Stylolites!!! - A diagnostic tool?



The chances are that the request "tiles or slabs with stylolites" from your supplier, will draw a blank stare at best, even if he has supplied the slabs to you for who-knows-how-long. A benign nod to your request still does not mean that he is spending much thought on stylolites. Why should we?

During the last century the use of stone slabs instead of solid masonry in buildings, facades, claddings has been significantly increasing, in addition to flooring and paving, presently the most used product accounting for nearly 40% of the global stone consumption.

In the selection of stone for the application we are helped by checking the geotechnical features, mainly mechanical and physical properties and awareness of these is the more important since flooring and paving, has been much in the limelight lately for several reasons, besides being a leading product.

Such attention included: firstly safety issues concerning slip proofing, resulting after many discussions, in a Slip Resistance Standard EN 14231; secondly the quest for the best way to consolidate with resins quarry blocks with open veins or cracks, and thirdly how to prevent opening of sutures, veins and avoid spalling in floor surfaces by choosing the right rock orientation. The latter depends on the inherent stone properties, the genesis and tectonic history of the rock mass, to which little attention is paid by the industry.

Little consideration is given to the internal structure of the stone. And still less has been written about the influence of stylolites, a major parameter, as a means to select a suitable stone, to foresee its effect on applications and their performance thereafter, not to speak of using stylolites as a diagnostic tool already during prospecting for stone deposits.

This disregard resulted in defaults, especially in limestones slabs and tiles with pre-existing discontinuities in the stone, like stylolites. The same applies to installation defaults, ie proper attachment and prevention of slabs breaking during processing, installation, or after placing. Actually a great deal, if not most, litigations and complaints in the stone sector concern floor coverings containing problematic stylolites cut with-the bed, in addition to slipping, tripping and stumbling accidents.

In a sense the industry can be hold partly responsible for this state of affairs, by the lack of geotechnical attention to stylolites during prospection and extraction at the quarries. In addition proper attention to possible defaults provides the quarry owner or stone supplier with a better understanding of the deposit and ensures the clients satisfaction.

What happens after the stone leaves the quarry, during and beyond processing, receives little dialogue, a situation partly due to limited outreach by the geo-technologists who in turn have their difficulties, not surprisingly in making geological gobbledegook palatable to industry. Mea culpa. The terms used for geological processes lean heavily on tectonical and petrographical terms which do not evoke the same connotations to the industry as the more commonly used mechanical and structural parameters of daily use in industry and construction. Demystifying the complex terminology used by geoscientists and harmonization with terms of the industry or interested outsiders, will be an important contribution. Informal explanations for some terms used are given in the boxes*


Whereas the use of stylolites to diagnose tectonics during for opening quarries may appear esoteric, the important role of bedding planes, one of the more obvious genetically features of a deposit or quarry face is easily explainable. A regular bedding plane facilitates extraction of blocks. The relation of stylolites to bedding planes is clear when considering cutting and sawing directions of stone. Stylolites act as orientation indicators and have a bearing on performance tests requirements, especially in stone strength testing, where testing in two directions of the stone to be used is mandatory. This requirement exists in several other stone standards tests; one test in the bedding direction and the other at right angles to the bedding plane.

These properties are known colloquially as "with the bed" or "on-bed", "against the bed", perpendicular-to-the-bed or edge bedded as the case may be. Environmental uses or conditions play an important part. For example, the same stone performing perfectly in cladding on-bed may have unsightly open seams when used in flooring where water action, by continuous dampness, ponding, residual water, or toxic cleaning may slowly dissolve the clayey or marly stylolite infillings. Surface spalling is another result from using stylolitic limestones cut on-bed. The interpretation of the stylolite geometry may eventually help to minimize these effects.

Stylolitic stone varieties

Stylolites are found in many rock types including sandstones, to a lesser extent igneous rocks, and other deposits metamorphosed to various degrees. However, it are the limestones and dolomites, used in building and construction that form the largest group influenced by the presence of stylolites, especially those stones with a high pure calcareous content.

