site, and not the A (2) site, by Na imposes strong constraints on the possible patterns of short-range order in these amphiboles. The site populations of the A ( m ), M (4) and O(3) sites are in accord with the local arrangement A ( m )(Na + K)–O(3)F– M (4)Na, with lesser amounts of A ( m )(Na + K)–O(3)OH– M (4)Na and major to minor amounts of A □–O(3)OH– M (4)Ca. The coupling of elements to specific short-range-ordered arrangements in these crystals suggests that the overall chemical variability of amphiboles is strongly constrained by local bond-valence restraints in addition to overall electroneutrality.

Major issues involved in the classifi cation of the amphiboles are examined: (1) the role of (OH), Li and Fe 3+ , (2) the formal defi nition of a root name, (3) irreducible charge-arrangements and distinct species, (4) the use of prefi xes, (5) the principal chemical variables used in a classifi cation procedure, and (6) the use of the dominant-constituent principle. The current IMAapproved classifi cation scheme is based on the A, B and T groups of cations in the amphibole formula: AB2C5T8O22W2. We argue here that classifi cation should be based on the A, B and C groups of cations as (i) it is in these groups of cations that the maximum variation in chemical composition occurs, and (ii) as a result of (i), the scheme is more in accord with the IMA-sanctioned dominant-constituent principle, which governs the recognition (and approval) of distinct mineral species. Two new classifi cations are presented here; one is based on the A, B and C groups of cations, and another on the dominant-constituent principle. These two schemes were produced to illustrate (i) the problems inherent in the classifi cation of a group of minerals as complicated as the amphiboles, and (ii) the sometimes disparate needs of crystallographer, mineralogist, petrologist and geochemist. Scheme 1 conserves current formulae and names as much as possible, whereas scheme 2 minimizes the number of formulae and names as much as possible. The differences between the current classifi cation and the two schemes presented here are discussed, and we highlight the problems associated with each scheme.