When explaining the distinctive character of wine, the discussion often relates to terroir. Soil is one of the elements that constitute terroir.
There are numerous types of soil. Alongside clay and limestone, Schiefer occupies a significant position among soil types. Schiefer is known as slate in English, though the term schist is also used depending on the degree of metamorphism.
Schiefer soils are widely distributed in Germany, and many wineries vinify their wines separately according to the type of Schiefer present. Schiefer exists in several varieties, distinguished primarily by color. Wines made from grapes grown in soils containing differently colored Schiefer exhibit distinct aromatic and flavor profiles.
Despite being classified under the same category of rock called Schiefer, differences in color correlate with markedly different characteristics. This article examines Schiefer, a soil type with these intriguing properties.
Formation of Slate
Understanding the characteristics and color variations of Schiefer—slate—requires some knowledge of geology.
Slate is a rock characterized by its tendency to split into thin, plate-like sheets. This property has led to its long-standing use in Europe as roofing material, similar to tiles, and occasionally as writing surfaces (slate boards).
Slate forms primarily through low-grade metamorphism of sedimentary rock and is classified as either sedimentary or metamorphic rock depending on its state.
Sedimentary rock forms mainly from sediments deposited on water bottoms through a process called diagenesis. Diagenesis involves compression by the weight of overlying sediments and the gradual bonding of mineral particles. The Schiefer found in Germany’s Mosel region is reported to have originated from marine sediments deposited approximately 400 million years ago during the Devonian period.
Slate is generated when sedimentary rocks such as shale, mudstone, and basite undergo regional metamorphism under relatively low-temperature, low-pressure conditions of 200–450°C and 2–10 kbar. When this metamorphism proceeds further, the rock becomes schist, a more highly metamorphosed rock.
During metamorphism, compression of the sedimentary rock aligns the orientation of contained minerals in a uniform direction, producing slate’s characteristic property of splitting into thin sheets along cleavage planes.
When pelitic rocks—a type of sedimentary rock—undergo metamorphism at pressures higher than those forming slate, the result is phyllite or pelitic schist (black schist).
Characteristics of Slate
Slate is a hard rock. Because the original sedimentary rock has undergone dehydration through heat and pressure during metamorphism, slate exhibits high crystallinity and possesses almost no water retention capacity. However, due to its extremely fine crystalline grain structure, slate is also brittle and prone to weathering. Slate is considered to have high thermal retention capacity.
When present as soil, slate exists as overlapping fragments of broken plate-like pieces. This configuration provides excellent aeration but does not constitute an environment conducive to root penetration.
The compositional elements of slate include primarily quartz, along with mica, clay minerals, feldspar, hematite, and pyrite. Since slate originates from sedimentary rock, its compositional components vary considerably depending on the sediments deposited at the time of formation.
The characteristic color of slate is a grayish-blue that can appear almost black, but depending on the components present, colors range widely through brown, green, red, purple, and blue.
Slate Color and Wine Characteristics
At German wineries with slate soils, wines are often vinified separately according to the color of Schiefer in their holdings. At such wineries, comparative tastings are possible of wines from the same vintage and grape variety, differing only in the type of Schiefer.
Blue and green cool-colored slates tend to produce wines with firm, pronounced acidity, while warm-colored slates trending toward red or black tend to yield softer wines with greater body.
What accounts for these color differences in slate? The following sections examine the three colors most frequently encountered in wine names: blue, red, and green.
Blue Slate: The Role of Glaucophane
The color of blue slate derives from glaucophane, a member of the amphibole group.
Glaucophane is a silicate mineral containing magnesium and iron, and its color ranges from blackish-blue to grayish-blue to violet-blue depending on its composition. Consequently, the color of slate varies from near-black through gray to blue, depending on which color variants of glaucophane are present and in what quantities.
In Germany, this blue Schiefer is distributed from the Mosel, Saar, and Ruwer regions through the Mittelrhein and into areas near the lower reaches of the Rheingau.
Red Slate: The Color of Iron
The color of red slate derives from hematite (Fe₂O₃). Although the brownish tones sometimes suggest the presence of copper, copper is not a constituent.
Red Schiefer is relatively soft compared to other Schiefer types and is reported to possess higher water retention capacity.
In Germany, red slate is found predominantly in the Mosel, Rheinhessen, and Pfalz regions, though it also occurs in scattered locations elsewhere. The Roter Hang at Nierstein in Rheinhessen owes its character to this red slate.
Green Slate: Multiple Contributing Minerals
The distinctive green color of green slate derives from various green-colored minerals including chlorite, epidote, actinolite, and tsavorite (green garnet).
Green slate originates from igneous and sedimentary rocks rich in calcium and magnesium, and forms under somewhat lower metamorphic pressures compared to blue slate. Green slate is also a rock type typically generated through volcanic activity.
In Germany, green slate is found predominantly in the Nahe region.
Summary | Minerals, Color, and Wine Character
The color differences in slate are based on differences in mineral composition.
However, grapevines do not accumulate minerals absorbed through their roots in the juice. In other words, based on current understanding, differences in slate composition do not directly influence wine flavor. Even when grapes are cultivated in red-colored Schiefer soil, their juice does not contain elevated iron levels.
Therefore, the differences perceived in wines made from differently colored slate soils—excluding the effects of site-specific microclimate—are thought to originate from the physical characteristics of the rock as indicated by its color. The mineral composition responsible for the color difference itself is not the cause.
Knowledge of why Schiefer has different colors may hold no significance beyond providing conversation topics over drinks.
Nevertheless, there is an intuitive appeal to associating Schiefer color with wine flavor and aromatic characteristics. In winemaking practice, wines named after their Schiefer type are often finished to match the expected profile associated with that image.
And through this practice, the perception that color differences correspond to differences in wine character becomes progressively reinforced.
