Konrad B. Krauskopf

Dissolution and precipitation of silica at low temperatures

Abstract The dissolution of amorphous silica as monosilicic acid and the establishment of solubility equilibrium with silica gel and with colloidal silica have been demonstrated in recent chemical papers. The rates of dissolution and precipitation are slow at ordinary temperatures; at temperatures near the boiling-point both solubilities and rates of dissolution are much higher. The solubility is little affected by pH between values of 1 and 9, but rises rapidly at pHs over 9. On the basis of this work and supplementary experiments, the geologic literature on the low-temperature solubility of silica is reviewed and various geologic applications are suggested. Much of the confusion in the literature can be traced to failure to recognize the slowness with which silica dissolves and precipitates or polymerizes. Opal dissolves in hot water to about the same extent as other forms of amorphous silica; probably it has a comparable solubility also at lower temperatures, but the rate of dissolution is so extremely slow that massive opal and diatomite can exist almost indefinitely in the presence of natural waters. In sea water amorphous silica has about the same solubility as in fresh water. Silica in true solution is not precipitated by electrolytes; colloidal silica may be precipitated by electrolytes, the rate and completeness of precipitation depending on pH and on the kind and concentration of the ions. Most of the silica in natural waters is in true solution rather than in colloidal dispersion. This means that silica brought to the sea by streams cannot be coagulated by the electrolytes of sea water. The factors that keep the concentration of silica below its equilibrium solubility (with respect to amorphous silica) in most natural waters are not completely understood; the slowness of dissolution, the use of silica by organisms and slowly-established equilibria with crystalline silica or with authigenic silicates doubtless all play a part. The origin of sedimentary chert may be plausibly ascribed to dissolution of remains of siliceous organisms and reprecipitation of the silica (initially as amorphous silica), but not in general to direct inorganic precipitation.

Separation of manganese from iron in sedimentary processes

Thermochemical data on compounds of manganese and iron are in general agreement with reported mineral associations, provided that the more complex mineral compounds, for which data do not exist, be assumed to have somewhat larger stability fields than their nearest simple chemical equivalents. The data show that the iron compounds to be expected in nature are uniformly less soluble than the corresponding manganese compounds and that ferrous ion is more easily oxidized than manganous ion under any naturally occurring pH-Eh conditions. Thus, inorganic processes should always lead to precipitation of iron before manganese from a solution containing both metals, unless the Mn/Fe ratio is very high. The oxidation of manganous and ferrous ions by atmospheric oxygen takes place by slow reactions which can be utilized as an energy source by bacteria. Selective oxidation and precipitation by different species of bacteria can lead to partial separation of the metals, but this is probably not a major factor in the formation of large, nearly pure deposits of manganese or iron compounds. Selective dissolution of the metals from igneous rocks, a mechanism of separation often postulated in the literature, was tested by treating basaltic andesite with a number of solvents at temperatures ranging from 25 to 300°C. The ratios of Mn to Fe in the resulting solutions were all approximately the same as in the original rock, showing that this assumed process of separation is ineffective. Isolation of manganese in solution can be accomplished by precipitating the iron first. This is most effectively done by adding alkali gradually to a solution containing both metals, keeping the solution in contact with atmospheric oxygen. The reaction can be demonstrated in the laboratory under conditions similar to those in nature by letting dilute acid percolate through crushed lava and then through limestone: iron dissolved from the lava is precipitated in the limestone and the solution is left with a high Mn/Fe ratio. This suggests a possible explanation for the origin of many manganese deposits, especially those associated with lavas and tuffs, but it requires that iron oxide in amounts many times that of the manganese be deposited in the rocks through which the solutions have passed.

