EVOLUTION OF BORON MINERALS: HAS EARLY SPECIES DIVERSITY BEEN LOST FROM THE GEOLOGICAL RECORD?

Of the 240 valid and 20 prospective mineral species containing essential B, 4 are fluorides and 256 are O compounds (borates) of which 128 contain only BΦ₃ triangles and/or BΦ₄ tetrahedra (Φ = O, OH) and 128 contain additional oxyanionic complexes of Be, C, Si, P, S, or As. The greatest diversity of B minerals is found in evaporites (80 species), followed by skarns (67), granitic pegmatites (41), alkaline rocks (33), Mn-rich rocks (19), and fumaroles (10). Volcanic and sedimentary processes, together with regional metamorphism, concentrated B sufficiently for B minerals to appear early in the geologic record; the oldest reported are dravite and schorl in the Isua belt (3700 Ma); bonaccordite, foitite and magnesio-foitite in the Barberton greenstone (3230 Ma); a Cr tourmaline in the Singhbhum craton (3100 Ma). Highly differentiated granitic pegmatites and their exocontacts (2520-2670 Ma), hydrothermal activity associated with gold deposits (2680-2700 Ma), and metamorphic rocks (2800 Ma, Fiskenæsset, Greenland) added 13 more species in the Archean. The borate deposits in Liaoning and Jilin provinces, China (2050 Ma), Mn skarns in the Långban deposit, Sweden (1825 Ma), Mg skarns in the Tayozhnoye deposit, Russia (1950 Ma), granitic pegmatites, and granulite-facies metamorphics (e.g., 2000 Ma, Magondi belt, Zimbabwe) contributed to 32 new species in the late Paleoproterozoic, whereas Mn deposits and Mg skarns were largest contributors to 38 more species in the remainder of the Proterozoic. However, the most species, 171, are reported to first appear in the Phanerozoic, including 37 species in Mg skarns and 20 in alkalic rocks. Evaporites, fumaroles and secondary minerals in extreme deserts contributed another 78 species. However, these ephemeral B minerals could have formed much earlier, and with rare exception, failed to survive later geologic events. The only borate reported from a Precambrian evaporite is chambersite (Mn₃B₇O₁₃Cl) associated with algal dolostone in the 1500 Ma Gaoyuzhuang Formation, China, but relict casts and B isotopes are cited as evidence for an evaporite origin for the Barberton tourmaline-rich rocks and Liaoning-Jilin borates, implying a much greater diversity of borate minerals as early as the Paleoarchean; i.e., the geologic record provides but an incomplete picture of B mineral evolution.