Where is basalt in the Goldich dissolution series?

Question

In relation to this experiment: https://www.remineralize.org/rem_publications/action-of-microorganisms-in-basalt-powder/

It is said that applying basalt rock dust to soil can improve soil fertility especially so if a complete soil food web is present due to biological weathering by bacteria and fungi - which help to release plant nutrients into the exchangeable and soluble pool.

Does basalt have a high rate of weathering and is it in the ‘discontinuous series’ or ‘continuous’ series?

Goldich dissolution series

Answer 1

To answer the other parts of you question, the main minerals composing basalt are calcic plagioclase feldspar, amphibole, and pyroxene. Olivine can be also be a significant constituent. The two main minerals are plagioclase and pyroxene. Plagioclase is in the continuous series, whereas pyroxene and amphibole are in the discontinuous serious.

There are two types of basalt:

• Tholeiitic basalt which contains pyroxene (augite and orthopyroxene or pigeonite) and calcium-rich plagioclase are common phenocryst minerals. Olivine may also be a phenocryst. The groundmass, or matrix,contains interstitial quartz or tridymite or cristobalite
• Alkali basalt which contains mineral assemblages that lack orthopyroxene but contain olivine.

Regarding weathering of basalt, basalt on the surface of the Earth weathers rapidly. Because of the low potassium content of most basalts, weathering converts the basalt to calcium-rich clay (montmorillonite) rather than potassium-rich clay (illite).

Chemical weathering also releases readily water-soluble cations such as calcium, sodium and magnesium, which give basaltic areas a strong buffer capacity against acidification. Calcium released by basalts binds CO₂ from the atmosphere forming CaCO₃ acting thus as a CO₂ trap.

Answer 2

The Goldich series does not contain rocks such as basalt. It contains the minerals that make up the rocks, generally igneous rocks such as basalt. Thus we have to be somewhat general, but we should expect to see basalt in the upper range of the Goldich series.

As with most rocks, the mixture of minerals includes both continuous and discontinuous components in the series.

Felsic vs mafic

Igneous rocks commonly found at the surface are classified as felsic or mafic, which correlates with their silica content: felsic is higher silica an mafic is lower silica. The latter corresponds to minerals that are higher on the Goldich series, such as pyroxenes.

The big reveal: basalt is mafic

So basalt, being a rock, does not have its own entry in the Goldich series, but it is the classic example of a mafic rock. Therefore relatively low silica, and high on the Goldich series. A typical weight-percent composition is given by Shrivasta et al. 1:

The silica level is similar to that in pyroxenes and in calcium-rich plagioclase feldspar (which are themselves major component minerals in basalt), so we should expect basalt to appear in the upper range of the Goldich series. Since basalt contains both pyroxenes and feldspar, it has both discontinuous and continuous components.

Reference

1. Shrivastava, J. & Rani, Nishi & Pathak, Vamdev. (2016). "Geochemical Modeling and Experimental Studies on Mineral Carbonation of Primary Silicates for Long-term Immobilization of CO 2 in Basalt from the Eastern Deccan Volcanic Province." Journal Indian Geophysical Union. Special volume -1. 42-58.