IAOS Bulletin No. 72, Summer 2024 Pg. 9 A SPECIATION MODEL FOR WATER IN OBSIDIAN Alexander K. Rogers a and Christopher M. Stevenson b a Maturango Museum, Ridgecrest, California, USA b Virginia Commonwealth University, Richmond, Virginia, USA Abstract Water is embodied in obsidian in the process of magma formation at high temperatures and pressures, and a residual amount is trapped as the melt cools to form glass. Water exists as two species, hydroxyl (OH) and molecular water (H2Om), which have differing effects on obsidian hydration. We develop a simple speciation model based on the Langmuir equation and a data set of N= 184 data points representing obsidian sources of interest archaeologically. The resulting fit is OH = 2.093*[0.489*H2Ot/(1 + 0.489*H2Ot)], in which both OH and H2Ot (= OH + H2Om) are in wt%. The R 2 for the fit = 0.9960, the rms residuals are 0.019 wt.%, and the range of validity is 0.05 wt% < H2Ot < 2.5 wt%. A further equation allows predicting the wt.% OH in a specimen from absorbance measurements on the strong absorption band at 3570 cm -1 . Introduction Water is embodied in obsidian (rhyolitic glass) in the process of magma formation at high temperatures and pressures, and a residual amount I s trapped as the melt cools to form glass (Shelby 2005; Zhang 2008). Water exists as two species, hydroxyl (OH) and molecular water (H2Om), which have differing effects on obsidian hydration. This paper develops a simple speciation model based on the Langmuir equation and a data set of N= 184 data points representing obsidian sources of interest archaeologically. Data are from Newman et al. (1986: 1535, Table 6), Stevenson et al. (2019: 233, Table 1), and recent data on Bodie Hills obsidian from Mono County, eastern California (N = 114). The resulting model is useful in studies of the obsidian hydration process. Obsidian Chemistry The water in obsidian is originally trapped as molecular water (H2Om) during cooling of the magmatic melt. Hydroxyl (OH) is formed by a chemical reaction between molecular water and oxygen atoms bound to the glass matrix, SiOSi+H2O = SiOH+HOSi (1) forming pairs of adjacent silanol (SiOH) groups (Doremus 1994: 198; 2002: 129; Ihinger at al. 1999). This process is known as speciation. The OH groups are immobile, so diffusion is by molecular water. Doremus (2002: 129ff.) further argued that the bonding in equation (1) is to non-bridging oxygen atoms, whose numbers are limited, in a process analogous to chemical adsorption on a surface. Chemical adsorption on a surface can be described by the Langmuir equation of the form y = C1*C2*x/(1 + C2*x) (2) where y is the number of reacted sites or species, x is the number of unreacted sites or species prior to the reaction, C1 is a proportionality constant, and C2 is the ratio of the forward and reverse reaction coefficients for equation (1) (Doremus 2001: 129).