Valongo slate is known for the specific characteristics of the region where it is found (Valongo – Douro Coast) and the geologic age of its quarries (Upper Landeilian).

Slates are defined as rocks that cleave into small sheets along a cleavage plane resulting from a foliation flow caused by very low- or low-grade metamorphism due to tectonic compression. They originate from argillaceous sedimentary rocks and petrographically belong to a class that begins at the border between sedimentary and metamorphic formations and ends in epizonal-phyllite metamorphic formations.

The most significant predominant components are micaceous minerals and occasionally also chlorotic minerals. These appear mixed with fine-grained quartz and clay minerals, depending on the degree of metamorphism in each case.


Physical and Mechanical Characteristics

1. Mechanical resistance to compression : kg/cm2 656
2. Mechanical resistance to compression after freezing test: kg/cm2 494
3. Mechanical resistance to bending: kg/cm2 582
4. Bulk density : kg/m3 2 809
5. PAtN Absorption: % 0,45
6. Intercommunicating porosity : %1.25
7. Thermal linear expansion coefficient: x10-6 per ºC 8.0
8. Wear resistance: mm 8.3
9. Impact test: minimum fall height cm: > 110 a)

a)Tested maximum height.

Notes: The results relate to average quality slates. Acid resistance (standard DIN 52206) is satisfactory, except in lower quality slates. At the end of the 25 freeze-defrost cycles the samples showed no significant changes in colour or structure.

Chemical Analysis

SiO2: 53.77%
Al2O3: 24.79
Fe2O3 (total): 8.55
CaO: 1.28
MgO: 2.33
Na2O: 1.05
K2O: 3.45
TiO2: 1.12
P.R.: 5.54
SO3 *: 0.84

* Traces in some of the samples analysed.

Macroscopic Description: Grey-blue to dark grey shale, homogeneous, compact, finely granular, showing clear schistose cleavage.