Next Article in Journal
From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)
Next Article in Special Issue
The Relationship between Surface Roughness, Capillarity and Mineral Composition in Roofing Slates
Previous Article in Journal / Special Issue
Nephrite-Bearing Mining Waste As a Promising Mineral Additive in the Production of New Cement Types
Open AccessArticle

Slatecalculation—A Practical Tool for Deriving Norm Minerals in the Lowest-Grade Metamorphic Pelites and Roof Slates

1
Lehrstuhl für Geologie, Universität Trier, Im Nettetal 4, D 56727 Mayen, Germany
2
Mineralogisch-Petrographisches Institut Universität Hamburg, Grindelallee 48, D 20146 Hamburg, Germany
3
Lehrstuhl für Geologie, Universität Trier, Behringstrasse 21, D 54296 Trier, Germany
4
Institute of Geography, Kiel University, D 24118 Kiel, Germany
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(5), 395; https://doi.org/10.3390/min10050395
Received: 3 March 2020 / Revised: 24 April 2020 / Accepted: 26 April 2020 / Published: 29 April 2020
(This article belongs to the Special Issue Minerals and Other Phases in Constructional Geomaterials)

Abstract

Roof and wall slates are fine-grained rocks with slaty cleavage, and it is often difficult to determine their mineral composition. A new norm mineral calculation called slatecalculation allows the determination of a virtual mineral composition based on full chemical analysis, including the amounts of carbon dioxide (CO2), carbon (C), and sulfur (S). Derived norm minerals include feldspars, carbonates, micas, hydro-micas, chlorites, ore-minerals, and quartz. The mineral components of the slate are assessed with superior accuracy compared to the petrographic analysis based on the European Standard EN 12326. The inevitable methodical inaccuracies in the calculations are limited and transparent. In the present paper, slates, shales, and phyllites from worldwide occurrences were examined. This also gives an overview of the rocks used for discontinuous roofing and external cladding.
Keywords: mineralogy; roof slates; shales; phyllites; norm mineral calculation; hydro-micas; illite mineralogy; roof slates; shales; phyllites; norm mineral calculation; hydro-micas; illite

1. Introduction

Under the term “roof and wall slates”, rocks that have good cleavage and high suitability for overlapping and discontinuous roofing as well as external cladding. Most of them are transversely schistose, very-low-grade metamorphic silt, and clay slates (more precise definitions in [1]). Occasionally, shales are included as well although they are stones that have no schistosity or slaty cleavage due to folding and orogenesis. Their good cleavability originates only from a fine sedimentary stratification (diagenetic foliation in the sense of [2]). Nevertheless, they are also used as roof and wall slates with a usual low thickness with an average of three or five millimeters [3].
Slate deposits are usually of a Proterozoic or Paleozoic age and usually originate from Caledonian and Variscan orogenesis. Unfolded parallel shales sometimes occur on old cratons (platforms). The fine grain size and the fine slaty cleavage, however, make it difficult to determine the composition quantitatively (e.g., polarization microscopy) [3]. Therefore, it can only be estimated by X-ray diffractometry (XRD).
Wagner et al. [4] tried to define very-low-grade metamorphic slates with the metamorphic phase diagram ACF/A’KF (A = (Al2O3 + Fe2O3) − (Na2O + K2O), A’ = (Al2O3 + Fe2O3) − (Na2O + K2O + CaO), K = K2O; C ≈ CaO, F = MgO + MnO + FeO). However, this attempt did not go beyond a simple description. In a research project from 1989 to 1991, an attempt was made to use a full chemical analysis similar to the CIPW (Cross, Iddings, Pirsson, Washington; [5]) norm for magmatic rocks to make a rather inaccurate norm mineral evaluation for roof and wall slates ([6] and Table 2 therein). Sericites (muscovite and paragonite), chlorite, quartz, and total carbonates were estimated as the main minerals. Ward and Gómez-Fernandez [7] used the Rietveld method-based Siroquant data processing system for X-ray powder diffraction analysis for the determination of the slate main minerals quartz, feldspar, micas, and chlorites. However, the application of the method was limited to low carbonate Spanish roofing slate. The determined feldspar (albite) values were higher than those of chlorite and are likely to be too high. Jung and Wagner [8] created a calculation method similar to the CIPW norm that was ready for practical use. They managed to determine the mineral constituents, and in particular, the content of free quartz with sufficient accuracy—for the first time—to some essential practical statements. The quartz content of a slate is, in addition to its structure and grain size, the most decisive factor for its workability (shape easily or heavily) [9,10]. It affects the brittleness and influences the edge straightness and flaking, punchability (making holes), smoothness and flatness of the cleavage surface, and thus, the cleavage thickness. In the slate industry, these characteristics affect the speed and profitability of production.
The petrographic description within EN 12326 [11] provides a rough estimation of the main and accessory constituents from thin sections and a measurement of minerals involved in mica layers and schistosity. Such estimated values are increasingly used to determine the quality and origin while ignoring inaccuracies, natural variations, and the limited sample of a thin section (first criticism in [3]). The norm mineral calculation presented here could replace this method. The results of such norm calculations have already been used not only in test certificates, but also in a manual [10]. Other authors cited such norm calculations together with results of other analyses and found good matches [9]. The results of more than 20 years of application of “slatenorm” [6,9,10,12,13,14,15] have shown that the inclusion of additional ore minerals, color-giving minerals, and hydro-micas (especially illite) in the new slatecalculation is reasonable.

2. Materials and Methods

2.1. Colors

In addition to the normal black to bluish gray colors (generally short “black slate”), there are also so-called “color slates” with green and purple and/or red hues. It has been well known since [16] that the color of a slate depends on the Fe2+/Fe3+-ratio. Therefore, FeO or Fe2O3 should be distinguished in wet-chemical roofing slate full analyses, as done in the extensive analyses by [6,16]. The entire data set of black, green, and red/purple slates and shales is shown in Figure 1. Black slate samples show Fe2O3 contents of 0% to 3%, green samples from 0.7% to 7.7% and red/purple samples from 4% to about 10%. The green color is caused by Fe-chlorite, and the red/purple colors by hematite. In the black color, however, the carbon content is more important [17]. This should also be reflected in the results of a norm mineral calculation.

