The R chondrite meteorite LaPaz Icefield (LAP) 04840 is unique among metamorphosed, non-carbonaceous chondrites in containing abundant OH-bearing silicate minerals: ∼13% ferri-magnesiohornblende and ∼0.4% phlogopite by volume. Other minerals include olivine (Fo₆₂), orthopyroxene (En₆₉Fs₃₀Wo₁), albite (An₈Ab₉₀Or₂), magnetite, pyrrhotite, pentlandite, and apatite. Ferromagnesian minerals are rich in Fe³⁺, as determined by Mössbauer spectrometry and electron microprobe chemical analyses. Fe³⁺/Fe_tot values are olivine ⩽5%, amphibole 80%, phlogopite 65%, and magnetite 42%. Mineral compositions are nearly constant across grains and the section, except for a small variability in amphibole compositions reflecting the edenite exchange couple (^ANa+^IVAl↔^A□+Si). These mineral compositions, the absence of Fe–Ni metal, and the oxygen isotope data support its classification as an R (Rumuruti) chondrite. LAP 04840 is classified as petrologic grade 5, based on the chemical homogeneity of its minerals, and the presence of distinctly marked chondrules and chondrule fragments in a fine-grained crystalline matrix. The mineral assemblage of LAP 04840 allows calculation of physical and chemical conditions at the peak of its metamorphism: T=670±60°C from a amphibole–plagioclase thermometer; [Formula: see text] between 250 and 500bars as constrained by the assemblage phlogopite+orthopyroxene+olivine+feldspar and the absence of diopside; [Formula: see text] unconstrained; [Formula: see text] at QFM+0.5log units; [Formula: see text] . The hydrogen in LAP 04840 is very heavy, an average δD value of +3660±75‰ in the magnesiohornblende. Only a few known sources of hydrogen have such high δD and are suitable sources for LAP 04840: ordinary chondrite phyllosilicates (as in the Semarkona chondrite), and insoluble organic matter (IOM) in ordinary chondrites and CR chondrites. Hydrogen from the IOM could have been released by oxidation, and then reacted with an anhydrous R chondrite (at high temperature), but it is not clear whether this scenario is correct.