BENITOITE AND BLUESCHIST: KEEPING UP WITH JO LAIRD

The brilliant blue gemstone benitoite is a rare Ba-Ti-silicate, known principally from its type locality within the New Idria serpentinite, located in California. In one of her early papers Jo Laird, then a Caltech student, reported the chemical and physical properties of benitoite and other associated (and rare) minerals neptunite and joaquinite, determined using an electron microprobe (Laird & Albee, 1972). At that time the origin of benitoite and its geologic context was poorly understood. Today the formation of benitoite is explained by the reaction barite + titanite + albite + chlorite + H₂O = benitoite + actinolite + clinozoisite + natrolite + SO₂(aq) that takes place at the boundary between two distinct tectonic blocks entrained by the New Idria serpentinite.

Jo Laird went on to spend much of her career studying amphiboles, including glaucophane - the blue in blueschist - and their tectonic settings in the Appalachians and elsewhere. I originally met Jo on a field trip somewhere in these Appalachians, and our discussions focused on amphiboles in those rocks. Some of my own researches have involved the serpentinite at Vermont's Belvidere Mt., located in what we might call Laird Country. This serpentinite includes multiple generations of distinctive minerals, including tremolite. Later, however, I had the opportunity to work on the amphiboles within the New Idria serpentinite, of which there are a surprisingly large number. In addition to hosting benitoite and other rare minerals, the New Idria serpentinite contains naturally occurring glaucophane/crossite, tremolite/actinolite, kaersutite, as well as another blue amphibole similar to winchite, whose composition is best depicted in Laird Space. These minerals are mostly associated with tectonic blocks within or adjacent to the serpentinite. Finally, two other amphiboles introduced in recent times by humans: hornblende and amosite, occur. In addition to their petrologic interest, some of these amphiboles have been the subject of studies by EPA and others due to the health risks they allegedly present.