What is the difference between silicate and carbonate minerals

This is a preview of subscription content, log in to check access. AIChE J 52 1 — Chem Geol 1—2 — Energy — CrossRef Google Scholar. Int J Greenhouse Gas Control 5 4 — Alexander G, Mercedesmarotovaler M, Gafarovaaksoy P Evaluation of reaction variables in the dissolution of serpentine for mineral carbonation.

Fuel 86 1—2 — Appl Geochem 25 10 — Science — CrossRef Google Scholar. Geological Survey Google Scholar. Min Eng 61 2 —32 Google Scholar. Exercise 2. Previous: 2. Next: 2. Share This Book Share on Twitter. Sulphates have the SO 4 —2 ion while sulphides have the S —2 ion. Fluorite calcium flouride CaF 2 , halite sodium chloride NaCl Halide minerals have halogen elements as their anion — the minerals in the second last column on the right side of the periodic table, including F, Cl, Br, etc.

Quartz SiO 2 , feldspar sodium-aluminum silicate NaAlSi 3 O 8 , olivine iron or magnesium silicate Mg,Fe 2 SiO 4 Note that in quartz the anion is oxygen, and while it could be argued, therefore, that quartz is an oxide, it is always classed with the silicates. They can, however, be divided into two broad classes, the silicate and non-silicate minerals. The silicates are more abundant, although non-silicates are very common as well. Not only do the two exhibit differences in their composition but also in their structure.

The structure of silicates tends to be more complex, while the structure of non-silicates features a great deal of variability. Silicate minerals all contain silicon and oxygen -- the two most abundant elements in the Earth's crust. Silicates are by far the more abundant of the two groups of minerals, comprising some 75 percent of all known minerals and 40 percent of the most common minerals.

Virtually all igneous rocks are made from silicate minerals; most metamorphic and many sedimentary rocks are made from silicates as well. They can be subdivided into smaller groups on the basis of their structure. Moreover, sulfide weathering rates increase with erosion, and the consequent titration of alkalinity has been proposed to compensate for increases in silicate weathering rates during mountain building 12 , 13 , 14 , Based on these studies, existing global carbon cycle models assume that the weathering rates of all mineral phases, including sulfides, carbonates and silicates, increase with erosion and proceed independently of each other 1 , 9 , 10 , However, where these weathering processes operate side by side, the acidity produced by sulfide oxidation may modulate silicate and carbonate weathering rates, with consequences for the carbon cycle 13 , 14 , 21 , Critically, we lack quantitative constraints on silicate, carbonate and sulfide weathering rates in parallel across extensive and continuous erosion rate gradients without accompanying variations in runoff, temperature and lithology.

In existing compilations where erosion rate and runoff are correlated 4 , 8 , 18 , unravelling the impact of mineral supply on weathering rates is challenging, because runoff impacts weathering 23 , Here we present water chemistry data from streams in southern Taiwan that span an erosion rate gradient of nearly three orders of magnitude in a relatively homogeneous substrate with minimal variations in runoff.

These data allow us, for the first time to our knowledge, to quantify the impact of erosion rates—independent of lithology and runoff—on the concomitant variation of carbonate, silicate and sulfide weathering and, thereby, on the sequestration and emission of CO 2 from these weathering reactions.

We present water samples from 40 separate catchments that span a striking gradient of northward-increasing relief, channel steepness and erosion rates 25 , 26 , 27 Fig. Erosion rate estimates from cosmogenic nuclide data are available for a subset of the sampled catchments 26 , These 10 Be erosion rate data are well correlated with the normalized catchment-averaged steepness index k sn , and we used a regression to predict the erosion rate for each sample from its catchment-averaged k sn estimate Extended Data Fig.

In contrast to this erosion rate gradient of three orders of magnitude, average annual rainfall and inferred runoff vary by no more than a factor of three within the study area 28 , The bedrock in the sampled catchments is dominated by mildly metamorphosed calcareous argillites and shales of the Cenozoic slate belt 30 Fig. In the northernmost part of the study area, these rocks unconformably overlie high-grade metapelites, whereas the southernmost tip of the island is characterized by alternations of sandstones and shales of the Mutan formation Minor carbonate is found as interstitial cement, clastic grains or in veins, in all formations of the study area, and pyrite occurs as a minor rock building mineral 30 Supplementary Text 1.

Map of the southern tip of Taiwan showing all sampling locations. Major lithological units and structures are modified after refs. Symbols for water samples indicate dominant lithology outcropping in the catchment.

Light grey lines mark the 40 unique catchments sampled here. Two rivers mentioned in the main text are marked. For all samples, we measured concentrations of major dissolved cations, anions and silica.

We used an inverse approach to estimate the cation concentrations contributed from silicates [Cat] sil ; Fig. We further estimated the concentration of sulfates contributed from pyrite oxidation [SO 4 ] w ; Fig. Inputs from evaporites and anthropogenic pollution are considered negligible Methods. All results are plotted against the catchment-averaged steepness index lower x axis and inferred erosion rates upper x axis.

