Smith, CW, Aptroot, A, Coppins, BJ, Fletcher, J, Gilbert, OL, James, PW, Wolseley, PA, eds. (2009). Lichens of Great Britain and Ireland. British Lichen Society.

Thallus to 1 cm diam., or several thalli confluent, continuous to rimose, somewhat uneven, white, pruinose, matt; prothallus indistinct.  Apothecia to 1 mm diam, black, +/- immersed; disc usually not pruinose, orbicular, remaining +/- flat; true exciple uneven, persistently pruinose, dark brown at the rim, inner part paler brown, K+ purple-red, containing crystals dissolving in K, crystals K+ yellow; epithecium dull grey to +/- red-brown, K+ purple-red; hymenium colourless to pale brown, containing crystals at least partly dissolving in K; hypothecium brown, K-.  Ascospores 12-16 X 6-10 µm, 1-septate, long remaining colourless, usually becoming unevenly green or brown with age.  Thallus reactions negative (stictic acid in apothecia). 

On epidiorite and other calcareous rocks, montane; very rare.  C. & N. Scotland (Angus, Caenlochan, Glen Isla; Inverness, Ben Alder).  Scandinavia, N. America.

R. chioneum forms small, chalk-white, orbicular patches with +/- immersed apothecia.  The combination of small, 1-septate, colourless ascospores and a brown, K+ purple-red epithecium is diagnostic.  See R. caeruleoalbum for differences from that species and R. expallescens.