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Pohorje tonalite

The quarry of the Pohorje granodiorite - tonalite - is situated in the town of Cezlak on Pohorje.

The tonalite from Pohorje is appreciated for its special mechanical and physical characteristics, such as high compressive and bending strength as well as resistance to freezing and wear. Geologically, the stone is technically a granodiorite, which means it is of high quality. Due to all above-mentioned characteristics the Pohorje tonalite is highly suitable for outside use. A special trait among the "granites" is its distinctive white vein that makes it unique.

The Pohorje tonalite has become a part of the identity of Slovenian towns, as it has inspired numerous architects and sculptors, and that is why it is an important part of the Slovenian cultural heritage. 

Quarry location Slovenia / Cezlak, Pohorje
Use Zunanja, notranja
Compressive strength 241 MPa
Compressive strength after freezing 202 MPa
Bending strength 24 MPa
Coefficient of thermal expansion 0,0084 mm/m°c
Water uptake 0,3%
Density 2687 kg/m3 
Modulus of elasticity 406,50 kN/mm2 
Slip resistance (polished) 13,6 units
Slip resistance (cut) 78,8 units 

The most common and well-known of Slovenia's igneous stone is the Pohorje tonalite, which composes the central part of Pohorje. In the past it had been known as Pohorje granite, but today it is generally known as tonalite. In petrographic terms the name, tonalite, derives from Passo Tonale, a pass in the Adamello Massif in northern Italy, a historic location where the stone was found. Detailed analyses showed that the composition of the Pohorje tonalite in somewhat different as that of the classical variety tonalite. The most suitable name for this Pohorje plutonic stone is granodiorite.

By using the K-Ar method, geochronological research showed that the rock was formed around 16 miliion years ago, in miocene. The Pohorje granodiorite is grey, with a slight bluish hue. The homogeneity of the stone is interrupted by white aplitic and pegmatitic veins of various widths and distributions. The most prominent mineral constituents are white Na-Ca feldspar (plagioclase) crystals; from 50 to 60%. Next is the greyish flint; from 20 to 30%. There are also some biotite grains (from 10 to 15%), K-feldspar (orthoclase; from 5 to 10%) as well as hornblende. Alongside the main minerals there are also various trace minerals - apatite, zircon, pyrite, titanite and epidote. The size of the mineral grains is up it several millimetres, but on average it is less. The stone structure is medium-grained and hypidiomorphic. The texture is directional, mainly due to parallel lines of biotite and partially flint. Directionality of the texture impacts production, especially the production of cut products.

Tonalite differs from granite due to the latter consisting of a higher proportion of alkaline feldspar and less plagioclase, which amounts to more K-feldspar (ortoclase), Na-feldspar (albite) as well as less Ca-feldspar (anortite).

Aplitic and pegmatitic veins play the biggest role in the outward appearance of the stone. They are composed mostly of different varieties of feldspar and flint. If these minerals are fine-grained, the veins are aplitic, and if they are large-grained, the veins are pegmatitic. In addition to feldspar and flint, other minerals, such as mica, pyrite, garnet, among others, can also be present occasionally. The contact between the veins and the stone is always clear. Generally it is lithified, which means it does not weaken the stone.