The international stone trade is diverse, complex and engulfed by misinformation and product inaccuracies. The basis for this commercial circus stems from the fact that stone is a natural product – one that can vary on any number of scales. And the knowledge necessary to more-closely define and categorize the huge number of individual products is empowered to only a few scientists and stone specialists. As with many products there is a human tendency to keep things simple and in the absence of correct classifications, broad generalizations and often incorrect terminology have crept into, and become established, in this industry. Some of the terminology is as simple as being based on colour (such as black granite), but other terminology comes from a visual perspective (i.e. texture), or the ease with which the product can be worked (e.g. marble).
Compounding the problem of inaccurate classification of stone has been the rapid global expansion of this industry. China has been at the forefront of this expansion followed by countries such as India, Brazil and Turkey. All of these countries do not have a stone culture and stone is virtually treated as a singular product. Education of this natural product in these countries is seriously lagging and being over-ridden by financial pressures.
Reason for providing this information
Historically there have been limitations as to what could be done with certain types of stone and because of the weight factor there were serious questions asked with respect to transport. Yet it is amazing at the achievements that occurred in several cultures. Modern transport, particularly shipping, has transformed the stone industry and the world of construction, and products can now be easily sourced from all corners of the globe. But limitations on the use of natural stone still remain even though there are now many avenues available for the dissemination of critical information and for the testing of stone.
Categorization of stone remains a major obstacle to this industry and issues of stone performance and nomenclature are frequently before courts. One of the minor categories of stone is basalt. Although a much smaller category than granite, marble, sandstone and limestone it nevertheless has a share of problems in the performance of many of the varieties. Usually the problems are due to the ignorance of the architect and/or specifier in not having sufficient knowledge about the product. G684 is a special type of basalt that has mineralogical characteristics that need to be recognized and matched to specific applications.
G684, is produced in Northeastern Taimu Mountain, Bailin Town, Fuding City, Fujian Province, eastern China. The basalt forms a high ridge that supports at least 3 quarries, one of which is truly spectacular (see photograph 1) by virtue of a cathedral of vertical columns up to 2m in diameter and rising over 60m in height. The quarries reportedly have a potential annual production capacity of 10 million m2 (doubtful) supplying up to 500 stone processing factories.
The town of Bailin, at the base of the range, runs the original three basalt quarries jointly under a unified mining exploitation license. It contains about 50 processing factories dedicated to this individual rock type.
G684 is the official Chinese code given to this rock type. In error, it has been given the descriptor G which designates granite. Marketing has generated a variety of names for this stone including Fuding Black, Fujian Black, Raven Black, China Black, Black Granite and even Absolute Black. Its correct scientific name is olivine-rich ankaramitic basalt.
Physical description of rock type
G684 is a tough, dense, quite strong, blackish, porphyritic rock of basaltic composition. It has abundant, rather large and prominent crystals of well-formed, black, calcic pyroxene phenocrysts to 15mm, giving a spotted appearance. Around the black pyroxene crystals is a “groundmass” of smaller, dark-greyish plagioclase feldspar and noticeably greenish crystals of olivine. Textural heterogeneities are not uncommon with patches that are poor in phenocrysts and others that have an abundance of phenocrysts and only a minor amount of groundmass.
Veinlets and patches of a white zeolite and minor calcite are not uncommon together with weathered cooling surfaces that are coated with greenish chlorite. These structural features are planes of weakness that might not be evident until pressure is exerted at a later date. Frequently the tiles or slabs will break along these planes of weakness at the exfoliation stage. Some projects have encountered failure rates exceeding 20%.
This porphyritic, medium- to coarse-grained basaltic to doleritic rock consists mostly of elongate prismatic crystals of plagioclase feldspar, rather large and prominent, black, calcic pyroxene crystals, clear and unaltered, commonly prismatic crystals of green olivine, and a healthy scattering of opaque iron-titanium oxide. Nestling between the large crystals are numerous small pockets of very fine-grained, darkish brown and messy material containing an abundance of needle-like crystals as well as small concentrations of minerals with a very low refractive index and birefringence. One of the minerals appears to be analcime and another is probably nepheline. There is also the possibility that some zeolite is admixed. Additionally, there is a reasonable abundance of acicular apatite.
Texturally this rock can also be described as intergranular to interlocking, in addition to porphyritic. The interlocking of the feldspar crystals with the calcic pyroxene provides much of the strength to the rock.