Thorium and rare-earth metals as analogs for actinide elements

Th is a good analog for Pu in its tetravalent state, which is the common oxidation state in most subsurface geologic environments. For both elements the stable solid form under most conditions is the dioxide, and the principal dissolved species in equilibrium with it is the undissociated hydroxide. Stability constants for complexes with common ligands are similar, those for Pu being generally somewhat larger because of the smaller size of the Pu4+ ion, but ligand concentrations in most groundwaters are too low for complex formation in the tetravalent state to increase concentrations appreciably. Differences in chemical behavior arise because Pu has stable oxidation states above and below 4+. In particular, carbonate complexes of tri-, and penta- or hexavalent Pu are stable enough to make the dioxide more soluble than Th dioxide under conditions of low Eh and pH, and of high Eh and pH, respectively. Unless carbonate concentrations are unusually high, however, the predicted solubilities of the two dioxides are similar and well below maximum permissible concentrations over a wide range of Eh and pH expectable in and near a repository. Thus the low mobility of Th demonstrated at Morro do Ferro, Brazil, is good evidence that the leakage of Pu from a breached subsurface repository will be exceedingly small. The rare-earth elements are even better analogs for the heavier actinides Am and Cm. These actinides under repository conditions can exist only in the trivalent state, and stability constants for their compounds and complexes are very similar to those for the trivalent rare-earth metals. The similarity reflects the close resemblance in ionic radii, especially the radii of the lighter rare-earth elements. Hence the observed low concentrations of La and Nd in groundwater and surface water at Morro do Ferro are a good indication that neither Am nor Cm will escape from a breached repository in unacceptable amounts.

Natural analogues for the transuranic actinide elements: An investigation in Minas Gerais, Brazil

A highly weathered deposit of thorium and rare earth elements located near the summit of a hill in the state of Minas Gerais, Brazil, is being studied as an analogue for a radioactive waste repository that sometime in the distant future may be eroded to the surface or intruded by groundwater Thorium serves as an analogue for Pu4+, and La3+ as an analogue for Cm3+ and Am3+ The mobilization rate constants of the analogue elements by groundwater are so slow (of the order of 10−9 per year) as to suggest that essentially complete radioactive decay of the transuranic actinides would occur in place even under the relatively unfavorable conditions that exist at a site such as this

LAVA MOVEMENT AT PARÍCUTIN VOLCANO, MEXICO

The lava flows of Paricutin Volcano, Mexico, offer an exceptional opportunity to study details of lava movement, because many flows are accessible throughout their length and can be watched from their birth to the cessation of their movement. Petrographically the lavas show no significant difference from one flow to another or in different parts of the same flow. The maximum lava temperature observed was 1070°C.; the maximum estimated rate of flow was 60 meters per minute; the viscosity is probably on the order of 105 to 106 poises. The flows start in fissure zones cutting older Paricutin lavas near the southwest base of the volcano. Within a short distance of its orifice a flow is covered with cooled blocks which are broken and slightly abraded as they are carried downstream. Differential movement in a flow takes place principally at its edges, where a reddish breccia and grooved and slickensided surfaces form. Like a glacier, a lava flow develops transverse crescentic ridges and lateral moraines. At its front movement is normally effected by the rolling forward of molten lava within, which carries down cooled blocks from the surface and piles them up ahead of the flow. As a flow dies the lava level lowers in the channel, and small side tongues break through the moraines. Some Paricutin lavas move considerable distances beneath older lava. The longest flow, which covered the town of San Juan in 1944, has a surface of rough curved slabs very different from the blocky surface of other flows but similar to the surface structures developed near lava orifices and by movement of viscous lava through fissures. The complexities of this small lava field, produced simply by the movement of successive flows, would be difficult to interpret if found in older rocks.

Search for Organic Compounds in the Lunar Dust from the Sea of Tranquiblity

A sample of lunar dust was examined for organic compounds. Carbon detected in concentrations of 157 micrograms per gram had a δ13C per mil (PDB) value of + 20. Treatment with hydrochloric acid yielded hydrocarbons of low molecular weight, suggesting the presence of carbides. The gas chromatogram of the acylated and esterified derivatives of the hydrolyzate was similar to that obtained for the Pueblito de Allende meteorite. There were no detectable amounts of extractable high-molecular-weight alkanes, aromatic hydrocarbons, isoprenoid hydrocarbons, normal alkanes, fatty acids, amino acids, sugars, or nucleic acid bases. Traces of porphyrins were found, perhaps arising from rocket exhaust materials.

