Ophicalcite
Ophicalcites are metamorphic rocks with strongly fluctuating mineral composition and changing texture characteristics . Their main components are carbonate minerals and magnesium-containing silicates, preferably of the serpentine group . The technically used ophicalcites are serpentine- containing marbles and carbonate-containing serpentinite breccias.
term
The term ophicalcite is made up of the Greek root ophítēs (snake-like) and the Latin word calx (lime), specifically the mineral name calcite here . In the same sense, certain ribbon-like winding fine structures within a homogeneous mineral stock of rocks are characterized by the word ophiolitic (snake-like impression). The thematically closely related term "serpentine" or " serpentinite " comes from Latin and refers to the same visual appearance (snake: Latin serpens ).
Origin and mineral inventory
Because serpentinite minerals arise from complex transformations of ultramafitic parent rocks, an ophicalcite can also contain very different color and texture-forming mineral components.
Ophicalcites then form when Mg-silicate metasomatoses (transformation through mineral exchange) occur on carbonate rocks ( limestone , dolomite , magnesite ) through metamorphic processes of varying intensity. Numerous new minerals are formed in the process, e.g. B. olivine , serpentine , magnesium chlorite , tremolite , talc and representatives of the humite group . These minerals have a significant influence on the color, structure and other optical and mechanical properties of the resulting rock.
Another and different formation process of ophicalcites occurs when, through processes of rock mechanics, i.e. through tectonically caused mixtures, the already separately existing components (e.g. in large ophiolite complexes of the southern Alpine zone) come together. A known form is the mixing of serpentinite Brecciated with limestone by mechanical and hydrothermal (by water and temperature caused solvents) operations. Rocks of this origin can be found in Switzerland and northern Italy , for example, and are mined there as so-called green “marbles” (in the Aosta Valley ).
features
Ophicalcites are in the rock classification between various other rock groups. These are serpentinites, peridotites , talc, chlorite and amphibole rocks . The main mineralogical characteristic consists in being composed of approximately equal proportions of carbonate minerals (mainly calcite) and serpentinite and olivine minerals (magnesium silicates). The rock designation is also used in the case when a carbonate rock contains only minor amounts of serpentinite minerals.
The colored features are very differentiated, because the group of silicates are already diverse from their initial state. The serpentinization of peridotites produces chrysotile fibers (fiber serpentine). Further forms antigorite (Blätterserpentin). The serpentine types as well as peridot and chlorite are responsible for the green color. The decomposition of olivine parts leads to brown colors, because the iron ions of the olivine change into independent Fe minerals. Talc deposits form a white to light greenish color component. The calcite or other carbonates are mostly white stone components. In terms of rock-physical features, the cracks and fissures of those ophicalcites that have formed through tectonic processes are to be emphasized. The former mechanical stresses on a deposit are documented in the form of open cracks and hairline cracks. In the best case, they are subsequently healed with calcite and in this way re-solidify the deposit.
The deposits of talc and chrysotile fibers result in mechanically unstable zones at higher local concentrations (risk of breaking or flaking). These areas take little or no polish. They can be in the form of veins, ribbons, or clouds.
The complex mineral composition described leads to differentiated patterns and colors. In some quarries, these features vary within a few meters of each other. The varieties to be expected in the depth of such a deposit can hardly be foreseen, since exploratory drillings only give a very limited visual impression of the material to be expected. If a quarry is built on a steep slope, the question arises as to the justifiable expense of such drilling. In practice, the procedure is often that the rock extracted is sorted according to optical characteristics and sold under different trade names.
Occurrence, extraction and processing
In some European countries, Ophicalcites are offered, mostly under the common group name “green marble”. Note that not all green marble-like rocks are automatically ophicalcites. We find important deposits in Italy ( Aosta region ), in Ireland region Galway ( Connemara ), in Sweden on Lake Vättern ( Brännlyckan ), in Turkey near Afyon (Uşak), in Greece on Euboea ( Styra ), in Portugal in the Alentejo ( Alvito and Santiago do Escoural ), in Spain in the province of Huelva (Aroche) and in Brazil in Minas Gerais (Campos Altos).
Quarrying is usually done with a helicoid saw (a certain type of wire saw ). In the underground mining, a cutter (sword or chainsaw) is also used. The raw blocks obtained in this way usually have rectangular shapes and can now be sawn into individual raw panels. The surfaces are polished, sanded, sandblasted, bush hammered or brushed or antique.
Foreign language terms
English: ophicalcite, serpentine marble , French: ophicalcite , Spanish: oficalcite ; Portuguese: oficalcite , Russian: офикальцит, Czech: ofikalcit , Polish: ofikalcyt .
Use and durability
The ophicalcites are used similar to the marbles / limestones or serpentinite stones for decorative and structural purposes in the interior and exterior. They are rarely used for road construction. In landscaping, they are occasionally used for heavy and light stone walls . In the north Italian Aosta region you can admire numerous and remarkable facade designs in the form of perfectly crafted quarry stone masonry on chalets and single-family houses. It is a mixture of serpentinite and ophicalcite bricks. Occasionally, tombs and other monuments are made from ophicalcites.
The ribbon, stripe and network structure of some varieties are highly attractive. These types are used in exclusive interior design. These are floor and wall coverings, furniture parts (table tops, vanity tops, basins, kitchen worktops) as well as individual pieces for design purposes and other artistic purposes.
swell
- Raymond Perrier: Les roches ornementales. Ternay (Edition Pro Roc) 2004, ISBN 2-9508992-6-9
- Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks. Enke, Stuttgart 1985, ISBN 3-432-94671-6