2.2. Slate Samples

Based on experience, 363 chemical analyses of roofing slates from all over the world were subdivided into groups (Table 1) (e.g., [12,21], using mostly average values, see Supplementary Materials: Tables S1a to 1h). From these, the standard norm minerals were determined with slatecalculation (Supplementary Materials: Tables S2a to 2h).
The “normal” slate originated from the Iberian—(59 samples) and Central Europea Variscides (69 samples). They are cleavable to usual thicknesses of 3 mm or 5 mm on average. In addition, they have good rock and selection quality, proven durability, and low oxidation susceptibility. The slates of the Iberian Variscides are very common in the world market. Central European as well as Iberian slates have been used for roofing since Roman times (see Figure 2, Figure 3 and Figure 4).
The type “carbonate slate” (21 samples) is defined in EN 12326 [11] as slates with at least 20% carbonate. They are the exception in the slate market and only of regional significance. The most well-known deposit of this type is in Liguria (Italy) and has been used locally for roofing slates in thicknesses of more than 6 mm since Roman times (Figure 2b; [13]). This slate changes its color from black to light grey at the latest after a few years on the roof. This characteristic is aesthetically favored in the region. The Ligurian slate has a good reputation even outside of the region and is used as freestone (natural stone) or for pool tables. The sample from Central Europe (Germany, Supplementary Materials: Tables S1c, S2c) comes from a now closed deposit. The slate example from China is still imported to UK but has some quality problems.
The 94 samples of the group “high carbon content“ are slates with organic carbon content higher than 1.0%. EN 12326 [11] excludes roofing slates with carbon contents above 2% (Supplementary Materials: Tables S1d and S2d). This decision goes back to experiences in the well-known roofing slate deposits in Thuringia (Germany). Here high-quality roofing slates with normal C contents and a high lifetime on the roof were used (so-called “Blauer Stein“). In addition other varieties with darker colors and higher C contents have been used (so-called “Dunkler Stein“ and “Dunkelkiesiger Stein“). The so-called “Rußschiefer” had the highest C content of more than 2% (from 3.3% to 4.9% after [22]) and a roof lifetime of only a few years. The other samples of this group come from the Himalayan and Southeast Asian Alpides. For samples from Bhutan, lifetimes beyond 70 years are declared [23]. The slate from Lai Chau (Vietnam) was lying on the roof of the opera-house of Hanoi for more than 50 years before it was renewed with the same slate.
Most (26 out of 29) samples of the group “with carbonate” originated from the Middle Devonian in the Central European Variscides (Supplementary Materials: Tables S1e and S2e). These always have carbonate contents above 5% and up to a maximum of 20%. They change in color from dark blue-grey to grey after approximately 10 years and can develop slightly reddish and brownish oxidation colors (Figure 5b,c). Their durability on the roof is sometimes lower than that of other “normal” slates from the Central European Variscides or “normal” slates from North-America or China (Figure 6).
Rocks of the group “Shales” (19 samples) lack schistosity (tectonic or slaty cleavage) (Supplementary Materials: Tables S1h, S2h). Their cleavage results only from a fine sedimentary stratification (Figure 7). All samples with this structure have inferior workability, especially punchability, compared to normal roofing slates. The most economically important “shale” deposits worldwide are located in the Neoproterozoic Bambui formation in Brazil [19]. This is the most important source for slates used for flooring and facades in the world. Roofing slate is a by-product and is exported mainly to countries using predominantly rectangle slates, including the USA and UK. The “shales” from the Neoarchean of South Africa are also of transregional importance. All other samples, like the ones from Central Europe, have or had only local importance. An example for this is the “Tafelfleins”. This is a layer in the Lias ɛ in the Lower Jurassic, which was once used as roofing slate, but fails to meet today’s requirements.
The 52 samples of the group “schists and phyllites“ are slates of a higher metamorphic state (“low-grade metamorphism“ instead of “very-low-grade metamorphism”, see Figure 8) (Supplementary Materials: Tables S1g and S2g). In the classical slate areas of Central and Western Europe, their use is rare, or they are only used in ornamental covers solely because of their color (e.g., green). Nevertheless, they are typical for some mountainous regions in the Alps or Scandinavia.

2.3. Algorithm Slatecalculation

The extended method slatecalculation presented here is based on a previous, unpublished program called “slatenorm” [8,25] (see Supplementary Materials).
In a first step, the extended algorithm includes the distinction of sulfides. So far, pyrite was the only sulfide calculated in “slatenorm”. This is inaccurate, as pyrite prevails only in less metamorphic slates (like in the Ardennes or Rhenohercynian Zone, Supplementary Materials: Tables S1b and S2b). In many higher metamorphic slates (e.g., from Spain) phyrrotite is predominant and should be included in the calculation because it is more susceptible to oxidation. Therefore, in a first step, the extended algorithm differentiates various sulfides.
The basic calculations of the algorithm, fundamental norm minerals, and chemical formulas are described in a very simplified form below. The detailed calculations can be found in the Supplementary Materials: a flow chart of procedures as well as the computing program in a screenshot of the Excel program “slatecalculation“, and they are numbered as follows:
P2O5 → ap = apatite = 3.3CaO P2O5
S (approx. half, so far macroscopically or microscopically determined) → pn phyrrotite ≈ FeS
Residual S → pt = pyrite = FeS2
Fe2O3 and TiO2 → tm = titanomagnetite = FeO Fe2O3 TiO2, frequently occurring mixture mineral in slates (cf. [4,6]) At the deficit of FeO in some cases a back calculation (28–31 or 49–51) tm in ru = rutile = TiO2 is needed.
TiO2 → ilm = ilmenite = FeO TiO2
Fe2O3 → he = hematite = Fe2O3
C → gr = graphite = C
CO2 → carbonates (cc = calcite = CaO CO2, dol = dolomite = CaO MgO 2CO2, sid = siderite = FeO CO2)
Residual CaO → an = anorthite = CaO Al2O3 2SiO2
K2O → mu = muskovite = K2O 3Al2O3 6SiO2 2H2O
Na2O → pa = paragonite = Na2O 3Al2O3 6SiO2 2H2O
The very variable minerals of the chlorite group require more complicated considerations with regard to their composition:
MgO → mc = 3MgO 2SiO2 2H2O = mc/3 (serpentine)
FeO → fc = 3FeO 2SiO2 2H2O = fc/3 (greenalite)
MgO and Al2O3 → mac = 2MgO Al2O3 SiO2 2H2O = mac/2 (amesite)
FeO and Al2O3 → fac = 2FeO Al2O3 SiO2 2H2O = fac/2 (daphnite)
With the negative rest of Al2O3 → (ab, or) = feldspars:
instead of pa Na2O → ab = albite = Na2O Al2O3 6SiO2
instead of mu → K2O → or = orthoclase = K2O Al2O3 6SiO2
In case of high Al2O3 – contents the calculation of chloritoid might be necessary:
instead of fac → ct = chloritoid = FeO Al2O3 SiO2 H2O
Residual SiO2 → qz = quartz = SiO2
Residual H2O → water → aq
The remaining H₂O = aq may have a positive or negative value, and this is the basis for an extended algorithm determining hydro-micas. In the input data, carbon compounds (e.g., CO2 and organic C) and elementary Sulphur (S) are not included in the glow loss (or loss of ignition (LOI)), but rather subtracted. Furthermore, in the case of predominant FeO compounds, the description of the total amount of Fe as Fe2O3 leads to an unrealistic oxidation gain at the expense of the LOI. Only a carefully corrected LOI can be incorporated in the norm calculation as H₂O = aq but will still be less precise. Thus, the calculated data will have a higher range of variation.

