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.