MECHANISM OF ERUPTION AT PARÍCUTIN VOLCANO, MEXICO

Paricutin Volcano (Mexico) is situated on a northeast-southwest zone of weakness, which is reflected in the location of the lava vents and a satellite cone. That the lava rises from considerable depths is shown by inclusions of several rock types not known to crop out anywhere in the vicinity. A detailed 4 month record of activity at the crater and in the area of lava outflow at the side of the cone shows little correlation between them. Only once, when the eruption became unusually vigorous, was there a direct response in the lava movement. No cyclic change was observed in the eruptive behavior of the crater except a vaguely defined alternation of moderate activity and quiet in 3 week periods; no periodicity whatever appeared in the lava movement, and the amount of flowing lava remained roughly constant during the 4 months of observation. A mechanism of eruption is suggested in which lava stands continuously at a high level in the conduit. Dissolved gas bubbles out through the crater, while the liquid spills over into a network of fissures which connect with lava vents at the side of the cone. This mechanism accounts for the simultaneous emission of huge amounts of gas from the crater and emission of lava with little gas from fissures near the base of the cone. It explains also why SO 2 is prominent in gas from the crater, while the small amount of gas accompanying the lava contains practically no sulfur compounds but is rich in HCl. Because the lava vents connect with the main conduit by narrow, winding channels, the lava flows are responsive only to major changes in the activity of the crater.

Protoclastic border of the Colville batholith

In the Okanogan Highlands of Washington a complex plutonic mass called the Colville batholith intrudes folded and dynamometamorphosed sedimentary and volcanic rocks of late Paleozoic and Triassic age. Along the sharply discordant contact the wall rocks are much fractured and granulated, but contact metamorphism is slight or absent. The batholith is remarkably heterogeneous both structurally and petrographically. A central mass of structureless granodiorite grades outward into a belt of foliated igneous rock which commonly shows intricate swirling of the foliation. These swirled rocks grade into a peripheral belt of variable but well-foliated migmatitic gneisses characterized by severe granulation of the constituent minerals. Over broad zones this rock is a mylonite; locally recrystallization has produced types resembling metamorphic granulites. That the crushing was protoclastic and not due to dynamic metamorphism following batholithic solidification, is proved by the relations with the wall rocks and by the widespread cementation of the broken materials by films and stringers of undeformed quartz and microcline. Along the contact between the approximately contemporaneous Osoyoos and Colville batholiths occurs a narrow belt of heterogeneous syenite with highly complicated internal structure. This is believed to be a hybrid rock formed by the action of magmas and emanations from both batholiths upon a thin wall rock septum. Deep erosion revealed the Colville rocks over wide areas. Flows of spilite and keratophyre were poured out, and after another erosion interval were covered by Tertiary sediments and volcanics. These were then folded and deeply eroded. Ice sheets covered the area during the Pleistocene.

Aqueous geochemistry of radioactive waste disposal

Abstract Present plans for disposing of high-level radioactive waste include converting it to a stable solid form, enclosing it in metal canisters, placing the canisters in mined cavities several hundred meters below the surface, and filling and sealing all entries to the cavities. The chief hazard in such disposal is dissolution of radionuclides from the waste in the ground water that will eventually fill the cavity and may carry the dissolved material to surface environments. To prevent or delay release of radionuclides in this manner, the form of the waste must be made as insoluble as possible, the canister metal must be resistant to corrosion, both the cavity filling and surrounding rock must be impermeable and highly sorbent, and the repository must be located at a site where ground water is scanty and slow-moving and its path to the ground surface is long. The effectiveness of these barriers in isolating the waste can be predicted for long times in the future by models based on experimentally determined solubilities and sorption coefficients and on measured rates of ground-water movement. Much uncertainty is involved in such predictions, and additional research is needed to lower the uncertainty. But there seems little question that a well-located and well-engineered repository of this sort will provide adequate protection against harm from radioactive releases far into the future.

Calaveras reversed: Westward younging is indicated

Samples of limestone collected from strata in the southern part of the western metamorphic belt of the Sierra Nevada, which traditionally have been assigned to the Calaveras Formation, have yielded Early Triassic conodonts, and samples of metavolcanic rock indicate an Early Jurassic Rb-Sr age. These ages, together with previously published fossil and isotopic data, are consistent with general westward younging of units rather than with eastward younging as has been assumed until recently by many workers, chiefly on the basis of sparse observations of bedding facings. The rocks are strongly deformed, and the possibility that tectonism rather than stratigraphic succession accounts for the age pattern cannot be dismissed.