Sample Analyses: Samples were analyzed for major and trace element concentrations of the unreacted and reacted sinter using a Thermo ARL Perform’X X-ray fluorescence (XRF) spectrometer at the Hamilton Analytical Laboratory at Hamilton College, New York, following protocols described in Johnson et al. (1999). One-part powdered sample is mixed with 2 parts Li-tetraborate flux and fused at 1000 °C in graphite crucibles. The cooled wavelength dispersive XRF (WDXRF) pellets are re-ground to powder and re-fused at the same temperature. Flat analytical surfaces are ground on steel-backed diamond laps. XRF determinations of 44 element concentrations are calibrated with approximately 70 reference materials. Net peak intensities are corrected for all potential spectral interferences; matrix correction was done with LOI-eliminated influence coefficients. Uncertainties in the XRF determinations depend upon element and concentration and range from approximately 0.2% for SiO2 to several percent for trace elements such as Ga and Cs. Samples were heated overnight at 900 °C in SiO2 crucibles in order to determine approximate water concentrations by loss on ignition (LOI). LOI measurements are reproducible to ~1%. Database Contents: The data file (XRF_Data_Supplementary.csv) contains the major and trace element concentrations measured from three sinter samples from Giant Geyser and three sinter samples from Castle Geyser. The entries in the data file appear in the following columns: A. Sample ID B. Location C. SiO2 (wt% oxide) D. TiO2 (wt% oxide) E. Al2O3 (wt% oxide) F. FeO (wt% oxide) G. MnO (wt% oxide) H. MgO (wt% oxide) I. CaO (wt% oxide) J. Na2O (wt% oxide) K. K2O (wt% oxide) L. P2O5 (wt% oxide) M. LOI (%) N. sumMaj+LOI (%) O. sumAll (%) P. F >= (ppm) Q. Cl >= (ppm) R. SO3 >= (ppm) S. Br >= (ppm) T. As >= (ppm) U. Ni (ppm) V. Cr (ppm) W. V (ppm) X. Sc (ppm) Y. Cu (ppm) Z. Zn (ppm) AA. Ga (ppm) AB. Ba (ppm) AC. Rb (ppm) AD. Cs (ppm) AE.Sr (ppm) AF. Y (ppm) AG. Zr (ppm) AH. Hf (ppm) AI. Nb (ppm) AJ. Ta (ppm) AK. Mo (ppm) AL. La (ppm) AM. Ce (ppm) AN. Nd (ppm) AO. Sm (ppm) AP. Dy (ppm) AQ. Yb (ppm) AR. Th (ppm) AS. U (ppm) AT. Ti (ppm) AU. Pb (ppm) AV. Sn (ppm) AW. Bi (ppm) References Johnson, D., R. Hooper, P. and Conrey, R., 1999. XRF Analysis of Rocks and Minerals for Major and Trace Elements on a Single Low Dilution Li-tetraborate Fused Bead. Adv. X-ray Anal. 41, 843-867.