THE ROCKS | Microscopic Classification

The classification is based on microscopic petrographic description of the stone. This description is essential not only to define its microscopic ranking, but also to highlight the characteristics that may influence its chemical behavior, physical and mechanical. In this context, it is essential to know the mineral characteristics and the structure of the stone, and any information characterizing. To ensure the objectivity of microscopic classification (petrographic), characterization of the material should be as far as possible (semi) quantitative.

The petrographic description of a rock sample is to identify its macroscopic structure, then make one or more thin sections, which we consider to polarizing light microscope to perform a microscopic characterization.

The methodology used in this description and how to write the minutes are described in detail in the European standard EN 12407.

Besides the macroscopic description, the information provided is essentially:

the composition and the identification of the stone:

main components

binder

biogenic residues

microstructure and texture

accessory components, possibly analyzed with a scanning electron microscope (SEM) or reflection microscope

the porosity of the surface, condition (micro) cracking and the presence of gaps

homogeneity

identification of minerals that could affect sustainability.

The classification of the rock follows the rules of the NBN EN 12670.

All technical data presented in this NIT includes a microscopic description of the stone in question, thus enabling identification beyond dispute.

1 Magmatic Rocks

In magmatic rocks, the classification is based on the mineralogical composition, based on the relative share of the following mineral groups:

Q = Quartz

A = alkali feldspar (including albite)

P = plagioclase (feldspar, excluding albite)

F = foïdes (feldspathoids)

M = mafic and related minerals (mica, amphibole, pyroxene, ...).

The distribution of minerals is represented in diagrams Streckeisen below, taken from the NBN EN 12670. These diagrams are used to define precisely granite and basalt sl (wider) the commercial classification on the basis of mineral composition. More detailed diagrams to define the magmatic ultramafic rocks.

Main plutonic rocks (granites)

Granites

Granites (strictly) are plutonic rocks (volcanics equivalent: rhyolites) whose essential minerals are feldspar and quartz, and secondary mineral mica, pyroxene and amphibole.

They are rock hard, grainy, usually of light color (light gray, pink, reddish or bluish) and speckled appearance following the content of the constituents. This is the type of rock texture grainy, but the elements can have widely varying dimensions. In the porphyritic granite, feldspar are in a microgranular paste quartz and feldspar.

Granite and its varieties represent 5 to 10% of the rocks from the surface of the continents.

The alteration of granites largely depends on that of feldspars, allowing a disintegration of the rock and leads to the formation of granitic arena. Generally, granites are highly resistant to weathering. Because of their hardness, granites, like most igneous rocks, are difficult to carve stones, which makes shaping and polishing expensive.

Syenite

Syenite are plutonic rocks (volcanics equivalent: trachytes), often referred to without quartz granites name. They are composed mainly of potassium feldspar and amphibole minerals pyroxene and accessories are biotite.

These rocks appear the same as granite, but are usually darker, with gray shades, pink or greenish. Syenite generally highly resistant to weathering.

The term "syenite" is derived from Syene, the old Aswan, Egypt, which were extracted the stones of most ancient Egyptian monuments. The larvikite (or laurvikite), named after Larvik in Norway, of which great use was done at home (under the name Labrador), is actually a special structure syenite and composition.                                                                                                                                                                                                                      

Diorites

Diorites are plutonic rocks (equivalent volcanic rocks: andesite) composed for the essential minerals, plagioclase and amphibole, an accessory mineral, biotite and sometimes accidental mineral pyroxene or quartz. Muscovite and potassium feldspars are lacking.

Their texture is grainy, microgranular or porphyry. They have a speckled appearance and color are overall gray-green or pinkish gray. The transition between the diorites and granites is gradual through quartzitic diorites and granodiorites. Diorites normally highly resistant to weathering.

Gabbros

Gabbros are plutonic rocks (equivalent volcanic rocks: basalts) composed of basic plagioclase (labrador, anorthosite, bytownite) and pyroxene as essential minerals, hornblende, biotite and olivine as accessory minerals. They contain neither quartz nor alkali feldspar or muscovite.

The texture is generally grainy. They are compact rocks, overall dark green more or less mottled with white. The weathering of gabbros causes saussuritisation plagioclase (epidote training, albite, ...) and ouralitisation pyroxene (pale green amphibole formation) in turn driving training sphene (titanite).

Major volcanic rocks (basalts S.L.)

Basalts

The basalts are volcanic rocks containing minerals such as plagioclase and augite, and as accessory minerals olivine, magnetite and ilmenite, with or without a glassy residue. Phenocrysts of augite and olivine can be present. They are black rock base; altered, they are dark green, dark brown, reddish or black. Classification and nomenclature are varied and related to their mineralogical and chemical composition and their genesis: basalts with or without olivine basalts saturated or under-saturated.

Obsidian

Obsidian is an entirely volcanic glass, black, smooth shiny conchoidal fracture, usually of rhyolite composition.

pumice

Pumice is a glass and highly porous volcanic rock, to rhyolite composition (pechstein).

Other types of magmatic rocks

Diabase or dolerite: intermediate rock between the gabbros (grainy) and basalts (microliths).

Porphyry: microdiorite quartzite porphyritic dacitic composition and structure. The term "porphyry" means all magmatic rocks (especially volcanic) with large crystals (eg. Feldspar) on a fine-grained substance (Microlithic). In Belgium, the only rock of this type is the Quenast porphyry-Bierghes-Lessines, mainly operated at present in the form of crushed aggregates.

2 Sedimentary Rocks

The names of sedimentary rocks are entirely prepared on the basis of the NBN EN 12670. In general, there are classification Kraeft (1994) and the Folk.

For limestone and dolomite, we used the classification of Folk presented in the European standard. Dunham's classification does not appear in the above standard but is known internationally of geologists and is therefore easier to apply.

Folk classification considers the major components of carbonate rocks, namely:

the allochems; it is the chemical or biochemical origin elements formed in the sedimentation basin and having suffered some transportation as separate fragments. Among allochems, there are:

the intraclasts: slightly rounded or angular fragments from a bad neighbor consolidated sediment redeposited at close range by forming a new revamped said sediment

oolites and pisolites: small spheres of a diameter of 0.5 to 2 mm (oolite) or greater than 2 mm (pisolites) whose center is comprised of a debris wrapped in thin layers giving a concentric structure to which may overlap a radial structure

pellets: small ovoid masses of less than 200 microns (40 to 80 microns on average), often formed of microcrystalline mud rich in organic matter and considered to be of sub-millimeter faecal

fossils, and skeletal grains bioclastes

the orthochèmes; they consist of:

a matrix (micrite calcite or mud, in particles of 1 to 4 microns)

of cement (sparite, in more than 10 microns crystals, often from 20 to 50 .mu.m).

microsparite (usually resulting recrystallization micrite) crystals from May to October microns.

3 Metamorphic Rocks