The grainsize for the majority of feldspar crystals is between 1.5mm and 3mm with only odd larger crystals. Individual crystals of calcic pyroxene in this thin-section are commonly between 0.5mm and 2mm with some rare larger ones to 10mm. Although in hand specimen the large crystals of calcic pyroxene appear to be single entities they rarely occur discretely and commonly form in “clots” with up to 30 smaller crystals that appear to be connected but have slightly different orientations. The olivine crystals have a much more restricted size range with a commonly occurring size between 0.4mm and 0.6mm. Some larger crystals approach 2mm and there are numerous smaller ones throughout and as inclusions within the calcic pyroxene.
Plagioclase feldspar is quite abundant occurring commonly as well-formed prismatic crystals (laths). It is moderately calcic and generally shows well-developed multiple twinning. Locally abundant transverse fractures that occur in many crystals appear to be systematic and together with irregular compositional zoning are additional features. Many of the crystals are slightly affected by alteration along grain boundaries and show a dirty type of brownish-grey dusting. Often the adjacent interstitial material appears to be penetrating the feldspar.
The calcic pyroxene varies from pale pinkish-brown cores to distinctly pinkish rims. This indicates a compositional zoning with increasing titanium content towards the rims. Compositionally, these pyroxenes are augite becoming titanaugite near the rims. Because most of the phenocrysts are glomeroporphs an abundance of original magmatic material has been trapped as inclusions between the individual crystals. It is unaffected by alteration.
There is an abundance of clear, commonly euhedral crystals of olivine. Much of it occurs discretely but it also has a tendency to form groups of crystals (glomeroporphs). Because it has been the first silicate mineral to crystallize it also occurs as inclusions within the calcic pyroxene. Small pockets of original melt are occasionally preserved within some larger olivine crystals but interestingly there are no inclusions of opaque oxides (e.g. chromite or chrome spinel).
There are at least two varieties of opaque minerals in this rock. One is an oxide of irregular shape and likely to be magnetite whereas the other is skeletal and elongate and indicative of ilmenite. Much of the opaque oxide has crystallized between the principal crystals of calcic pyroxene, plagioclase feldspar and olivine, and very little of the main oxides occurs within the interstitial patches.
As noted above, small interstitial patches of darkish material occur between most of the major crystals. These patches (also called mesostasis) are residual – they have crystallized late in the formation of the rock and therefore have concentrated certain elements and volatiles. The result is a crystallization and concentration of minerals that are less-stable physically and chemically than the principal minerals and in equilibrium with a silica undersaturated environment. Unfortunately, most of these late-formed minerals in this rock type are reactive with low-pH fluids, including the apatite which is a phosphate. Some of the minerals gelatinize with the application of hydrochloric acid (or its commercial equivalent muriatic acid) and some produce a whitish enamel-like product (see photograph 3).
plagioclase feldspar 30%
calcic pyroxene 35%
opaque minerals 2%
interstitial material 18%
Because of its widespread usage this basalt has been tested numerous times in Australia and in China for the most useful geotechnical properties.
A simple hardness/scratch test indicates that it is fairly resistant to scratching with common materials but on a small scale there is clearly some variation in hardness with patches of soft material between the larger crystals. These are pockets containing late-stage or secondary minerals, such as zeolites, and these cause the petrographically determined hardness to fall below 6 on the Moh’s scale of hardness.
Compressive strengths, flexural strength and values for modulus of rupture are typically quite high (around 250, 22 and 20MPa, respectively), reflecting the close-knit, interlocking texture. The specific gravity is high (approaching 3.00) – reflecting the density and abundance of ferromagnesian minerals. Also reflecting the dense nature of this rock type is a low imbibition coefficient (i.e. measure of water absorption). This value which basically measures the connected and open pore space is typically around 0.10% by weight. Individually, these strength values are towards the upper end of most building stone.
However, as with all testing, there is considerable variability in the results stemming from uncontrolled sampling procedures, uncontrolled sample integrity, and various levels of technical expertise at different laboratories. Testing stone for the sake of testing, at laboratories that have no expertise in stone, is a very common problem. Laboratory technicians only need to have expertise in the equipment that they use for testing – not in the preparation of stone samples.
Being a product derived from the upper mantle basalts are very low in natural radiation and this stone is no exception.
