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The elemental analyses whose basis is described in the preceding two papers represent the composition of samples of Martian fines; the only undetermined major constituents thought to be present are H2O, CO2, Na2O, and possibly NOx. The samples are principally silicate particles, with some admixture of oxide and probably carbonate minerals; the fines appear to have been indurated to a variable degree by a sulfate‐rich intergranular cement. The overall elemental composition is dissimilar to any single known mineral or rock type and apparently represents a mixture of materials. Close chemical similarity among samples at each site, and between the two sites, indicates effective homogenization of the fines, presumably by planetary windstorms, and further suggests that the samples analyzed represent the fine, mobilizable materials over a large part of the planet's surface. Low trace element, alkali, and alumina contents suggest that the great preponderance of the materials in the mixture is of mafic derivation; highly differentiated, salic igneous rocks or their weathering products are insignificant components of the samples. Normative calculations, comparisons with reference libraries of analytical data, and mathematical mixture modeling have led to a qualitative mineralogical model in which the fines consist largely of iron‐rich smectites (or their degradation products), carbonates, iron oxides, probably in part maghemite, and sulfate minerals concentrated in a surface duricrust. The original smectites may have formed by interaction of mafic magma and subsurface ice, and the sulfates (and carbonates?) may have been concentrated in the surface crust by subsurface leaching, upward transport, and evaporation of intergranular moisture films. Testing and refinement of this and competing models will accompany continuing acquisition of samples and data and refinement of the analyses, particularly with respect to the critical light elements Mg, Al, and Si.

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Elemental analyses of fines in the Martian regolith at two widely separated landing sites, Chryse Planitia and Utopia Planitia, produced remarkably similar results. At both sites, the uppermost regolith contains abundant Si and Fe, with significant concentrations of Mg, Al, S, Ca, and Ti. The S concentration is one to two orders of magnitude higher, and K(<0.25 percent by weight) is at least 5 times lower than the average for the earth's crust. The trace elements Sr, Y, and possibly Zr, have been detected at concentrations near or below 100 parts per million. Pebblesized fragments sampled at Chryse contain more S than the bulk fines, and are thought to be pieces of a sulfate-cemented duricrust.

Acid‐base accounting (ABA) is a common procedure to predict the alkaline or acid‐producing potential of overburdens. Neutralization potential (NP) as currently written in ABA overestimates alkalinity when siderite (FeCO3) is present in the overburden. Siderite initially yields alkalinity upon digestion, but with time the alkalinity is neutralized by acidity from ferric iron (Fe3+) hydrolysis and precipitation. Thirty‐one overburden samples containing varying amounts of siderite, calcite, pyrite, and quartz were analyzed by four NP digestion methods and titrated either by hand or by autotitration. The NP methods were: (i) standard Sobek method (Sobek); (ii) a method that boils the sample for 5 min (BOIL); (iii) a method similar to BOIL but it includes filtering and treating the sample with hydrogen peroxide before back‐titrating (H2O2); and (iv) a modified Sobek method that adds H2O2 after the first hand titration (SobPer). For samples containing primarily calcite, quartz, or clays, the NP values for a particular sample were similar among digestion methods. For samples containing pyrite, the SobPer method (no filtering) produced the lowest NP values. Siderite‐containing samples showed wide variation in NP values among methods. The H2O2 method decreased NP values of siderite samples compared to Sobek and BOIL methods. Lower NP values were generally obtained with autotitration vs. hand titration because autotitration added the base slowly, which allowed concurrent oxidation and hydrolysis of iron. Hand‐titration of siderite samples requires H2O2 treatment to accelerate iron oxidation. Variation in NP values for a particular sample was high among three laboratories using the Sobek hand titration method, but the average variation in NP values among labs decreased by 66% when using the H2O2 hand method. Variation in NP values among labs was also due to the same samples being assigned different fizz ratings by laboratory technicians, which changed the concentration of acid added in the digestion procedure. With more acid, NP values generally increased, especially for siderite samples. A more quantitative approach is needed to determine the amount of acid to add for NP digestion, and the percent insoluble residue of the sample used in this study may be a good alternative but requires more testing and multilaboratory screening. The ABA values (using %S and NP from the various methods) were compared with soxhlet leachate pH and cumulative alkalinity. The ABA values with H2O2 digestion were consistent with soxhlet leachate quality in 13 out of 13 samples. It is suggested that laboratories conducting NP in the ABA procedure use the H2O2 method.