2.4. Hydro-Micas

A positive aq value is the basis for the hydro-micas calculation (see Supplementary Materials: flow chart, steps of calculation 40 to 60). The original values, as well as the results of calculation steps 9 (Al2O3 and SiO2) and 17 or 19 (MgO and FeO) are used as the starting point.
aq (share K2O) → ill = illite = 0.65K2O 3Al2O3 6SiO2 2.35H2O
aq (share Na2O) → br = brammallite = 0.65Na2O 3Al2O3 6SiO2 2.35H2O.
The calculation of hydro-micas requires a recalculation of the remaining phyllosilicates, like micas (mu, pa, steps 41 and 42) and chlorites (mac, mc, fac, fc, steps 47 to 59).
If aq—contents > 0, the calculation of limonite is necessary as well:
instead of he → lm = Limonit = Fe2O3 H2O
Residual SiO2 → qz = quartz = SiO2
Residual H2O → aq
If the ill and br values are exceptionally high, in rare cases, MgO and FeO can remain after the calculation and lead to an Al2O3-deficit. In this case, the following varieties may be calculated as hydro-micas:
aq (share K2O) → maill = 0.65K2O 1.05MgO 1.95Al2O3 6SiO2 2.35H2O (Mol. Wt of Mineral: 705.21)
aq (share K2O) → faill = 0.65K2O 1.05FeO 1.95Al2O3 6SiO2 2.35H2O (Mol. Wt of Mineral: 738.32)
or in combination:
aq (share K2O) → mfaill = 0.65K2O 0.525MgO 0.525FeO 1.95Al2O3 6SiO2 2.35H2O (Mol. Wt of Mineral: 721.77)
aq (share Na2O) → mabr = 0.65Na2O 1.05MgO 1.95Al2O3 6SiO2 2.35H2O (Mol. Wt of Mineral: 684.27)
aq (share Na2O) → fabr = 0.65Na2O 1.05FeO 1.95Al2O3 6SiO2 2.35H2O (Mol. Wt of Mineral: 717.39)
or in combination:
aq (share Na2O) → mfabr = 0.65Na2O 0.525MgO 0.525FeO 1.95Al2O3 6SiO2 2.35H2O (Mol. Wt of Mineral: 700.83).
If the first calculation steps yield negative values or additional information about other minerals is available, further calculations may be considered (e.g., Alta-Quartzite-schist and others):
CaO → epi = epidote = 4CaO Fe2O3 2Al2O3 6SiO2 H2O
CaO → wo = wollastonite = CaO SiO2
TiO2 → tit = titanite = CaO TiO2 SiO2
Fe2O3 → mt = magnetite = FeO Fe2O3
MnO → sp = spessartine = 3MnO Al2O3 3SiO2.
As a final step the norm minerals are added up to minerals and/or mineral groups:
an + ab + or = feldspars
cc + dol + sid = carbonates
mu + pa = micas
ill + br = hydro-micas
mac + fac + mc + fc = chlorites
pn + pt = sulfides
pn + pt + tm + ilm + ru + he (+ mt) = ore-minerals
qz + an + ab + or (+ sp) = rigid minerals
mu + pa +ill + br + mc + mac + fc + fac = elastic minerals.

2.5. Analysis of the Fabric of Phyllosilicates

The petrographic analysis in EN 12326 [11] can be used to estimate the structure of the slate. An important measurement is the mica layers count (better: phyllosilicate layers) per mm in thin sections. This indicates the possible splitting thickness of the slate. Furthermore, the perfection of the phyllosilicates fabric (often simply called the mica fabric) is important for the weathering resistance (perfection in terms of EN 12326-2 [11]: “phyllosilicates are continuous and tied together they will form a net”). In simple terms, the phyllosilicates, with a perfect net structure enclose the weatherable minerals, and protect them against weathering [4,6].

