Figure 2 SEM micrograph of pyrite inclusion in an unused marble tile. X marks spot of EDX analysis (see Fig. 3). Secondary electron image.

A routine petrographic examination usually involves an escalating series of magnification steps. A macroscopic examination is first undertaken to note features such as cracks, stylolites, veins, voids, fillings, bedding, cleavage, fossils, grain size, colour range, weathering and alteration, staining, inclusions, fabrication issues, etc. The macroscopic examination may include observations aided by a hand lens or a low-powered stereomicroscope. At least five polished thin sections (slices of stone carefully ground down to a thickness of 0.03 mm and then mounted on microscope slides) should then be prepared from representative samples. These are then viewed under the petrographic (polarising) microscope in both transmitted and reflected light modes in order to determine the detailed mineralogy and texture. More powerful microscopic techniques, such as the SEM (and EDX), can then be used to further differentiate potentially deleterious minerals (if required). An accurate and detailed petrographic assessment should only be undertaken by a qualified geologist/petrographer with experience in the dimension stone industry.

Provided sampling is adequately representative, petrography can be used in assessing the potential durability of newly worked stone, as well as in the forensic analysis of existing stone structures. It is important to note that a petrographic examination should always complement (and not replace) other standard physical test methods, such as those described by the author in a previous paper [1].

Figure 3 EDX spectra of pyrite mineral (FeS) shown in Fig. 2

The marketing of dimension stone has often led to the use of "descriptive" names that are of misleading non-technical nomenclature. For example, many of the stones typically referred to as granite are not granite at all. Geologically, granite is a visible granular, light coloured plutonic igneous rock consisting of quartz, alkali feldspar and mica minerals. In the trade, however, the term usually encompasses nearly all coarse-grained crystalline igneous and metamorphic rocks despite their origin, and includes such varieties as gabbro (black granite), diorite, syenite and gneiss. Similarly, the geological definition of a marble is a calcareous rock (usually a limestone) that has undergone recrystallisation due to heat and/or pressure (metamorphism). In the building trade many marbles are actually dense limestones, a sedimentary rock composed of calcium carbonate (often fossiliferous) capable of taking a polish. In Victoria, Australia, bluestone is a dark basaltic volcanic rock, while in South Australia and the USA the term is reserved for blue-coloured siltstones or fine-grained sandstones (sedimentary rocks). Stones that have been cleft (i.e. split) are sometimes all referred to as slates (e.g. some split-faced sandstones, limestones). However, a true slate is a fine-grained metamorphic rock, derived from sedimentary shales, and is composed mainly of planar micas, chlorite and quartz. True slates can be easily split into thin sheets because of the planar orientation of the minerals. To add to the confusion, some stones are named after their place of origin (e.g. Sydney Blue) or special visual characteristics of the stone (e.g. Desert Lilac) and bear no relationship to any geological names. Accurate petrographic definitions are now required in European Standard (CEN) natural stone test methods (BS EN 12407 and prEN 12670).

Figure 4 Macrograph of faded "black granite" showing porphyritic texture of black unaltered phenocrysts of pyroxene (augite) set in grey (faded) fine-grained groundmass. Width of field: 80 mm.

An accurate petrographic name and description should convey to the stone specifier information in terms of its mineralogy, grain size and texture. Where possible, the characterisation of the material should be in quantitative terms. An accurate petrographic description will assist the specifier in:

• selecting the most appropriate testing regime for the intended application;
• the interpretation of results from other standard test methods;
• the interpretation of service performance where there is no standard or reliable test data; and
• the evaluation of conformity (with other standard test methods).

Table 1 provides a summary of the type of information that can be provided by a detailed petrographic examination of dimension stone.

Figure 5 Petrographic micrograph of the faded "black granite" in thin section. The volcanic glass (dark areas) is interstitial to the mainly pyroxene, olivine and plagioclase feldspar minerals. The stone is classified as a porphyritic (volcanic) basanite. Transmitted-light microscopy, crossed polars. Width of field: 5.7 mm.

Figure 6 Petrographic micrograph of the unaltered "black granite" in thin section. The stone is mainly composed of alkali and plagioclase feldspar and pyroxene crystals. Note that volcanic glass is absent. The stone is classified as a (plutonic) monzonite. Transmitted-light microscopy, crossed polars. Width of field: 5.7 mm.

