A thin section of Basalt, with large plagioclase crystals in grey and olivine and pyroxene as coloured minerals.

Hacking through on of the geography text books I was a bit dismayed by what I read when it came to the subject of weathering.  I know that weathering is not the most exciting topic – it was boring when I was at school and it still is kind of boring.  So here is an attempt to make it more exciting.

Magic takes place in that pot

Imagine a confectioner making fudge.  Into the pot goes sugar, water, chocolate, cream and maybe some liqueur.  The whole lot gets brought to boil and magic takes place in that pot.  Then it gets poured out to cool, but imagine now if we could cool it really slowly.  I think you will agree with me that sugar has a different melting point to chocolate which has a different melting point to water, and alcohol has the lowest melting point of all and will have evaporated off at the start of the process. Slow cooling will allow for crystals to grow, and we could then analyse them.

Denying them to other minerals further down the cooling curve

By way of explanation, the different crystals of the fudge will form from the various constituents as the temperature falls and they pass through their ‘freezing’ points. A similar thing happens when you thaw a frozen bottle of milk – the water thaws first and the solids come later.  Igneous rocks are the same – the minerals that freeze at higher temperatures are the first to form as the magma or lava cools.  As the temperature falls, different minerals crystallise out of the melt as certain cooling thresholds are crossed, bearing in mind that the chemistry of the melt – which is the term that igneous petrologists use for the cooling magma – changes as minerals form, taking specific elements out of the mix and denying them to other minerals further down the cooling curve.  Eventually only silica is left. 

Goldich’s diagram is the exact reverse of a diagram that every first-year geology student has to learn

A quartz crystal

So, what has this got to do with weathering?  Well, everything.  I see Goldich’s weathering system in the text books which mostly explains things.  This is okay, but what does it mean? There is a list of minerals – olivine – augite-hornblende-biotite – orthoclase-quartz on the right-hand side, and lime plagioclase-sodic plagioclase on the left.  Do you know that Goldich’s diagram is the exact reverse of a diagram that every first-year geology student has to learn?  It was put together by a very famous Canadian igneous petrologist called Norman Bowen, who along with another geologically famous man called Orville Tuttle worked out the various minerals that formed as a magma cooled – which is essentially Goldich’s system in reverse. Or as the geologists would say, the correct way around. Bowen’s work established the direct line of descent of minerals forming in a melt. 

Those that crystallise at lower temperatures are more stable

Minerals that crystallise in the early stages of cooling do so at higher temperatures.  For the record, the pressures will probably be the same during the process unless there is an eruption.  As temperatures fall, new minerals form as they pass through their freezing points, as we have seen earlier.  Those that crystallise at lower temperatures are more stable than those that come out of the melt at higher temperatures.

Those minerals that formed at higher temperatures start to weather first

When these same rocks are exposed at the surface and subject to the agents of erosion – water, heat, wind, mechanical break up to name an obvious few, it is those minerals that formed at higher temperatures that start to weather first because the ambient temperatures and pressures in which the rock finds at surface are very different to those temperature and pressures found in the magma chamber. 

A magma chamber frozen in time and now exposed at surface.  This is also known as a batholith

They are also more susceptible to weathering thanks to their inherent softness and complex chemistry.  For example the formula for augite – a pyroxene mineral – is (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O₆ which weathers earlier on in the process, as compared to that of the most stable mineral, quartz, with a formula of SiO₂.

A list of minerals which is exceptionally boring

The trouble is that a list of minerals which the kids have to learn - namely olivine, augite, hornblende, biotite, orthoclase, plagioclase, muscovite and quartz – is just that – a list of minerals, which is exceptionally boring. 

Find some local limestones and drop some dilute sulphuric acid on them

I suggest we take bring some sand and clay into the classroom to have a closer look at weathering and the products thereof. Also it would be worthwhile finding a rock crystal shop and buying some quartz crystals and possibly some of those other minerals that are in Goldich’s/Bowen’s series, and it may be worthwhile finding some local limestones and dropping some dilute sulphuric acid or hydrochloric acid onto them to see how they react to acid rain.

Rock Cycle and Weathering Course

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