Obsidian Na,K,Al,Fe silicate glass

Locality: unknown

Element	%	Oxide	%
H	0.09	H₂O	0.80
O	48.63
F	0.07	F	0.07
Na	3.01	Na₂O	4.06
Mg	0.04	MgO	0.07
Al	6.94	Al₂O₃	13.11
Si	34.56	SiO₂	73.94
Cl	0.36	Cl	0.36
K	4.18	K₂O	5.04
Ca	0.54	CaO	0.76
Ti	0.06	TiO₂	0.10
Mn	0.05	MnO	0.06
Fe	1.34	FeO	1.72

Sum	99.9	Sum	100.1


	200 µ m

This is a reflected light micrograph of a polished, carbon coated obsidian grain. The identifying etched label, which is a feature of our standards, is invaluable for navigating in an electron beam instrument. Obsidian is a volcanic glass containing the full range of rock-forming elements from hydrogen to iron. Because of its glassy state, the alkali elements Na and K are not tightly bound and may therefore migrate, giving instability in intense electron bombardment. However, the composition has been well established through repeated analysis by classes of students, it is a most useful standard for general use in silicate analysis.

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Cautionary Note

The compositional and other data shown here are intended to convey typical values for this item and do not neccessarily correspond in detail to standard mounts supplied in the past or to be supplied in the future. Items are changed from time to time as stock is exhausted or alternate choices are preferred. These data should, however, give someone contemplating the purchase of an SPI standard a fair indication of what they may expect to receive. Records are kept of all mounts sold so that, should you lose your documentation at some time in the future, we will be happy to replace it at no charge.

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