This paper presents the nomenclature of the rhodonite group accepted by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA). An overview of the previous studies of triclinic (space group P$\bar{1}$) pyroxenoids belonging to the rhodonite structure type, with a focus on their crystal chemistry, is given. These minerals have the general structural formula VIIM(5)VIM(1)VIM(2)VIM(3)VIM(4)[Si5O15]. The following dominant cations at the M sites are known at present: M(5) = Ca or Mn2+, M(1–3) = Mn2+; and M(4) = Mn2+ or Fe2+. In accordance with the nomenclature, the rhodonite group consists of three IMA-approved mineral species having the following the general chemical formulae: M(5)AM(1–3)B3M(4)C[Si5O15], where A = Ca or Mn2+; B = Mn2+; and C = Mn2+ or Fe2+. The end-member formulae of approved rhodonite-group minerals are as follows: rhodonite CaMn3Mn[Si5O15]; ferrorhodonite CaMn3Fe[Si5O15]; and vittinkiite MnMn3Mn[Si5O15].

Oxidative dissolution of a primary Mn-silicate phase (rhodonite) was studied via synchrotron X-ray techniques. The study was designed to combine the element-specific chemical technique of Glancing Incidence X-ray Absorption Spectroscopy (GIXAS) with the surface structural technique of X-ray scattering in order to produce the first depth resolved study of Mn-silicate low-temperature reactivity. A chemo-mechanically polished polycrystalline rhodonite sample was characterized and then reacted with pH 3.5 nitric acid. The surface originally had a mosaic structure and 15.5 (±1) Å r.m.s. roughness. Surface composition was not measurably different from bulk rhodonite before reaction, indicating that the surface preparation regimen had not produced an altered surface. After 1 h of reaction, the roughness of the mineral surface decreased and reflectivity oscillations developed, resulting from the formation of a leached layer. This layer was 74.7 (±2) Å thick with an electron density equal to 72% of that of bulk rhodonite (equal to the loss of ~1 in 2 Mn atoms). Both the primary and the buried interfaces had similar roughnesses; 4.9 and 4.5 (±1.0) Å , respectively. Diffuse scatter indicated that the correlation length between surface features also decreased. The GIXAS analysis showed that the Mn remaining in the surface had become oxidized, with the degree of oxidation decreasing as a function of depth. Oxidation penetrated at least 140 Å into the structure. A further 2.5 h of reaction at pH 3.5 caused dissolution of the leached layer and reduced the thickness of this altered region to 16.0 (±2) Å , while surface roughness increased slightly to 6.2 (±1.0) Å . Depletion of Mn in this region increased only slightly relative to the first reaction step; the electron density was 67% that of bulk rhodonite, equivalent to the loss of 2 in 3 Mn atoms. The thickness of the oxidized region however, persisted. Analysis by XPS on the same specimen corroborates the X-ray results.

The Luziyuan Pb–Zn skarn deposit, located in the Baoshan–Narong–Dongzhi block metallogenic belt in SW China, is hosted by marble and slate in the upper Cambrian Shahechang Formation. Three skarn zones have been identified from the surface (1495 m above sea level (asl)) to a depth of 1220 m asl: zone 1 consists of chlorite–actinolite–calcite–quartz, zone 2 of rhodonite–actinolite–fluorite–quartz–calcite, and zone 3 contains garnet–rhodonite–actinolite–fluorite–quartz–calcite. The deposit formed in four distinct mineralization stages: an early anhydrous skarn (garnet, rhodonite and bustamite) stage (Stage 1), a hydrous skarn (actinolite and chlorite) stage (Stage 2), an early quartz (coarse barren quartz veins) stage (Stage 3) and a late sulphide-forming (fine sulphide-bearing quartz veins) stage (Stage 4). The Stage 1 skarn-forming fluid temperature was at least 500 °C according to the geothermometer with rhodonite/bustamite trace elements measured by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS). A decrease in ore fluid temperatures with time is consistent with the decreases in the δ18Ofluid and δDfluid values from Stage 3 to 4. This trend suggests that the ore fluid was mainly derived from magmatic water and mixed with large amounts of meteoric water during mineralization. The δ34S values of Stage 4 chalcopyrite, sphalerite and galena are similar to those of an Ordovician gypsum layer, and together with the high-salinity fluids in Stage 4 indicate the dissolution of evaporites in the Luziyuan region. Overall, the results of this study suggest that the Luziyuan deposit is a distal Pb–Zn skarn deposit that formed in response to multi-stage alteration associated with a combination of magmatic water and meteoric water.