3. Results and Discussion

The mineral contents computed by slatecalculation show proper values for the different slate varieties: The Roman slate shown in Figure 2a is a normal slate from the Lower Devonian, for which slatecalculation calculates the normal mineral quartz 30.0 +/− 4.4%, micas 41.3 +/− 2.5%, hydro-micas 5.5 +/− 4.2%, chlorites 16.8% +/− 2.9 and carbonates 1.6 +/− 0.5. Roman Slate, Liguria/Italy (Figure 2b) is a carbonate slate with quartz 18.3 +/− 1.8%, micas 19.0 +/− 1.7%, hydro-micas 4.0 +/− 3.8%, chlorites 5.4 +/− 0.6% and carbonates 50.2 +/− 4.3% (based on [13]). The 21 roof slate samples from the Casaio formation (Upper Ordovician, Figure 3) in Valdeorras are normal low-carbonate slate with an average mineral content of quartz 31.3%, micas 51.1%, hydro-micas 0.1%, chlorites 12.6%, carbonates 0.2%, sulfides 0.2%, and phyrrotite 0.1%. The sample from the Luarca formation (Valdeorras, Figure 4, Middle Ordovician.) shows quartz 28.6%, micas 40.3%, carbonates 0.4%, sulfides 1%, and a higher phyrrotite content of 0.6%. The partly filigree slate coverings of ornaments (Figure 5) require well workable slates. Norm minerals of three different varieties are (slatecalculation): Black, hard (Figure 5a: quartz 53.0%, micas 21.8%, hydro-micas 8.2%, Fe-chlorites 7.0%, Mg-chlorites 4.0%, carbonates 2.0%; black, with carbonate (Figure 5b): quartz 25.3%, micas 16.0%, hydro-micas 17.7%, Fe-chlorites 5.91%, Mg-chlorites 10.3%, carbonates 12.4%; green, with carbonate (Figure 5c): quartz 27.2%, micas 35.0%, hydro-micas 2.9%, Fe-chlorites 6.7%, Mg-chlorites 6.9%, carbonates 11.0%. The two import slates from the church in Plön (Figure 6) come from overseas: Ziyang/China (black) quartz 45.4%, micas 25.5%, hydro-micas 2.2%, carbonates 0.8%; Trinity Bay/Canada (purple) quartz 30.5%, micas 42.0%, h-micas 2.8%, carbonates 0.4%. The following norm minerals were calculated for the shale from Minas Gerais (oliv-grey, Figure 7): quartz 33.1%, feldspars 14.4%, micas 31.5%, hydro-micas 1.8%, carbonates 1.8%, sulfides 0.3%. The calculated norm minerals of Alta Quartzite (Figure 8) are quartz 57.9%, feldspars 25.4%, micas 7.8% and carbonates 3.0%. All results of the norm minerals calculated by slatecalculation are petrographically coherent, rational, and free of contradictions (for example, normally no negative results for norm minerals) (Supplementary Materials: Tables S2a to S2h).
The differences between the results of slatecalculation compared to the previous version slatenorm can be seen when comparing the results of the main minerals quartz, mica, chlorite, carbonate, and feldspar in Figure 9. The differences between slatenorm (blue) and slate calculation (red) are not significant (Figure 9). There are only significant differences if hydro-micas are calculated. In the case of slatecalculation, only a carefully determined LOI can be incorporated in the norm calculation of hydro-micas (slatecalculation). It will still be less precise and show a larger range of variation. The slates of the Iberian Variscides have only a low content of hydro-micas (0% to 7%), so there are only slight differences in the two methods (Figure 9). The differences are higher in the case of the samples of the Central European Variscides with hydro-micas contents of 5% to 24%. As more Al₂O₃ is used in the calculation of hydro-micas in slatecalculation, the calculation in a few cases results in higher contents of feldspars (ab and or; steps 44 and 46 in the algorithm, Section 2.3) than in the previous calculation slatenorm (see steps 23 and 25 in the algorithm, Section 2.2) (Figure 9c). The slightly lower chlorite content in the slatecalculation compared to slatenorm is due to the calculation of the norm mineral titanomagnetite (tm) instead of ilmenite (ilm) or rutile (ru) (Figure 9a). The differences between the results of the two methods lie in the range expected by the authors.
Table 2 shows a rough overview of the practical roofing slate characteristics and which minerals influence them [3,4,6,9,12,14,21,26]. The classification “positive“ or “negative“ can only be considered as a rough indication for slate quality. Different uses usually also require different assignments. The hardness of the slate, higher quartz content, and the associated higher bending strength are considered positive in UK as they allow the production of larger rectangular formats and the use in areas of high wind load. In contrast, this is different when round and even filigree shapes are desired, as a higher quartz content makes such a processing more difficult (e.g., in Germany, Figure 5b,c).
Moreover, weathering colors can be desirable in the conservation of monuments and historic buildings. Such colors in combination with carbonate percentages above 3% unfortunately bring a shorter lifetime for the roof. The quality of slate is not only determined by the mineral content, but also the fabric of micas (better: fabric on phyllosilicates) (schistosity and, slaty cleavage), which essentially determines the gap cleavability [3,4,6,9,11,12].
The shales without schistosity have only a fine stratification and no phyllosilicate net structure. As a result, they have no protective effect. If harmful components, such as sulfides are embedded in these fine layers of stratification, oxidation occurs (Figure 7).
The phyllosilicates calculated in slatecalculation show a total mica content from usually above 40% (up to a maximum of 60%) and a chlorite content from more than 10% (up to a maximum of 25%) in normal slates (Figure 10a). Only for samples with higher carbonate (“carbonate” and “with carbonate”) or higher carbon (“high carbon content”), are the proportions lower. The micas (mu+pa+ill+br) predominate over the chlorites (mac + mc + fac + fc) in a ratio of 3 to 1.
In “normal” slates, the Fe-chlorites content (fac + fc) outweighs the Mg-chlorites content (mac + mc).
The calculated proportion of hydro-micas (ill and br) could reflect (in addition to the Kübler Index or the organic matter reflectance [26,27]) the degree of metamorphism in most of the samples. That is why phyllites always have hydro-mica values of 0% in the calculation outputs.
The slates of the Iberian Variscides also show very low positive percentages of hydro-micas, while slates from the Central European Variscides (Ardennes and Rhenohercynian zone) have a lower metamorphic grade and show higher values of hydro-micas (Supplementary Materials: Tables S2a and S2b).
Some Mesozoic and Cenozoic examples of “shales” and “carbonate” slates have higher values, up to a prevalence of hydro-micas. There may be other phyllosilicates (perhaps with swelling capacity) that are not calculated in the context of slatecalculation (Figure 10b). There are also exceptions, e.g., when hydro-micas appear as new minerals due the weathering of metamorphic rocks. The Ordovician-Silurian Lederschiefer from Thuringia is deeply weathered because of unstable mineral constituents (hence the name), which can be seen by a high content of hydro-micas (see “!” in Figure 10a). The K-micas (mu + ill) are clearly more frequent than the Na-micas (pa + br).
There is a connection between the calculated “elastic” minerals and the mica-layers per mm in the sense of EN 12326-2 [11], which can be used for practical questions, such as possible thickness and cleavability. This relationship applies to “hard” slates only if the lengthening and orientation of the quartz is relevant as well (Figure 11).
There is a connection between the calculated “elastic” minerals and the mica-layers per mm in the sense of EN 12326-2 [11], which can be used for practical questions, such as possible thickness and cleavability. This relationship applies to “hard” slates only if the lengthening and orientation of the quartz is relevant as well (Figure 11).
When assessing the weathering resistance of slates on the roof, the location and orientation of the roof are important as well. Wagner [14] investigated military barracks in Germany that provided an ideal example for such weathering studies: Two different roofing slate types were studied on 29 similar buildings, all of similar age and exposed to similar weathering conditions.
The slates with carbonate (Central European Variscides) were considered to be less stable. This became clear from both the state of weathering and the water absorption of slates on the barrack roofs. For the “normal” slates (with less carbonates) (also from the Central European Variscides), the values are significantly better. The main wind and rain direction at the location is North–West.
When old roofs are examined in relation to their orientation to the main wind and rain direction side, the saying goes, “Roofing slate loves rain”. Since the different roofs of the barracks were built in different directions, it was possible to check this assumption. Roofing slates on the windward side were allegedly more stable than on the lee side.
The considerably better condition of roofs oriented to the windward side and covered with slates with carbonate confirmed the statement. Results for roofs covered with “normal” slates are similar but less distinct (Figure 12).
The norm mineral calculation slatecalculation (like its unpublished predecessor versions) has been developed to investigate important practical characteristics of roofing slate deposits and their products. Minerals that affect the durability or workability can be determined with this tool with sufficient accuracy. The result analysis is partly better than in the petrographic analysis of EN 12326 [11].
The open access publication of the algorithm online may encourage colleagues to test the method for other fields such as diagenesis, grade of metamorphosis, and provenance analysis, or for other fine-grained sediments and metamorphic rocks. The authors are very open to constructive criticism, improvements, and further experiences or results for further developments and improvements.

