CHARACTERIZATION OF TWO NEW REFERENCE MATERIALS FOR XRF ANALYSIS: THE STEWART LEPIDOLITE, STL-1, AND THE ZAPOT AMAZONITE, ZA-1

The XRF analysis of granitic pegmatites still remains a challenge because suitable (homogenous and well characterized) natural pristine standards are scarce. Granitic pegmatites are the focus of many studies due to the great variety of minerals they contain, their importance as major sources of gem materials and their unusual chemical composition.  Pegmatites are generated from highly evolved aqueous fluids derived from the fractionation of granitic magmas and may be enriched in rare alkalis (e.g., Li, Rb, Cs), high field strength elements (e.g., Nb, Ta, Sn), rare-earth elements and volatile components (e.g., B, F) in concentrations that are significantly higher than found in most igneous rocks.  These elements are key to establishing the fractionation level for individual pegmatites or pegmatite fields, as well as providing clues to the tectonic setting of the granite-pegmatite system. Two new natural reference materials were prepared and characterized at the Smithsonian Institution, to provide an avenue to pursue the geochemical analysis of granitic pegmatites: the Stewart Lepidolite (STL-1), USNM 174041, and the Zapot Amazonite (ZA-1), USNM 174042.  STL-1 was prepared with lepidolite collected from the lithium-rich Stewart pegmatite, San Diego County, California (33°22'52” N, 117°03'41” W). The ZA-1 was prepared with an amazonite from the topaz-bearing Zapot pegmatite, Mineral County, Nevada, (38° 41 ‘N, 118°33' W).  STL-1 and ZA-1 were analyzed by wet chemistry (Si, Al, Ca, Mg and Fe), spectrophotometry (Mn, P and Ti), flame photometry (Li, Na, K, Rb and Cs), ICP-MS (Ga, Nb, Sn, Cs, Tl),Tables 1 and 2. Table 1

(wt. %)	STL-1  6-2	STL-1  4-2	ZA-1 7-2	ZA-1 8-2
SiO2	51.36	51.56	64.75	64.44
Al2O3	26.1	26.1	19.1	19
TiO2	0.01	0.01	0.01	0.01
Fe2O3	0.04	0.04	0.11	0.11
MnO	0.37	0.37	b.d.l.	b.d.l.
MgO	b.d.l.	b.d.l.	b.d.l.	b.d.l.
CaO	0.01	0.04	0.01	0.03
Li2O	5.15	5.31
Na2O	0.29	0.29	1.66	1.66
K2O	10.84	10.73	12.92	12.95
Rb2O	1.76	1.76	1.34	1.35
Cs2O	0.24	0.23	0.04	0.05
L.O.I.	0.11	0.08	0.03	0.03

Table 2

(ppm)	STL-1 2-1	STL-1 4-2	STL-1 6-2	ZA-1 3-1	ZA-1 7-2	ZA-1 8-2
Cs	2390	2400	2320	341	342	292
Ga	79	83	82	46	46	40
Nb	122	126	124	2	1	2
Sn	47	48	44	<1	<1	<1
Tl	42.2	48.3	40.2	19.3	17.9	14.5

 

The XRF analysis of random splits showed a high degree of homogeneity (Table 3). Table 3                                                                     

		Ga			As			Rb			Nb			Sn			Cs			Tl			Pb
(ppm)	n	mean	sa	2 s	mean	std	2 s	mean	std	2 s	mean	std	2 s	mean	std	2 s	mean	std	2 s	mean	std	2 s	mean	std	2 s
SL-1	4	81	0.4	0.8	<4			17089	69.9	137.1	151	1.6	3.2	39	4.1	8.1	2340	18	34	42	0.2	0.4	<5
ZA-1	5	46	0.3	0.6	9	1.4	2.7	12224	32.6	64.0	2	0.3	0.5	<5			366	3.7	7.2	15	0.4	0.7	107	0.8	1.6