Our attention is therefore turned to the stylolitic limestones that are used for decorative and ornamental uses, and often marketed as marble.(the term is used in a commercial sense).

Geotechnically we are not concerned with colour aesthetics, except perhaps where fading and possibly discolouration may occur. However grouping by shade is helpful as most users specify in the first place colour rather than other properties in selection. We will consider some international or locally popular varieties (mainly from those denominated in Denomination Criteria Standard EN 12440) containing stylolites. The terms in brackets give, where typical, the stylolitic pattern/colour effect of the containing stylolites in the stone varieties listed: geometrical -single ,net, rich,+ veins ; seam colour - faint, yellow, greenish, black.

Amongst the lighter shades are:

Cream, beige to greyish:
Perlato Sicilia, Italy
Botticino Classico (limestone) Italy (photo D)
Asagio Perlatino (single), Italy
Topazio (single), Portugal
Santo Florient (net), Portugal
Rose de Brignoles (rich), France
Comblanchien, France
Rhodos Beige (greenish), Greece

Chiampo, Italy
Serpeggiante, Italy
Ioannina Grey, Greece
Rocheret Gris (net), France
Reddish, brown to yellow:
Jaune Imperial, France
Siklos, Hungary
Florida Rose, Spain
Rasotica, Yugoslavia
Kastoria (+veins), Greece
Troizina (net), Greece
Orquidea Sierra, Cuba
Bayamo Orquidea (net), Cuba

Dark red:
Rosso Colemandina (black), Italy
Saalburg Rot (+ veins), Germany

Black to greyish:
Portoro (yellow), Italy
CH234 (faint), China
CH056 (yellow), China
Tuhar (violet), Czech Rep
Gris Motrico (black)

Diagnostic potential of stylolites

Behaviour of stone materials can be traced to conditions during the rock genesis and to the tectonic features of the rock mass. Discontinuities (sudden changes of rock features) originate during the rock genesis and to the successive tectonic events in the quarry area. The quarry investigations and laboratory measurements have then to be linked to the limestone applications in building frontages or paving.

The material characterization is made by mechanical tests, petrophysical measurements and observations of the rock petrography by optical microscope or electronic instrumentation. The mechanical tests, particularly direct tensile and bending tests confirm the foresights made on the slab properties (poor, even nil, mechanical resistance for some discontinuities, breakage during handling etc.).

Although the overall mechanical properties may be acceptable, the inherent properties footprinted during genesis or those caused by historical tectonic discontinuities may at times disappoint during or after the stone application.
Early investigation of the stone material for potential disorders prevents default. This requires some knowledge of the stylolites, a structural examination of the quarry beds to look for discontinuities, avoiding excessive tectonic stylolites to facilitate commercial block extraction. The same applies to figuring out cutting orientation to reduce the 'stratigraphic' stylolites frequency.

Without going in too much details of stylolite geometry, the interpretation of the various types may provide clues about the genesis or tectonic history to assist in quality evaluation. Eg presence of short calcite veins sometimes positioned like ladder like rungs or in echelon, in association with tectonic stylolites, gives information of the stress evolution of the slab material, and fracturing proneness during exposure. The accelerated aging tests, like the Freeze and Thaw test (EN 12371) confirm this possibility.

Inconclusive discussions on the origin and formation of stylolites have been, and are still ongoing for over a century now. The subject though is not quite so academically as it seems. Although an extensive literature is available on stylolites, little attempt has been made to apply the data to the dimension stone industry. A lesson may be learned from oil prospection, where close attention is paid to the origin of stylolites resulting in important economic implications and where stylolitic porosity in carbonates is considered a critical factor for deep hydrocarbon production, and tracing oil traps. Stylolite development may have resulted in the preservation of early oil accumulation in place.