4. Conclusions

With the slatecalculation method for calculating norm minerals presented here, we introduced another method for determining the mineral content of lowest-grade metamorphic pelites and roof slates.
The calculated norm minerals are:
  • carbonates (cc = calcite, dol = dolomite, sid = siderite),
  • feldspars (an = anorthite, ab = albite, or = orthoclase),
  • micas (mu = muskovite, pa = paragonite),
  • hydro-micas (ill = illite, br = brammallite),
  • chlorites (mac+mc = Mg-chlorites, fac+fc = Fe-chlorites)
  • sulfides (pt = pyrite, pn = phyrrotite),
  • other ore-minerals (tm = titanomagnetite, ilm = ilmenite, ru = rutile, he = hematite, lm = Limonit),
  • others (ap = apatite, gr = graphite, ct = chloritoid),
  • and qz = quartz.
With the norm mineral computation, this becomes a better, more exact and more easily reproducible tool. The inaccuracies of this method remain limited and transparent. The results are also comparable and sometimes more precise than other methods, such as X-ray diffraction (XRD) (Supplementary Materials: Table S3). It is also a good complement to microscopic thin section analysis, which is usually difficult due to the fine grain of the slate.
The different varieties of slate, such as normal slate, slate with carbonate, carbonate slate, high carbon slate, or quartz-rich slate can be accurately and precisely determined using slatecalculation. Slatecalculation can also help with this to estimate the practical raw material qualities of roof and wall slates (see Table 2). The elastic mineral contents calculated using this method also allows conclusions to be drawn about the microscopic structure of the slate (Figure 11, Supplementary Materials: Table S4). In contrast to thin-section microscopy, slatecalculation can be used to distinguish between micas (muscovite and paragonite) and hydro-micas (illite and brammallite). The method could also provide information (in addition to the Kübler Index or the organic matter reflectance [26,27]) about the degree of metamorphism with the calculated hydro-mica (e.g., illite-) content.

Supplementary Materials

The following are available online at https://www.mdpi.com/2075-163X/10/5/395/s1. Flow chart of procedures in “slatecalculation“; a, b, c, … = conditions in the Excel program; 1, 2, 3, … = calculation steps in the program. Screenshot of the Excel program slatecalculation. Tables S1a to S1h, chemical full analysis, (taken from [6,9,12,13,15,18,20,22,23] with added information and own analyses), Italic s = estimated values; Ø 5 = average of five single analyses. Tables S2a to S2h, results of slatecalculation norm calculations. Table S3, Comparison of XRD data and slatecalculation. Table S4, Mica-layers per mm and elastic minerals in % (slatecalculation). slateNorm.exe (use only “Grundversion!”). slatecalculation.xls can be requested by email: [email protected] References [28,29,30] are cited in the supplementary materials.

Author Contributions

Conceptualization, H.W.W.; methodology, H.W.W. and D.J.; software, H.W.W.; validation, H.W.W.; formal analysis, H.W.W.; investigation, H.W.W.; resources, H.W.W.; data curation, H.W.W.; writing—original draft preparation, H.W.W.; writing—review and editing, M.P.W. and J-F.W.; visualization, H.W.W.; supervision, J.-F.W. and D.J. All authors have read and agreed to the published version of the manuscript.