Symbols mark dominant lithology see Fig. Samples with anomalously high sulfate concentrations are marked as darker points. Using our new water chemistry data, we investigate the limits to weathering of sulfide, carbonate and silicate phases in parallel across a single erosion rate gradient. Because runoff is inferred to vary by less than two- to threefold across the erosion rate gradient 28 , 29 , we interpret the solute concentrations to be proportional to weathering rates within a factor of two to three.

The steady, fold increase in concentrations of sulfates Fig. Such supply limitation is consistent with previous studies 14 and with the observed depletion of pyrite in the deepest parts of the weathering zone 31 , The supply of sulfuric acid by sulfide oxidation has been hypothesized to either increase both carbonate and silicate weathering rates 14 or to boost only carbonate weathering Our observations suggest that carbonate weathering rates are closely associated with sulfuric acid addition, whereas silicate weathering rates are decoupled Fig.

Carbonate weathering in Taiwan seems to be limited by the saturation of weathering fluids equilibrium limit , based on several observations that include supersaturation of stream waters with respect to calcium carbonate Extended Data Fig. Yet, despite this equilibrium limit, [Cat] carb increases across the erosion rate gradient Fig. This key observation implies that the solubility of carbonates must increase with erosion.

In contrast, acid addition can increase carbonate solubility by an order of magnitude per unit pH decrease Supplementary Text 2. Hence, based on the close association of sulfate concentrations with [Cat] carb Fig. We estimate the pH at which the sampled supersaturated river waters would be at saturation with calcium carbonate pH eq —a maximum estimate of the pH in the subsurface Fig. This inferred equilibrium pH of the weathering fluids generally decreases with increasing erosion rates, mirroring the pattern of [Cat] carb Fig.

In either case, carbonate weathering increases substantially with erosion and appears strongly coupled to sulfuric acid supply. In contrast to carbonate weathering, cation concentrations from silicate weathering do not increase or may even decline at higher erosion rates Fig. An equilibrium limit whereby silicates are saturated is unlikely, because, similar to carbonates, the decrease of the solution pH by at least one unit Fig. Hence, silicate weathering in the study area appears limited across almost the entire erosion rate gradient of three orders of magnitude wide by the slow kinetics of silica dissolution 3 , 4.

This interpretation implies that silicate weathering kinetics are constant or declining from south to north. In addition, the potential two- to threefold increase in runoff could dilute major cations, although the dilution is probably less than a factor of two to three see Supplementary Text 2.

Such changes would more than compensate for the potential impact of temperatures or runoff. The observed invariance or even slight decline of silicate weathering kinetics despite a fold increase in the supply of sulfuric acid Fig.

The subsurface pH appears to be sufficiently buffered to within circumneutral values to prevent any substantial increase in silicate weathering kinetics by the addition of sulfuric acid. Such buffering of the pH is consistent with subsurface pH values inferred from carbonate equilibria Fig.

By observing the weathering of sulfide, silicate and carbonate across a single erosion rate gradient, we find that rock mass weathering does not have a universal limit. Rather, across a large range of erosion rates in the mountains of Taiwan, silicate weathering is kinetically limited, carbonate weathering is equilibrium limited and sulfide weathering is supply limited.

Moreover, our data suggest that the supply of sulfuric acid does not increase weathering rates of both silicates and carbonates as previously hypothesized 14 , even though sulfuric acid probably contributes to weathering of both phases Instead, we propose that in metasediments with sufficient buffering capacity, the supply of sulfuric acid increases the equilibrium limit of carbonates, whereas buffering of weathering fluids at circumneutral pH prevents an increase in silicate weathering rates.

The details of this coupling depend on the relative positions of weathering fronts and the distribution of mineral phases in the subsurface 32 , but the dominance of carbonate weathering, especially at high erosion rates Fig. The observed trends continue across the lithologic boundary between the more sand-rich Mutan formation and the more shale-rich slate belt Fig.

Lithology may cause a change in the trend between carbonate weathering rates and erosion rates Fig. The similarity of samples collected under different hydrological conditions across two field seasons—a dry winter and a wet spring Extended Data Fig. Moreover, the chemistry of many streams is relatively insensitive to short-term variations in runoff 24 , 37 , and Taiwanese catchments typically show dilution by factors of less than five across discharge ranges of several orders of magnitude 38 , In apparent contrast to our findings, existing global compilations of river chemistry often find a positive link between silicate weathering fluxes that is, the concentration multiplied by runoff and erosion 4 , 8.

However, these positive correlations are frequently dominated by a strong co-variation of runoff and erosion rate. Because runoff can directly affect weathering rates 23 , this co-variation obscures the link between mineral supply and chemical weathering.