The durability and visual appearance of dimension stone is controlled by the constituent minerals present and by their textural and structural relationships. This paper has shown that petrographic techniques can be effectively used, in conjunction with other standard physical test methods, to assess the potential performance in service of natural dimension stone. Australian producers should be aware that accurate petrographic descriptions and names are now required in specifying dimension stone in Europe.

TABLE 1 - SUMMARY OF PETROGRAPHIC FEATURES AND OBJECTIVES

Petrographic features/objective	Examples/comments
Petrographic classifications	Based on mineralogical constituents, texture and origin of stone source. Petrographic definition based on numerous classification charts published in many textbooks. For some fine-grained volcanic rocks, classification based on XRF elemental analysis.
Primary mineralogy, grain size and texture	Includes volume estimates, dimensional size range, sorting, crystal habit and shape, distribution, orientation, mineral alteration, etc. (may require XRD or DTA analysis to determine clay type). Finer grained rocks are generally harder and stronger than coarser rocks. Granites with high proportions of mica may have fabrication problems, while those with high quartz contents will have increased microcrack porosities and decreased strength. Rocks showing preferred mineral orientations usually have anistropic properties in respect to strength, e.g. schists, gneisses.
Knoop indentation microhardness	A microscopic technique now commonly used to differentiate the hardness of minerals (and stone) on polished thin sections. Generally, the indentation hardness increases with decreasing grain size.
Nature of cement (binder) in sedimentary rocks	The composition, degree of cementation, amount, structure, and hardness of the cement sedimentary rockss(binder) in sedimentary rocks. Sandstones cemented with siliceous cements usually have high strengths, especially if the pores are filled with the cement. Sandstones with calcareous cements have lower strengths and are more susceptible to dissolution attack by acidic rainwater and to interactions with sulfate ions in industrial and urban atmospheres. Clay-coated grains may weaken the stone in the presence of water.
Weathering/mineral alteration	Extent of weathering and mineral alteration (decay) in: (a) existing structures, when compared to reference samples; and (b) proposed new stone.
Deleterious	Presence of potentially deleterious minerals may give rise to: Dimensional instability (swelling clays, sulfides, sulfates, some zeolites, volcanic glass). These minerals or phases may cause permanent expansion and physical disruption to the stone. Some clays are conducive to swelling/shrinking after periods of wetting and drying. The presence of clays infilling stylolitic (wavy crack-like) features in some dense limestones, may indicate planes of potential weakness and erosion. A tendency for some so-called "geen marbles" to warp on exposure to moisture, heat or UV radiation, has been attributed to the instability of the mineral serpentine. Discolouration (sulfides of iron, iron oxides). The oxidation of iron sulfide (pyrite) or iron carbonate (siderite) minerals may result in extensive brown staining. Delamination - the presence of planar micas along bedding or cleavage planes in sandstones may indicate potential zones of delamination.
Porosity, microcracking	Pores, voids, micro-cavities, microcracks - extent, size, shape, fillings and interconnection of void spaces. Porous and permeable rocks may be conducive to frost, salt attack, capillary water movement, or have the potential to carry harmful materials deep into the stone. The extent of microfracturing (microcrack density): (a) inherent; (b) post-processing; (c) post-installation; sand (d) in service.
Thermal variation	Minerals and textures that are potentially vulnerable to thermal variation may cause disruption to the stone.
Orign of stone	For conservation studies - to confirm or establish the origin of a particular stone for matching or replacement purposes.

Geoff Quick is a geologist and microscopist at CSIRO Manufacturing & Infrastructure Technology.

He is a member of the Standards Australia committees on natural and reconstituted stone tiles, and masonry units and segmented pavers. He is also a member of the ASTM C-18 Committee on Dimension Stone, and an office bearer of the Geological Society of Australia (Victorian Education Committee), the Australian Microbeam Analysis Society (AMAS), and the Victorian Electron Microscopy and Analysis Society (VEMAS).

REFERENCES

[1]	QUICK, G.W., 2002. Selective guide to the specification of dimension stone. Discovering Stone, Vol. 1, Issue 1
[2]	HUESTON, F.M., 1995. Stone Restoration & Maintenance Troubleshooting Guide. NTC Publishing Company, Florida, USA, p. 136.