Funding

The publication was funded by Open Access Fund of Universität Trier and the German Research Foundation (DFG) within the Open Access Publishing funding programme. The development of the computer program “slatenorm” was funded by Rathscheck Schiefer und Dach-Systeme—ZN der Wilh. Werhahn KG Neuss, D 56727 Mayen-Katzenberg. The publication here was permitted.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Arkai, P.; Sassi, F.; Desmons, J. Very low- to low-grade Metamorphic rocks. Recomm. IUGS Subcomm. Syst. Metamorph. Rocks 2003, 5, 1–12. [Google Scholar]
  2. Passchier, C.W.; Trow, R.A.J. Microtectonics, 2nd ed.; Springer: Heidelberg, Germany, 2005. [Google Scholar]
  3. Wagner, H.W. The basics of test methods of slates for roofing and cladding. Grundlagen für die Prüfung von Dach- und Wandschiefern. Z. Dtsch. Ges. Geowiss. 2007, 158, 785–805. [Google Scholar]
  4. Wagner, H.W.; Le Bail, R.; Hacar, M.; Stanek, S. European roofing slates part 1: Remarks to the geology of mineral deposits. Z. Angew. Geol. 1994, 40, 68–74. [Google Scholar]
  5. Winter, J.D. (n. s.). CIPW Norm Excel Spreadsheet. Available online: http://webspace.pugetsound.edu/facultypages/jtepper/MIN-PET/Norm%20Calculation.XLS (accessed on 21 February 2020).
  6. Wagner, H.W.; Baumann, H.; Negendank, J.; Roschig, F. Geological, petrographic, geochemical and petrophysical investigations on roofing slates. Mainz. Geowiss. Mitt. 1997, 26, 131–184. Available online: https://www.uni-trier.de/fileadmin/fb6/prof/GEO/baumann/pdf_files/slate.pdf (accessed on 21 February 2020).
  7. Ward, C.; Gómez-Fernandez, F. Quantitative mineralogical analysis of Spanish roofing slates using the Rietveld method and X-ray powder diffraction data. Eur. J. Mineral. 2003, 15, 1051–1062. [Google Scholar] [CrossRef]
  8. Jung, D.; Wagner, H.W. SlateNorm: Calculation of a normative mineral content of slates. Unpublished. 1998–2000. [Google Scholar]
  9. Cárdenes, V.; Rubio-Ordóñez, A.; Wichert, J.; Cnudde, J.P.; Cnudde, V. Petrography of roofing slates. Earth Sci. Rev. 2014, 138, 435–453. [Google Scholar] [CrossRef]
  10. Lorenz, W.; Gwodsdz, W. Chapter 3.2 Roofing slate. In Manual on the Geological-technical Assessment of Mineral Construction Materials; Geol. Jahrbuch, Sonderhefte Reihe H, Heft SH 15; BGR: Hannover, Germany, 2003; pp. 276–290. [Google Scholar]
  11. European Committee for Standardization (CEN). EN 12326-1 and -2 Slate and Stone Products for Discontinuous Roofing and Cladding—Part 1: Product Specification 2014, Part 2: Methods of Test 2010; Beuth: Berlin, Germany, 2010. [Google Scholar]
  12. Jung, D. Zur Frage der Qualitätsbeurteilung von Schiefer. Schr. Schiefer Fachverb. Dtschl. E. V. 2009, 10, 75–107. [Google Scholar]
  13. Wagner, H.W.; Schultheis, W. Römischer Dachschiefer—Neue Funde und neue Erkenntnisse. Anschnitt 2011, 63, 202–206. [Google Scholar]
  14. Wagner, H.W. Geologische Untersuchung und Materialprüfung an Dachschiefer-Altdeckungen. Mainz. Geowiss. Mitt. 2014, 42, 121–142. [Google Scholar]
  15. Morales-Demarco, M.; Oyhantçabal, P.; Stein, K.-J.; Siegesmund, S. Dolomitic slates from Uruguay: Petrophysical and petromechanical characterization and deposit evaluation. Environ. Earth Sci. 2013, 69, 1361–1395. [Google Scholar] [CrossRef]
  16. Franke, W.; Paul, J. Pelagic redbeds in the Devonian of Germany—Deposition and diagenesis. Sediment. Geol. 1980, 25, 231–256. [Google Scholar] [CrossRef]
  17. Cárdenes, V.; Prieto, B.; Sanmartín, P.; Ferrer, P.; Rubio, A.; Monterroso, C. Influence of chemical-mineralogical composition on the color and brightness of Iberian roofing slates. J. Mater. Civ. Eng. 2012, 24, 460–467. [Google Scholar] [CrossRef]
  18. Dale, T.N.; Eckel, E.C.; Hillebrand, W.F.; Coons, A.T. Slate Deposits and Slate Industry of the United States; Bulletin 275, Series A, Economic Geology; U.S. Geological Survey: Reston, VA, USA, 1906; Volume 63, pp. 1–154.
  19. Grossi, J.H.; Chiodi, C.; Kistemann, D. Panorama do Setor de Ardósias do Estado de Minas Gerais; Textos e Anexos; Governo de Minas Gerais: Belo Horizonte, Brasil, 1998; Volume 1, pp. 1–90.
  20. Krosse, S.; Schreyer, W. Comparative geochemistry of coticules (spessartinequartzites) and their redschist country rocks in the Ordovician of the Ardennes Mountains, Belgium. Chem. Erde 1993, 53, 1–20. [Google Scholar]
  21. Ministerio de Industria y Energia. Pizarras de Espana; Instituto Geolocico y Minero de Espana: Madrid, Spain, 1977; p. 83. [Google Scholar]
  22. Zimmerle, W.; Stribrny, B. Organic carbon-rich pelitic sediments in the Federal Republic of Germany. Cour. Forsch. Inst. Senckenberg 1992, 152, 1–144. [Google Scholar]
  23. Jung, D. Mineralogisch-petrographische Untersuchung eines Schiefers aus Bhutan. Unpublished. 2001; 13. [Google Scholar]
  24. Stribny, B.; Urban, H. Classification of sedimentary rocks—Black shale series based on their normative minerals compositions. IGCP Proj. Newsl. 1989, 254, 30. [Google Scholar]
  25. Jung, D.; Geisler-Wierwille, T. Program “SlateNorm” Version 1.1. Unpublished. 1999–2000. [Google Scholar]
  26. Cardenes, V.; Rubio Ordonez, A.; Lopez Munguira, A.; Monteroso, C. Petrografía y mineralogía de las pizarras para cubiertas de la Península Ibérica en relación con su calidad. Trab. Geol. Univ. Oviedo 2010, 30, 412–420. [Google Scholar]