Hence, a direct comparison between these data and our observations is difficult. Existing carbon cycle models typically assume that the reactivities of all mineral phases increase with rates of erosion and mineral supply, and evolve independently of each other 1 , 9 , 10 , As a consequence, decreasing atmospheric CO 2 concentrations during the Cenozoic era have been linked to increased silicate weathering rates with uplift and erosion of the Alpine—Himalayan system 7 , 10 , In contrast, our data suggest that in the absence of a strong co-variation of mineral supply and runoff, increased erosion of marine siliciclastic sediment sequences may lead to a constant or even decreasing reactivity of silicate minerals, whereas carbonate and sulfide reaction rates increase Fig.

We postulate that this co-variation of silicate, carbonate and sulfide weathering rates applies to erosion rate gradients in many active mountain ranges. Catchments in southern Taiwan are underlain by meta sedimentary rocks of an inverted passive margin sequence that is typical of rocks uplifted along active orogens.

Moreover, solutes from carbonates and sulfides dominate total weathering budgets from freshly exposed metasediments in the Southern Alps of New Zealand 18 , the Himalaya 17 , the Rocky Mountains 14 , the Andes 13 and even in slower-eroding continental interior settings such as northern Texas Finally, it is likely that sulfuric acid addition acts to increase silicate weathering rates only in rare cases, where acid production exceeds the buffering capacity of the sediment, for example in acid mine drainage 41 , 42 , or in headwater streams draining pure silicate lithologies Relative changes in sulfide, silicate and carbonate weathering rates modulate the sequestration and release of CO 2 on millennial to multi-million-year timescales In particular, the increase of sulfide weathering relative to silicate dissolution as erosion rates increase Figs.

This contrast between CO 2 release and sequestration may be even more pronounced where runoff increases in parallel with erosion rates 4 , 8 , As a result, the total CO 2 release from mountain building may overwhelm CO 2 drawdown from more slowly eroding landscapes, even if areas of high erosion rates do not cover the majority of exposed land area Moles of CO 2 emitted or sequestered per volume of water against catchment-averaged steepness index lower x axis and inferred erosion rates upper x axis.

Ultimately, sulfuric acid generation by sulfide oxidation is balanced by marine sulfate reduction, although the timescale for achieving such a balance probably exceeds 10 6 years These are the timescales of long-term climate change and mountain building.

Even beyond the timescales of sulfide compensation in the ocean, the rate of CO 2 sequestration from silicate weathering may be largely unaffected by mineral supply Extended Data Fig.

If our findings extend to metasediments globally, the Cenozoic decrease of atmospheric CO 2 concentrations cannot be associated with an increased supply of minerals within inverted passive margins and must, instead, be linked to increased organic carbon burial 47 , increased relief and orographic precipitation 23 , 44 , alkalinity generation in floodplains 48 or weathering of fertile igneous silicates Over the course of Earth history, the growing accumulation of carbonaceous shelf sediments on continental crust may thus have shifted orogenesis from a net sink of CO 2 to a net source of CO 2.

Subscripts carb, sil and sulf denote quantities calculated for the ions derived from carbonate, silicate and sulfide weathering, respectively, and the subscripts hs, cy, sc, m and wtot refer to contributions from hotsprings, cyclic sources, silicate and carbonate weathering, and to the total measured concentrations and the solutes from weathering reactions, respectively. We use the equilibrium constants and standard enthalpies as specified in Extended Data Table 2.

We collected water samples across southern Taiwan for chemical analysis in 1 litre high-density polyethylene bottles from river banks with access to the main flow. All samples were filtered the evening after sampling into a set of smaller high-density polyethylene bottles for anion and cation measurements using 0. We rinsed each bottle with filtered water before filling, and we acidified all bottles for cation analyses with concentrated ultrapure nitric acid HNO 3.

Every ten samples, a quality control sample mixed at the GFZ was measured to monitor machine drift. We used measurements only within the range of accepted standards, and estimated the uncertainty in the cation analysis from the maximum deviation of the calibration standards from the calibration line.

Uncertainty estimates were based on the standard deviation from three repeat measurements. Uncertainties were propagated from the analytical uncertainties of the measured concentrations. All raw measurements are reported in Supplementary Data 1. Catchment-averaged erosion rates are commonly well correlated with channel steepness, but the relationship has to be calibrated locally We used a regression between existing cosmogenic nuclide erosion rate data 26 , 27 Extended Data Fig.

First, we calculated mean steepness indices upstream of each cosmogenic nuclide sample and upstream of each water sample using TopoToolbox v2. Cosmogenic nuclide concentrations in southern Taiwan decrease northward, and concentrations in repeat samples collected in , and generally agree 26 , We used a regression through the catchment-averaged normalized steepness and catchment-averaged erosion rates to predict, from the steepness index, a catchment-averaged erosion rate for each of the water samples collected in this study Extended Data Fig.

Uncertainties in erosion rates were estimated from the confidence band of the regression. The decrease in 10 Be concentrations of the samples was interpreted to result from extensive landsliding triggered by Typhoon Morakot in 27 , and we excluded these data from the analysis blue points in Extended Data Fig.

Our water samples extend farther south than existing cosmogenic nuclide data.