THE ROCKS | Macroscopic Classification

1-Introduction

This classification, borrowed from technical requirements PTV 844 was developed in a pragmatic goal by a working group composed mainly of active geologists in the field of natural stone and its applications in construction1, as part of the activities of the UBAtc2 on natural stone as well as on the basis of advice and advisory opinions BCCA3 and COPRO4.

The proposed classification has rocks from a macroscopic perspective. It differs therefore a commercial or petrographic classification. It essentially aims stones used in construction (roads and buildings). The rock salt, carbon, phosphate, etc. are not considered.

This macroscopic classification is also based on the three main families of rocks previously defined: igneous, metamorphic and sedimentary.

Macroscopically, the color is also an important criterion to define the choice of a stone. This is a visual concept quite subjective, but it is hard to describe other than a qualitative perspective for rocks in heterogeneous structure, veined or layered. When the rock is sufficiently uniform in color, the latter can be defined in two ways:

        A measurement by colorimétrie5 coordinates L, a and b

        A measurement-based Color Rock Chart6.

1- T. De Ruyver (FPU), R. Van Rossum (Walloon Ministry of Equipment and Transport), J.-P. Cnudde (Universiteit Gent), Mr. Briessinck (Flemish Community), R. and D. Dreesen Lagrou (VITO-Flemish Institute for Technological Research), P. Bonmariage (BCCA), V. and B. Netels Misonne (Career Belgian Federation Blue Stone - Petit Granit), F. Tourneur (Stone and Marble of Wallonia), J. Elsen (KULeuven), D. Nicaise and F. de Barquin (CTSA).

2- Belgian Union for technical approval in construction.

3- Belgian Construction Certification Association.

4- Impartial body control products for construction.

5- DIN 6174 Farbmetrische Bestimmung von Farbabstanden bei Körperfarben nach der Formel-CIELAB.

6- Geological Society of America (Boulder, Colorado).

         

2- Magmatic Rocks

The classification of igneous rocks is a simplified classification into levels based on the classification of Streckeisen, when the NBN EN 12670 standard also refers. Established by the proportions of essential minerals present in the rock, it is used in microscopy to quantify exactly various minerals, but is also used for macroscopic descriptions, insofar as the quantity and the relative nature of the main minerals give rock a particular macroscopic appearance.

For example, a granite often has a pink dominant color (gray minerals and large roses) dotted with small black mineral gabbro a dark black color and a diorite a gray appearance.

Among the magmatic rocks are distinguished macroscopically:

plutonic rocks of slow cooling at depth

lode rocks (périplutoniques or subvolcanic)

volcanic rocks (effusive) to rapid cooling.

acid rock is further talks if the dominant mineral is quartz, and basic rock when the dominant mineral is a mineral pyroxene ferromagnésien kind.

3-Sedimentary Rocks

Sedimentary rocks cover 75% of Earth's surface, which causes significant variability again. This is disintegrated materials, altered and transported, or from chemical or mixed precipitation.

Rather than a genesis, the classification presented in Table 6 reflects the predominant nature of the rock, which can be mainly silica (quartz and silicates, such as clays) or carbonate.

Regarding the siliceous rocks of detrital origin (siliciclastic) or from precipitation (bio) chemical, classification takes into account the broad categories of rocks that can be found on the construction market.

In the case of carbonate rocks, classification focuses, first, on the presence of dolomite - element that can be important to the stone age and patina - and, secondly, on the presence of detrital components (quartz and glauconite).

For limestone and dolomitic limestone, the distinction is made on the abundance of macroscopically recognizable fossils (fossil limestone), the presence of oolites the brecciated texture or the nature of the stone (chemical precipitation like travertine or tufa). For fossil limestone fossil dominant specified (eg crinoids, brachiopods shells or shellfish).

In standard EN 12670, the classification used is that of Folk, classification used mainly under the petrographic examination (microscopic description). This classification can also be made on the basis of a simple examination under the microscope.

The classification presented in Table 6 does not address aspects such as geological age, although it is an important element in the sedimentary rocks. Generally, the intensity or the effect of diagenesis (consolidation and / or cementing) of a sedimentary rock increases with its geological age. An increase of diagenesis product, overall, a more compact rock, harder and less porous.

A white stone of Mesozoic age, softer and more porous, can therefore be in the same class that Paleozoic blue stone, harder and nonporous (eg Euville stone and Petit Granit).

However, the color and especially the technical characteristics (bulk density, porosity, compression, etc.) will easily distinguish stones.

The stones used in Belgium are mainly siliceous sedimentary rocks or carbonate, detrital, derived from a chemical or mixed precipitation, but it can be also bioclastic rocks (with fossil fragments).

4-Metamorphic Rocks

As mentioned above, the metamorphism phenomena are many and complex, and varied rocks, where a typology and particularly complex nomenclature, too.

Many classifications have been established on the basis of essential minerals or mineral accessories for defining the temperatures and pressures prevailing at the time of metamorphism. Such information requires a detailed petrographic analysis.

A feature finely grained metamorphic rock (clayey nature of the source) as the phyllites and quartzophyllades is their ability to be cut into sheets (cleavage).

Again, it was decided to consider only the families of the most common rocks, related to two texture facies minerals opposite easily recognizable from a macroscopic point of view: the metamorphic rocks foliated and non-foliated.

A somewhat similar approach was adopted in the European standard EN 12670.

In Belgium, the rocks of this type are, for example, the stone Ottré, the ferruginous shale Lienne, quartzite and Cambrian-Ordovician quartzophyllades...........