Sunburner basalt

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Sunburner basalt (also known as “sunburner”) is a weathering effect on gravel and paving made of ultrabasic basalts and other effluent rocks .

mechanism

The mechanism underlying the effect is controversial. It can be considered certain that all sunburner basalts are characterized by the presence of the zeolite mineral analcime in the rock matrix.

  • According to the simplest explanation, the analcime causes increased porosity through capillary cracks in its crystals, which facilitates the attack of weathering by the ingress of water.
  • Another explanation assumes that the analcime, when it crystallizes as the last primary excretion, exerts a "latent crystallization pressure" on the structure due to its volume 5.49% larger than the previously crystallized nepheline, which when the rock is exposed to the atmosphere still increasing.
  • It was pointed out early on, however, that the mere presence of analcime was not enough; rather, it must be present in a patch or network-like distribution within the rock. In order to explain the origin of such a distribution, the following model was used: During the fractional crystallization of a basaltic magma, a residual melt is created which, depending on the partial pressure of water vapor , can produce either analcime or a mixture of albite and nepheline during crystallization. If the stability field of analcime is reached, for example due to isobaric cooling, then its crystallization begins in a spatially static random distribution; this is said to be the reason for the patchy distribution. If analcime is excreted in the entire base mass as the cooling progresses, the rock should no longer have sunburn properties. Only in areas in which a certain ratio between the distance between the spots and the size of the spots (the so-called "stress number" of one or less) is achieved does the cooled rock show the undesirable instability. In this model, the analcime is a primary excretion of the basaltic magma.
  • Another approach starts with the assumption that the analcime is not a primary formation. Rather, a nephelinitic magma is supposed to form an analcimitic partial melt by assimilating SiO 2 -rich side rock at a shallow depth . This does not mix homogeneously with the magma, but rather distributes itself in the form of smaller droplets, which are recognizable in the later rock as analcim- rich globulites . Since this partial melt is also cooler than the starting magma, thermally induced stresses develop between these differently composed areas during the final cooling and crystallization, which promote the formation of microcracks.

Physico-technical criteria

The affected stones initially get stains and efflorescence and later cracks. At the same time, the density of the rock, its thermal conductivity and the speed of sound in the rock decrease. The corresponding gravel has a lower strength than expected and disintegrates under mechanical stress to gravel or stone earth. Stones made from sunburner materials are not suitable for more demanding technical purposes, such as gravel or paving material. They are often used for high-grade chippings , chippings or, to a limited extent, as concrete aggregate because they are easier to process. It has even been reported that concrete made with suitable grain sizes of broken basalt formed by sunburner decay is said to have a higher strength than concrete made of broken basalt regularly. The "critical grain size" of the fragments produced during disintegration is important here.

Various methods have been proposed for testing rough stones for sunburn property. The test according to the current DIN standard is based on the fact that the stains become visible when the rock is heated in demineralized water for a long time. Indications of the presence of a sunburner basalt include: a. the formation of granular, cellular fracture surfaces in the handpiece. The occurrence in basalt chimneys is regarded as a risk factor to be assessed in the field.

The sunburner property essentially determines the usability of basalt deposits if the rock is to be used for the production of paving stones or other larger building blocks (e.g. in hydraulic engineering). As these uses decline, the economic importance of the phenomenon also diminishes. However, it must be taken into account early on when developing and securing raw materials.

swell

  1. ↑ http://geolines.gli.cas.cz/fileadmin/volumes/volume15/G15-188.pdf
  2. ↑ a b A. Peschl: Natural stones . 1st edition. VEB German publishing house for basic industry, Leipzig 1977, p. 118 .
  3. ↑ D. Reinsch: Natural stone science . 1st edition. Enke, Stuttgart 1991, p. 126 .
  4. ↑ a b c Th. Ernst: Problems of the "sunburn" of basaltic rocks . In: Journal of the German Geological Society . tape 112 , 1960, pp. 178-182 .
  5. ↑ U. Schreiber: New research results on the characteristic patchy weathering phenomenon of basatic rocks (basaltic "sunburn") . In: New Yearbook for Geology and Paleontology, monthly books . No. 6 , 1990, pp. 353-366 .
  6. ↑ a b c Bernd Weiher: Characteristic value prognoses in the natural stone industry as an aid for exploration and quality assurance . In: Munich Geoscience papers, Series B . tape 16 . Verlag Friedrich Pfeil, Munich 2011, ISBN 978-3-89937-128-4 , p. 77-81 .
  7. ↑ [1]  ( Page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. EIA report as of 2006 for an Austrian quarry project, describes the separation of areas at risk from sunburn for grit@1@ 2Template: Toter Link / www.umwelt.steiermark.at  
  8. ↑ DIN EN 1367-3 describes the "sunburn test" or "boiling test for sunburn basalt"
  9. ↑ [2] Hesse raw material security concept, technical report natural stones and natural stone as of 2006