The first recorded occurrence of cummingtonite in the Scottish Dalradian is described. The mineral occurs in thin layers within a psammite-semipelite sequence forming the lower part of the Easdale Subgroup in which it coexists with bytownitic plagioclase, almandine-rich garnet and quartz. Textural and mineral chemical evidence suggest that the cummingtonite developed from hornblende during amphibolite-facies regional metamorphism. The protolith to this cummingtonite-bearing rock is believed to have been a basaltic tuff, possibly alkaline in character, containing a significant proportion of sedimentary detritus. The cummingtonite-bearing rock may provide the earliest record of basic volcanism in the Dalradian of the Ballater district.

The crystal structure of a manganoan cummingtonite, composition [M4](Na0.13Ca0.41Mg0.46Mn1.00) [M1,2,3](Mg4.87Mn0.13)(Si8O22)(OH)2, (Z = 2), a = 9.5539(2) Å, b = 18.0293(3) Å, c = 5.2999(1) Å, β = 102.614(2)° from Talcville, New York, has been refined at high temperature using in situ neutron powder diffraction. The P21/m to C2/m phase transition, observed as spontaneous strains +ε1 = −ε2, occurs at ˜107°C. Long-range disordering between Mg2+ and Mn2+ on the M(4) and M(2) sites occurs above 550°C. Mn2+ occupies the M(4) and M(2) sites preferring M(4) with a site-preference energy of 24.6±1.5 kJ mol−1. Disordering induces an increase in XMnM2 and decrease in XMnM4 at elevated temperatures. Upon cooling, the ordered states of cation occupancy are ‘frozen in’ and strains in lattice parameters are maintained, suggesting that re-equilibration during cooling has not taken place.

We detected late Pleistocene cummingtonite-bearing cryptotephras in loess deposits in NE Japan and correlated them with known tephras elsewhere by using major-element compositions of the cummingtonite. This is the first time cryptotephras have been identified by analysis of a crystal phase rather than glass shards. In central NE Japan, four cummingtonite-bearing tephras, the Ichihasama pumice, the Dokusawa tephra, the Naruko–Nisaka tephra, and the Adachi–Medeshima tephra, are present in late Pleistocene loess deposits. Because the cummingtonite chemistry of each tephra is different and characteristic, it is potentially a powerful tool for detecting and identifying cryptotephras. An unidentified cummingtonite-bearing cryptotephra previously reported to be present in the late Pleistocene loess deposits at Kesennuma (Pacific coast) did not correlate with any of the known cummingtonite-bearing tephras in central NE Japan, but instead with the Numazawa–Kanayama tephra (erupted from the Numazawa caldera, southern NE Japan), although Kesennuma is well beyond the previously reported area of the distribution of the Numazawa–Kanayama tephra. Three new cummingtonite-bearing cryptotephras in the mid and late Pleistocene loess deposits (estimated to be less than 82 ka, 100–200 ka, and ca. 250 ka) on the Isawa upland were also detected.

Analysed compositions of coexisting phases from cordierite-amphibole-bearing rocks (cordierite-orthoamphibole-cummingtonite-garnet-staurolite-spinel-opaque ores) and mica schists (garnet-staurolite-biotite-muscovite-plagioclase-andalusite-quartz) have been used in conjunction with experimentally determined equilibria to estimate pressures and temperatures of metamorphism for the rocks of the Outokumpu district. The data do not determine a single point in P-T space, but bracket an area such that T ≈ 600 ± 50°C and P ≈ 3·5 ± 1 kb. These conditions represent the maximum temperature attained by the rocks, and the associated sulphide ore bodies, during the Svecokarelian orogeny and are consistent with a regional palaeogeotherm of c. 50°C/km.

Assessment of the composition of the coexisting phases in the cordierite-amphibole-bearing rocks, particularly the Fe-Mg-bearing phases, indicates that it is unlikely that either iron or magnesium was mobile during metamorphism and that the equilibrium assemblage represents essentially isochemical metamorphism of a rock composition that predates the metamorphism. A similar genesis is considered likely for the cordierite-anthophyllite rocks of Orijärvi in southern Finland which forms the basis of the classic statement of the magnesium-metasomatism hypothesis.