University of Wisconsin-Eau Claire

Department of Geology

X-Ray Fluorescence Analytical Procedure

 

J. Brian Mahoney, University of Wisconsin-Eau Claire

 

Sample Preparation

Samples selected for whole rock analysis ideally consist of greater than 1kg unaltered rock to ensure the rock being analyzed is representative of the unit being examined.  Smaller rock samples may be used if necessary, provided the sample is homogeneous and representative of the unit under investigation.  Deeply weathered samples are avoided.

 

Large rock samples are reduced to fragments <8 cm diameter with a rock hammer and pedestal.  These fragments are reduced to gravel-sized (<1 cm diameter) chips in a BICO Chipmunk hardened steel jaw crusher.  Approximately 200-400 grams of rock chips are hand-picked to eliminate weathered surfaces, mineralized zones, or fracture fillings.  Rock chips are milled for approximately 2 minutes in a BICO shatterbox utilizing a tungsten carbide grinding mill.  Rock powder is ground to at least 200 mesh.

 

Major Elements

Major elements are analyzed using a fused pellet technique.  Rock powder is mixed at a 1:10 ratio with lithium tetraborate (B4Li2O7) (0.8 grams rock powder in 8 grams of lithium tetraborate), and thoroughly mixed in a 250 ml plastic vial.  Rock/flux mixture is fused in a 95%Pt/5% Au crucible at 1100°C for 6 minutes following a 4 minute pre-fusion stage on a Breitlander autofluxer.  Glass beads are formed by direct transfer of molten rock/flux mixture to a 95%Pt/5%Au mold.

 

Glass beads are analyzed for SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, and P2O5 on a SIEMENS SRS3000 in the Geochemistry Laboratory in the Department of Geology.  The SRS3000 is a high precision wavelength dispersive sequential X-ray fluorescence spectrometer.  The concentrations of these compounds are determined by comparison of the X-ray intensity of each element with calibration curves developed using eight (8) U.S. Geological Survey international standards (AGV-1, W-2, BIR-1, STM-1, QLO-1, RGM-1, SDC-1, BHVO-1), three Energy, Mines and Resources Canada Minerals Research Program (CANMET) international standards (SY-3, SY-4, MRG-1), four (4) University of British Columbia Mineral Deposit Research Unit (MDRU) standards (ALB-1, MBX-1, WP-1, P-1), and nine Siemens standards (SIEM 1-9).  Analytical accuracy is determined by comparative analysis of international standards; analytical precision is determined by replicate analyses.

 

Trace Elements

Trace elements are analyzed by a pressed pellet technique.  Rock powder is mixed with a binding agent (Spectroblend) with a rock:binder ratio of 10:1 (4.0 grams of rock powder:0.4 grams binder).  Pellets are pressed into an aluminum foil backing in a tungsten carbide die set in a 25 ton hydraulic press (Carver, Inc.).

 

Pressed pellets are analyzed for Ba, Ce, Co, Cu, Cr, Hf, La, Mn, Nb, Nd, Ni, Pb, Rb, Sc, Sr, Th, V, Y, Zn, Zr on a SIEMENS SRS3000 in the Geochemistry Laboratory in the Department of Geology.  The concentrations of these compounds are determined by comparison of the X-ray intensity of each element with calibration curves developed using eight (8) U.S. Geological Survey international standards (AGV-1, W-2, BIR-1, STM-1, QLO-1, RGM-1, SDC-1, BHVO-1), three (3) Energy, Mines and Resources Canada Minerals Research Program (CANMET) international standards (SY-3, SY-4, MRG-1), four (4) University of British Columbia Mineral Deposit Research Unit (MDRU) standards (ALB-1, MBX-1, WP-1, P-1), and nine Siemens standards (SIEM 1-9).  Analytical accuracy is determined by comparative analysis of international standards and interlaboratory comparison of known samples; analytical accuracy is estimate at £5%.  Analytical precision is determined by replicate analyses of international standards.

 

Loss on Ignition

The volatile content (LOI) of each sample is determined using a separate aliquot of rock powder from the same vial of rock powder used in major and trace element analysis.  A 1.0 gram portion of the rock powder is ignited in a tared porcelain crucible at 925°C for 45 minutes.  The crucible is then cooled in a dessicator and weighed.  The weight loss is then reported as percent LOI, and is reported with the major element analysis.