Agate Analysis by Raman, XRF, and Hyperspectral Imaging Spectroscopy for Provenance Determination

AbstractThe Getty Institute recently acquired the Borghese-Windsor Cabinet (Figure \ref{620486}), a piece of furniture extensively decorated with agate, lapis lazuli, and other semi-precious stones.  The cabinet is thought to have been built around 1620 for Camillo Borghese (later Pope Paul V).  The Sixtus Cabinet, built around 1585 for Pope Sixtus V (born Felice Peretti di Montalto), is of similar design to the Borghese-Windsor and also ornately decorated with gemstones.  Although there are similarities in gemstones between the two cabinets, the Sixtus and Borghese-Windsor cabinets vary in their agate content.  It was traditionally thought that all agate gemstones acquired during the 16th and 17th centuries were sourced from the Nahe River Valley near Idar-Oberstein, Germany.  It is known that Brazilian agate began to be imported into Germany by the 1800s, but it is possible that some was imported in the 18th century or earlier.  A primary research goal was to determine if the agates in the Borghese-Windsor Cabinet are of single origin, or if they have more than one geologic provenance. Agates are made of SiO2, mostly as the mineral quartz, but also as metastable moganite.  Both quartz and moganite will crystallize together as the agate forms, but moganite is not stable at Earth's surface and will convert to quartz over tens of millions of years \cite{Moxon_2004,Peter_J_Heaney_1995,G_slason_1997}, thus relatively older agate contains less moganite.  Agate from the Idar-Oberstein is Permian in age (around 280 million years old), while agate from Rio Grande do Sul of Brazil generally formed during the Cretaceous (around 120 million years old).  It is thought that Rio Grande do Sul would have been a primary source of material exported to Europe because it is one of Brazil's oldest and largest agate gemstone producers.  Since Cretaceous agate from Brazil is many millions of years younger than Permian agate from Germany, the quartz to moganite ratios between the two localities should be quite different.  The agate gemstones of the Borghese-Windsor Cabinet cannot be removed for detailed Raman mapping experiments.  Because of this, we first analyzed multiple agate specimens from the collections of the Natural History Museum of Los Angeles (NHMLA) and the Smithsonian Institution National Museum of Natural History (NMNH) using three different techniques: Raman mapping, XRF mapping, and hyperspectral imaging. Raman spectroscopy provides an easy method to distinguish the relative quartz:moganite ratios and XRF analysis provides a measure of bulk geochemistry in agates.  Maps have advantages over line scans and point analysis in that they give a better representation of the mineral content, can be used to exclude trace mineral impurities, and yield better counting statistics and averaging. Hyperspectral imaging provides a range of optical data from IR through UV wavelengths.