  27. Ferreiro Mählmann, R.; Bozkaya, Ö.; Potel SLe Bayon, R.; Šegvic, B.; Nieto, F. The pioneer work of Bernard Kübler and Martin Frey in very low-grade metamorphic terranes: Paleo-geothermal potential of variation in Kübler-Index/organic matter reflectance correlations. A review. Swiss J. Geosci. 2012, 105, 121–152. [Google Scholar] [CrossRef]
  28. Bushan, A. Dachschiefervorkommen Deutschlands-petrographisch-geochemische Charakterisierung mit besonderem Blick auf die Goslarer Dachschiefer. Masterarbeit Universität Potsdam. Unpublished. 2017; 1–112. [Google Scholar]
  29. Chemical Formulas of the Minerals with Changes after. Available online: http://webmineral.com (accessed on 21 February 2020).
  30. Molar Mass Calculator. Available online: http://www.chemeurope.com/en/tools/ (accessed on 21 February 2020).
Figure 1. The dependency of slate color on the Fe2O3/FeO ratio (analyses from [6,16,18,19,20] and own data).
Figure 1. The dependency of slate color on the Fe2O3/FeO ratio (analyses from [6,16,18,19,20] and own data).
Minerals 10 00395 g001
Figure 2. Roman slates: (a) Roman Slate, Central Europe; (b) Roman Slate, Liguria/Italy (based on [13]).
Figure 2. Roman slates: (a) Roman Slate, Central Europe; (b) Roman Slate, Liguria/Italy (based on [13]).
Minerals 10 00395 g002
Figure 3. Underground slate mining in Spain (Valdeorras, Casaio-formation).
Figure 3. Underground slate mining in Spain (Valdeorras, Casaio-formation).
Minerals 10 00395 g003
Figure 4. Opencast slate mining in Spain (Valdeorras, Luarca slate).
Figure 4. Opencast slate mining in Spain (Valdeorras, Luarca slate).
Minerals 10 00395 g004
Figure 5. The partly filigree slate coverings of ornaments with round forms in Central Europe require well workable slates; four different deposits and varieties: (a) black, hard, Upper Devonian; (b) black, with carbonate, Middle Devonian; (c) green, with carbonate, Middle Devonian; (d) purple, hard Lower Devonian; (slate covering at least 90 years old).
Figure 5. The partly filigree slate coverings of ornaments with round forms in Central Europe require well workable slates; four different deposits and varieties: (a) black, hard, Upper Devonian; (b) black, with carbonate, Middle Devonian; (c) green, with carbonate, Middle Devonian; (d) purple, hard Lower Devonian; (slate covering at least 90 years old).
Minerals 10 00395 g005
Figure 6. The imported slates (China and Canada) on the Saint Nicolai Church in Plön, arranged in “English” rectangle roofing with hooks (15 years old).
Figure 6. The imported slates (China and Canada) on the Saint Nicolai Church in Plön, arranged in “English” rectangle roofing with hooks (15 years old).
Minerals 10 00395 g006
Figure 7. The cleavage of the shale does not originate from real schistosity but from a fine stratification (a). (See picture detail on the lower right (b)) (At the time of the photo a nearly new cladding.); Minas Gerais (oliv-grey)/Brazil.
Figure 7. The cleavage of the shale does not originate from real schistosity but from a fine stratification (a). (See picture detail on the lower right (b)) (At the time of the photo a nearly new cladding.); Minas Gerais (oliv-grey)/Brazil.
Minerals 10 00395 g007
Figure 8. Phyllitic Quartzite facade panels (transverse length = 50 cm) (almost 60 years old); Alta Quartzite/Norway.
Figure 8. Phyllitic Quartzite facade panels (transverse length = 50 cm) (almost 60 years old); Alta Quartzite/Norway.
Minerals 10 00395 g008
Figure 9. Comparison of the main minerals calculated with slatenorm (blue) and slatecalculation (red). (a) quartz/micas/chlorites, (b) quartz/micas/carbonates, (c) quartz/micas/feldspars.
Figure 9. Comparison of the main minerals calculated with slatenorm (blue) and slatecalculation (red). (a) quartz/micas/chlorites, (b) quartz/micas/carbonates, (c) quartz/micas/feldspars.
Minerals 10 00395 g009
Figure 10. Phyllosilicates as calculated by slatecalculation (a) Types of phyllosilicates; (b) selection of different deposits.
Figure 10. Phyllosilicates as calculated by slatecalculation (a) Types of phyllosilicates; (b) selection of different deposits.
Minerals 10 00395 g010
Figure 11. The relationship between calculated “elastic minerals“ (see Table 2 and Section 2.4(i)) (slatecalculation) and the mica-layers per mm in the sense of EN 12326-2 [11].
Figure 11. The relationship between calculated “elastic minerals“ (see Table 2 and Section 2.4(i)) (slatecalculation) and the mica-layers per mm in the sense of EN 12326-2 [11].
Minerals 10 00395 g011
Figure 12. The dependence of weathering to the main wind and rain direction of slates “with carbonate” (turn blue) and “normal” slates (green): average values with inclusive standard deviation of some norm minerals. The distance from the center to the outside is the relative weathering intensity (altered based on [14]).
Figure 12. The dependence of weathering to the main wind and rain direction of slates “with carbonate” (turn blue) and “normal” slates (green): average values with inclusive standard deviation of some norm minerals. The distance from the center to the outside is the relative weathering intensity (altered based on [14]).
Minerals 10 00395 g012
Table 1. The geological information of the slate samples analyzed in this paper (also based on [4,6,9,12,14,15,18,24].
Table 1. The geological information of the slate samples analyzed in this paper (also based on [4,6,9,12,14,15,18,24].
CountryProvinceRegion/LocationØ of SamplesType of RockFormationSeries/System
“normal” slate, Iberian Varicides
SpainAsturiaOscosVilarchao2slateLuarcaMiddle Ordovician
Galicia/OrenseAlto BierzoAnllares2
