Geochemical analysis reports

I urge persons who really want to know if their rock is a meteorite to obtain a chemical analysis from a commercial rock testing laboratory and send the report to me to interpret.

Major elements

Reports from commercial labs can be misleading if they are interpreted literally by persons who are not geochemists. I discuss some of the pitfalls here.

Table 1. Typical results for a terrestrial igneous rock

For both terrestrial rocks and meteorites, only about 10 chemical elements account for 99+ % of the mass. Geochemists call these elements the major elements. Table 1 contains major-element data from a report prepared by a respected commercial rock-analysis laboratory. The report was sent to me by the finder of a rock who suspected the rock to be a meteorite. In such reports, the major-element data are usually expressed in units of percent (%) as most such elements occur in concentrations of more than 1% by mass. The full report contained concentration data for 55 elements. Minor and trace element concentrations are usually expressed in units of ppm (parts-per-million) or even ppb (parts per billion). “Percent” (%) is equivalent to “parts per hundred.”

Columns B and C of Table 1 are exactly as stated in the report. The sample weighed several grams and was finely powdered for analysis. Consequently, the composition of Table 1 is sometimes called the “bulk composition” or “whole-rock composition” to distinguish it from mineral or phase compositions obtained with electron-, ion-, or X-ray beam techniques. The data of Table 1 were obtained by a technique known as ICP-MS – inductively-coupled plasma mass spectrometry. The other common technique for obtaining research-grade, whole-rock major-element data is X-ray fluorescence (XRF) spectrometry.

I have not seen the rock but the composition does not correspond to that of any kind of meteorite. It is consistent, however, with a common type of terrestrial volcanic rock known as basaltic andesite. Basaltic meteorites occur but they do not have such high concentrations of Na2O and K2O. Also, the rock contained only 40 ppm Cr whereas basaltic meteorites (Mars, Moon, Vesta) usually contain 2000-8000 ppm Cr. Na, K, and Cr are three of the best elements for distinguishing terrestrial volcanic rocks from planetary basalts.

