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.
Clay minerals are secondary minerals, formed from the weathering products of primary minerals. Examples of clay minerals are montmorillonite and kaoline. Clay minerals have a layered structure, are brittle and can hold vast amounts of plant nutrients. Do not confuse clay minerals with clay soils.
Other important minerals in the the vineyards are gypsum and apatite. Gypsum ( CaSO4 ∗ 2H2O) is formed when sea water evaporates in the presence of carbonates. Apatite (Ca5OH(PO4)3) are fossiles extremely rich in phosforous.
Granite [Fr. granit] is a magmatic rock, generally composed of various primary silicate minerals. The composition is typically in the range 60% quartz, 30% feldspars and 10% dark minerals. Since granite, and its cousin gneiss, does not contain minerals that easiliy undergoes chemical weathering, soils formed from granite tend to be acidic. Famous Alsace Grands Crus on granite include Sommerberg, Schlossberg and Brand.
Limestone [Fr. calcaire] is composed of the mineral calcite with more less dolomite. Limestone is relatively soft, and weathers rapidly, both chemically and physically. If the dolomitic content is in the range 10-50%, one speaks of it as dolomitic limestone. Grand Cru Rosacker rests on dolomitic limestone.
Marl [fr. marne] is a sedimentary rock composed of small particles of carbonate minerals and clay minerals in roughly equal proportions. Marl is soft and is breaks up easily to form clay soils. Marls are alkaline and are naturally rich in plant nutrients.
Shale and schist [Fr. schiste] are formed in a similar way as sandstone. However, the grains are flaky bits of clay minerals that are stacked on top of each other. Abouth 1/5 of the grains are normally silicate minerals in the sand fraction. Since shales are layered they are quite brittle.