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Minerality: Gneiss-y Nonsense

There has been a lot of words written on minerality over the past few years; what it means, where it comes from, is it real, and so on. I think the terms minerally and minerality are useful descriptors for wine; I accept, however, that it can be difficult knowing exactly what someone (including me) means when we use one of these terms in a tasting note. I wanted to share some of my own thoughts on minerality over the next few days. In particular I will try and explain my own take on the term, although I will warn you now I’m not going to be as precise as you might be hoping.

Before I get to that though, I first wanted to consider the notion that minerality in wine reflects minerality in the soil.

The major problem with minerality is that some may have assumed that its presence in a wine reflects a direct relationship between the wine and/or fruit from which it was made, and the soil or rock in which the vines are planted. In other words, minerals from the ground somehow find their way into the grapes, and then to the wine. This has always seemed like nonsense to me. We probably all know that plants absorb water, nutrients and minerals through their roots; this is why the health and growth of plants can be influenced by adding fertiliser to the soil. The fertiliser contains nutrients and minerals valuable to the plant, which can be absorbed across the huge surface area offered by its extensive network of fine, branching roots. It is important to understand exactly what we mean by ‘minerals’ though. It is tempting to think they are rock minerals, a molecule of gneiss here, a granite molecule there. That’s always seemed like nonsense to me, for several reasons. Take gneiss – a commonly encountered metamorphic rock in the Muscadet region – as an example; this is made from a variety of minerals, but is likely to be a mix of the following:

  Feldspar, chemical formula : (Ca,Na)AlSi3O8 or KAlSi3O8

  Quartz, chemical formula : SiO2

  Muscovite, chemical formula : KAl2(AlSi3O10)(F,OH)2 or (KF)2(Al2O3)3(SiO2)6(H2O)

  Biotite, chemical formula : K(Mg,Fe)3(AlSi3O10)(OH)2

Gneiss, as an aside, is not defined by a specific mineral composition (so it can be a mix of the above, and other minerals too) but by a texture; it is a metamorphic rock, often granite-derived in the Muscadet region I believe, where the minerals are banded into dark and light strands a result of extreme heat and pressure.

No, I haven’t surreptitiously pulled out a geology degree you never knew about; I simply pulled these formulae off Wikipedia, which only seems fair as they have been mining Winedoctor for wine-related facts ever since a wiki author decided that Latour and Le Pin warranted entries just as much as Miley Cyrus and all the episodes of South Park and The Simpsons ever made did.

Looking at the above minerally formulae, several things spring to mind:

  1. Gneiss is made up of some huge molecules; it would be very difficult (impossible I think – but I’m not a plant scientist and I’m trying to remain open and balanced) to transport them across a cell membrane.

  2. There looks as though there is a lot of potential toxicity there – do plants really want to be hoovering up Aluminium-containing minerals?

  3. Active transport into the cell requires energy – why would the plant use energy to absorb such huge molecules, when it is the constituents (iron, manganese, etc.) that might be useful. Why not just transport in iron and manganese ions from the surrounding soil?

  4. Transport into the cell requires the nutrient or ion to be water soluble; any gardener knows this. In high-pH (alkaline) soils plants suffer from chlorosis. This disease was a big problem for those planting experimental grafted vines in the early years after phylloxera, as some rootstocks when planted on limestone couldn’t handle the conditions. In such conditions much-needed iron is sequestered as a solid, bound with calcium. Solids cannot be absorbed by roots, the ‘minerals’ need to be dissolved. There are two solutions; alter the pH, for a long term solution, or add chelated iron (iron in soluble compounds) for a quick fix. It is impossible to imagine the molecules of stone, described above, dissolving in water and being absorbed. Gneiss, and other rocks, just aren’t water-soluble. They erode, yes, but they don’t dissolve into water like sugar.

  5. Discount all of the above; assume gneiss is soluble, and magically taken up by the roots. How and why would this gigantic minerally molecule be ascended through the structure of the plant to be deposited in the grapes. Would the normal xylem channels achieve this? What would be the purpose of this energy-expensive process?

It all seems like gneiss-y nonsense to me, and seems to discount both the notions that ‘minerality’ originates with the absorption of minerals, and also – on a related issue – that a particular terroir might be expressed in a wine through the absorption of the very minerals that comprise that terroir.

More minerally thoughts on another day.

2 Responses to “Minerality: Gneiss-y Nonsense”

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    The elemental composition of gneiss is not that different from that of clays, which are generally regarded as less suitable for wines. And in any event, the major elements (Silicon, Aluminium) are not used biotically in any appreciable quantity (except by horsetails). The others (Magnesium, Calcium, Potassium) are called trace bioelements for a reason.

    My guess is that it all has to do with fertility drainage and root depth. Stressed plants make better grapes.

    There is a funny video of graduating Harvard seniors who, on being handed a piece of wood, are unable to accept that it is mostly derived from carbon dioxide — i.e., the air.

  2. Avatar

    Thanks for your response Frank. I will explore further what minerality is, and where it comes from, in coming days.