Basic principles of the geology of Alsace

Minerals and rocks

The Earth is built out of crystalline, chemical substances called minerals. The different minerals form various rocks. The distinction between rock and soil is simply that the soil is defined as the loose solids that lay on top of the solid bedrock.

To understand the difference between different Alsace terroirs, one needs to be able to differentiate between 4 main classes of minerals.

Primary silicate minerals totally make up 90% of the Earths crust. There are several groups of silicate minerals with different nutrient content, weathering rate and structure.

Feldspars is an important group of silicate minerals that weather quite slowly. One dominating feldspar is potassium feldspar (KAlSi3O8 ), that give many granites a pink colour. Different feldspars typically form solid solutions with each other.

The dark silicate minerals form a group of easily weatherable minerals, including amphibole, biotite and pyroxene. In granite, one can see these minerals as dark grains. The chemical weathering of these minerals is an important source of plant nutriens in soils with granite as parent material.

The hardest and least weatherable silicate mineral is quartz, SiO2. Quartz is semi-opaque and milky.

The most important carbonate minerals are calcite, CaCO3, and dolomite, CaMg(CaCO3)2. Carbonate minerals undergo chemical weathering very easily. During this process, acid hydrogen ions (H+) are consumed. The high weathering rate leads to pH-values as high as 7.8 in the soil water. Soils rich in carbonates are thus alkaline. As discussed later, low soil acidity (high pH) favours high acidity (low pH) in the grape must.