ValdeorrasMormeau8ArgüeiraCasaioUpper Ordovician
Los Molinos8
Riodolas5
Rozadais3Rozadais
Penedo Rayado6
Los Campos2Losadilla
Castile and LeónLa CabreraSan Pedro2Rozadais
Benuza2
La Baña2
SpainGalicia/OrenseSan Vincente2SoldonMiddle Ordovician
QuirogaPacios gris3Luarca
Pacios negra7
La Campa negra1
FranceAnjouAngersTrélazé2Grand-Auverné (Trélazé Mb.) ≈ Luarca
PortugalPortoValongo1Valongo ≈ Luarca
Arouca1
"normal" slate, Central European Varicides
BeLuxArdennesMartelange2slateSchiste ardoisierLower Devonian
GermanyNorth Rhine-WestphaliaEifelVenn21Revin 4/5Black shalesUpper Cambrian
Rhineland-PalatinateKatzenberg13SiegenianLeutesdorf or AubachLower Devonian
Bausberg2
Margaretha7
Xantenfrom a Roman building2
NetherlandNijmwegenfrom a Roman wreck2
GermanyRhineland-PalatinateHunsrückMoselberg2Lower EmsianHunsrück slates
Altlay1Altlay
Bundenbach5Hunsrück slates, Kaub
Middle RhineKaub-Bacharach4
GermanyRhineland-PalatinateLahnSinghofen1Lower EmsianSinghofenLower Devonian
HesseLanghecke1AdorfBänderschieferUpper Devonian
BayernFrankenwaldLotharheil1KulmBordenschieferMississipian Carboniferous
Czech Rep.MoraviaOlomouc5Moravice
carbonate slate
GermanyNorth Rhine-WestphaliaNuttlar1carbonate slateFlinzMiddle to Upper Devonian
ItalyLiguria3Val LavagnaUpper Cretaceous
SwitzerlandGlarusEngiLandesplattenberg1Nordhelvetischer Flysch, Engi SlateEocene
ChinaHubeiDaba ShanPingli1PingliLower Ordovician
UruguayLavallejaArr. Minas Viejas12Minas Viejas groupPaleoproterozoic
Arr. Mataojo3Mataojo group
high carbon content
GermanyNorth Rhine-WestphaliaEifel-VennElise7slateSalm 1/2TremadocLower Ordovician
Marsberg1HangenbergUpper Devonian to Lower Carboniferous
24KulmLyditLower Carboniferous
10Posidonia
15Clayshales (Marsfeld?)
28Lower Alum
ThuringiaRonneburg3Leder SlateUpper Ordovician to Lower Silurian
Lehesten Schmiedebach3Kulm Lehesten dunkelkiesigMississipian Carboniferous
BhutanWangdu Phodrang2above the ChekhaTriassic?
Vietnam Lai Chau/Dien BienQuang Chêng1schiste ardoisierTriassic
slate with carbonate
GermanyHesseLahn-DillHaiger2slateEifelianWissenbachMiddle Devonian
Batzbach1
Rhineland-PalatinateLahnRupbach2Rupbach (≈ Wissenbach)
North Rhine-WestphaliaBad FredeburgMagog12Fredeburg (≈ Wissenbach)
Felicitas7
HessenWillingenBrilon2Asten (≈ Wissenbach)
SwitzerlandSt GallenCalandaPfäfers-Vadura1KalkthonschieferEocene
U. S. A.New YorkRed1PoultneyCambrian to Ordoviciann
PennsylvaniaLehigh1MartinsburgMiddle to Upper Ordovician
Other slate UK, N-America, China
UKWalesPenrhynpurple5slateLlanberisCambrian
CanadaNewfoundlandTrinity Baypurple3Bonavista
green1
U. S. A.VermontSea green1PoultneyCambrian to Ordovician
Unfading green1
variegatedEureka1
purple1
black1Trenton
New YorkBright green1Poultney
ChinaHubeiHongshigou3Daguipings 1dsLower Silurian
ShaanxiZiyang2
shales
BotswanaSouthern DistrictKanye1shalePlatberg GroupRietgat Neoarchean
NigeriaEbonyeSouthern Benue trough3Asu River GroupAbakalikiLower Cretaceous
BrazilMinas GeraisPapagaios, Paraopeba, Sete Lagoasblack1Bambui, Santa HelenaNeoproterozoic
oliv2
green1
purple1
Santa Catarinadark green, black2TubaraoMafraPermian
GermanyBaden-WuerttembergDotternhausenTafelfleins1carbonate shalePosidoniaLias ɛLower Jurassic
NorwayFinnmarkVestertanared1shaleFinnmark SupergroupVestertana GroupRiphaen Proterozoic
green1
SwitzerlandZurichWeiach4shale (high carbon)Lower Rotliegendes, lakustrine seriesPermian
1Carbonate shale
schists and phyllites
SpainCastile and LeónSegoviaBernardos4phylliteSchists-Metagraywacke ComplexNeoproterozoic to Lower Cambrian
GaliciaA Terra ChaVerde Lugo2Cándana GroupLower Cambrian
PortugalTrás-os-Montes e Alto DouroFoz Coa1Desejosa
NorwayFinnmarkAlta1Quar-tziteAlta GroupRiphaen Proterozoic
Friarfjord2phylliteLaksefjord group
BelgiumWalloniaArdennesVielsalm1Salm GroupEarly to Middle Ordovician
Ottré redschist8
Ottré coticule25
GermanySaxonyVogtlandTheuma, Oelsnitz1PhycodendachschieferLower Ordovician
Czech Rep.Plzeňský krajRapštejn2Kralupy-Zbraslav groupProterozoic
Liberecký krajZelezny Brod1Radčice GroupCambrian to Ordovician
IndiaHimachal PradeshHimachal1Chamba and KatarigaliProterozoic
HaryanaBectel1Delhi SupergroupAjabgarh Group Neo- to Mesoproterozoic
ArgentinaCuyoSan Luis2San LuisOrdovician
Table 2. Minerals and their influence on practical roofing slate quality.
Table 2. Minerals and their influence on practical roofing slate quality.
Norm MineralsPositiveNegativeColor
QuartzqzRidged mineralsmechanical strength, hardnessresistance to weathering> 32% w/w brittleness, sometimes worse cleavability, punching quality and finishing quality in round forms
feldsparsOrthoclaseor(have a cleavability)
Albiteab
Anorthitean
micasMuscovitemuElastic mineralscause the cleavability, punching quality and finishing quality also in round formsresistance to weathering
Paragonitepa
hydro-micasIlliteill product of weathering
Brammalitebr
chloritesMg-Al-Chloritemachelp with cleavability, punching quality and finishing quality
Mg-Chloritemc
Fe-Al-Chloritefacblue, green
Fe-Chloritefc
carbonatesCalcitecc (have a cleavability)> 5% w/w
after EN 12326-1 thickness increase
with higher contents > 5% w/w less resistant to weatheringlight grey with oxidation colors weathering ("semiweathering")
Dolomitedol
Sideritesid
ore mineralsPyrrhotitepn > 0.3% w/w and in accumulations oxidation risks
Pyriteptin accumulations of micro pyrites oxidation risks
Hematitehe > 2% w/w
red, purple
Limonitelm product of weathering
Graphitegr2% w/w
after EN 12326-1 does not permit
black
Back to TopTop