Some nonintuitive features of the data in the report

  • The bulk chemical composition reflects the mineralogy of a rock, i.e., the identity and relative proportions of the major minerals composing the rock, but it does not actually identify just which minerals are present. The data of Table 1 is the chemical composition, not the mineralogy. Experienced geologists and geochemists can infer from the composition, however, that the rock of Table 1 is likely dominated by the minerals pyroxene and plagioclase.
  • Oxygen is the most abundant chemical element, both by mass and number of atoms, in most terrestrial rocks and all meteorites. So, literally, oxygen is THE major element in nearly all common rocks.
  • Neither ICP-MS nor XRF actually determines the concentration of oxygen (O), however, so there is no row for O in Table 1. Oxygen is a difficult element to measure accurately in rocks despite its high abundance. Ironically, we do not really need to know the oxygen abundance as we can infer (below) that the concentration of oxygen in the rock of Table 1 is 45% to 46% (100% minus 54.46%, column E).
  • Both ICP-MS and XRF only determine the concentrations of the metallic elements. The detected signals in the instruments (secondary X-rays in XRF and atomic mass in ICP-MS) are from atoms of the elements of column D. These techniques do not “see” SiO2 or CaO, for example. They only see the Si and Ca atoms; there is no information about the atoms are bonded to other atoms or even if the atoms occur in crystals or glass.
  • By geochemical convention, concentrations of the major elements in terrestrial rocks are not stated as % of the element (column E) but as the % of the corresponding oxide (column C). This conversion requires simple high school chemistry and a table of atomic masses: 10% Mg, if totally oxidized, corresponds to 16.58% MgO ([24.305+15.999]/24.305 = 1.6583); Table 2. In words, “the concentration is 16.58% magnesium expressed as magnesium oxide.” This convention is applied to terrestrial rocks because all the metallic elements are oxidized on earth. Scientists who study meteorites typically do not express concentrations as oxides because most meteorites contain some unoxidized iron-nickel metal.
  • The problem is that persons who are not familiar with this convention sometimes reasonably misinterpret geochemical reports. For example, 53.32% SiO2 does not mean that the rock contains 53.32% quartz (mineral formula: SiO2) and 11.99% Fe2O3 does not mean that the rock contains 11.99% hematite (mineral formula: Fe2O3). The rock in Table 1, if a basaltic andesite, contains little or no quartz or hematite. The rock certainly does not contain Na2O and K2O, chemical compounds that do not occur as minerals because they are unstable in nature. The Na and K occur mainly in feldspars, which are aluminosilicate minerals.
  • Then there is the iron (Fe) problem. Iron occurs in three oxidation states in nature: Fe0 (metallic iron), Fe2+ (ferrous iron or iron (II)), and Fe3+ (ferric iron or iron(III)). On earth, all Fe in rocks is oxidized to Fe2+ or Fe3+; metallic iron does not occur in metallic form in natural earth rocks. In common igneous minerals like olivine and orthopyroxene, the iron is Fe2+. In sedimentary minerals like hematite the iron is Fe3+. In some minerals (clinopyroxene, magnetite) iron occurs in both forms. By convention, labs that specialize in testing terrestrial rocks report “total iron as Fe2O3,” that is, all the iron is assumed to be Fe3+. It is possible to determine the concentration of iron that is in the Fe2+ oxidation state independently by titration or Mössbauer spectroscopy, but this is not usually done. In contrast, researchers who study stony meteorites and lunar samples typically report “total iron as FeO,” that is, all the iron is assumed to be Fe2+ as only a small proportion of the iron is oxidized to Fe3+ in rocks from oxygen-poor Moon and asteroids. People who study chondrites often do the “right” thing and simply report the concentration as % Fe (or mg/g Fe) as some of the iron is Fe2+ in silicate and oxide minerals and the rest is Fe0 in iron-nickel metal. So, again, a report that lists “Fe2O3 should not be interpreted to mean that all the iron is in the 3+ oxidation state.
  • Sedimentary rocks, most notably limestone, often contain a lot of calcite, a carbonate mineral. Thus in carbonate-rich rocks, carbon (C) is also a major element. As with oxygen, however, ICP-MS and XRF do not determine the concentration of carbon. Similarly, rocks often contain water and hydrated minerals. ICP-MS and XRF do not determine the concentration of hydrogen (H), water (H2O), or OH.
  • Why report metal concentrations as metal-oxide concentrations? Because in the minerals that are composed of the elements of Table 1 all of the metallic elements occur as cations (positive charge) and oxygen is the only important negatively charged ion. (Sulfur as sulfide is also important in some rocks). As a consequence, the sum of the oxides is usually about 100% in igneous rocks. This is a great test. If the sum of oxides is not 99-101%, then the discrepancy indicates (1) that there is some error in the analysis, (2) the rock contains a significant proportion of calcite, water, or hydrated minerals like hornblendes and micas, (3) some of the iron does not occur in the oxidation state assumed, or (4) the rock contains an unexpected mineral dominated by elements not usually included among major elements, e.g., chromite, zircon, or sulfides like pyrite.

Bottom line: A geochemical report stating, for example, that a rock has 15% Fe2O3 does not necessarily mean that the rock contains the iron oxide mineral hematite. The rock may contain some hematite, particularly if the rock is sedimentary. For igneous rocks, however, most of the iron is carried by silicate minerals like pyroxene and olivine unless the rock is highly weathered.

I have received reports of rock analysis by labs that do not specialize in rocks and consequently do not follow the oxide convention described here; results are reported as % element. Most of these analyses have been done by energy dispersive XRF. EDXRF is not the best way to determine if a rock is a meteorite. See more about this here.

Loss on ignition

Table 1 contains a row labeled LOI – loss on ignition. LOI is the mass change when a powdered rock sample is heated to a high temperature, up to 1000° C, in a furnace until the mass stabilizes, usually several hours to overnight. Baked rock samples lose mass as volatiles are released. The volatiles are usually liquid water, water from hydrated minerals like hornblendes and micas, carbon dioxide from carbonate minerals and oxidation of carbon (e.g., coal) in organic-rich sedimentary rocks. Even igneous rocks may lose several percent water. LOI is a proxy (substitute) for the absence of direct analytical data for carbon and hydrogen.

LOI values can be negative, reflecting a gain in mass when metals oxidize or when Fe2+ oxidizes to Fe3+.

Sum-of-oxide values are usually nearer to 100% if LOI is include as an “oxide,” as in Table 1.

Element-oxide conversions

Table 2 can be used to convert data expressed as percent element to percent oxide. For example, if a magnesium concentration is stated as %Mg, multiply the value by 1.6583 to convert to %MgO. Similarly, if a concentration is expressed as %MgO, multiply by 0.6030 to convert it to %Mg.

Table 2. Element-oxide conversions