Reactivity to acidic products
An acid test shows that the stone contains no reactive carbonate. However, a short-term application of 10% HCl had a significant effect on the colour (but not surface morphology) of the stone tile in that a prominent whitish mark was formed. This indicated the presence of minerals such as nepheline. analcime and/or zeolite in the rock. Such minerals are not uncommon in chemically undersaturated basaltic rocks.
This high degree of sensitivity to acidic products (and the resultant change to a light grey appearance) is a major negative aspect to the usage of this stone. Chosen as a dark (almost black) stone the architect or client has an expectation for the stone to remain dark in colour. However, the application of anything acidic (such as wine, tomato sauce, vinegar, fruit juices) will cause a dramatic and irreversible whitening, compromising the original aesthetic intent. So for use, whether it be internal or external, this change must be recognized (photograph 4). Of lesser, but even more widespread impact, is the use of this stone in applications that are exposed in urban environments. Most larger cities have an element of pollution which causes the (light) rainfall to have a pH less than 5, and not uncommonly approaching 4. Exposure to this mildly acidic rain over longer periods will cause a whitening of this basalt and a loss of contrast with any adjacent stone or structure.
Enter the after-market products. Recognizing that there are both real and perceived flaws with a variable natural product that few people know anything about has spawned a peripheral industry that is probably more profitable than stone itself. In a short period of time it has generated an army of salesmen willing to sell an amazing array of similar products to the client “to protect the stone” – without knowing anything about the stone.
When sent to a laboratory for testing to evaluate the suitability of a stone for certain applications, it is usual for only the strength parameters and the amount of absorption to be tested. That is because the client is unaware of the characteristics of the stone and does not know exactly what to request and the laboratory does not have the expertise, or permission, to carry out tests outside of their brief.
A recent debate on an American forum highlighted the ignorance that exists with after-market products such as sealers and enhancers, and their use on certain types of stone. Fabricators, tilers, and sealer salesmen were arguing about the virtues of different impregnating sealers applied to G684 and how they bond to quartz. The sad truth is that there is no quartz in this stone, the stone is too dense to accept a sealer or enhancer, and if applied properly, should be removed from the surface because it would be in excess. It therefore would provide little or no protection to the stone. Yet the client has paid, or is expected to pay, a considerable sum to protect the stone. A good example is a recent installation in Australia where exfoliated G684 was laid on a walkway into a prestigious high-rise. It was darkened with an enhancer but because of the high density and low absorption coefficient it was probably not uniform. A decision to clean and strip the enhancer resulted in the following artwork (Photograph 4).
Comparison with other basalts
Basaltic rocks are the most abundant rock type on Earth occurring as voluminous outpourings on all continents and covering most of the ocean floor. Basaltic magma is generated in the upper mantle at varying depths and varying quantity leading to compositional variation. The magma might also interact with other rock types which results in additional compositional and mineralogical variation. Physical conditions at the point of eruption driven by volatile content largely determine the morphology of the lava outpouring and of the rock itself. Given that most basalt lavas extrude at temperatures between 1000oC and 1100oC small lava volumes will crystallize quickly. Natural cooling forces result in the formation of closely spaced joints. It requires the formation of a lava pile to allow some of the lava to cool more slowly, whether from successive flows or intrusive forms such as dykes and sills.
In the stone trade the most conspicuous and discriminating features are the size, abundance, and distribution of vesicles (voids/holes) in the basalt, as well as the degree of freshness. Vesicularity is a measure of volatile content and activity and ranges from essentially zero to over 50% (scoria). Because holes have no strength their abundance (and distribution) impact on the strength and behaviour of the rock in construction. Also impacting on the quality of the basalt is the degree of freshness. Due to their mineralogy basalts are subject to alteration and rapid weathering. As the lava flows are cooling, trapped volatiles (CO2, H2O) will readily modify some of the mineralogy of the basalt at even modest temperatures (200o – 300oC) and produce secondary minerals. Olivine is particularly susceptible to alteration and will produce expansive clays (smectite). Excess CO2 will produce carbonate (usually calcite but may also be other varieties).
Another volatile, though less common, is SO2 that may combine with the abundant free iron in basalt and form sulphides that are susceptible to rusting. Once the secondary mineralogy has formed, groundwater activity can further modify the mineralogy. Basalts containing alteration are less stable dimensionally and have a reduced durability therefore must be treated differently and with care. In vesicular varieties some of the alteration forms veinlets and may fill both small and large vesicles. Veinlets also compromise rock strength.
Generally, three varieties of basalt for building stone are distinguished, namely no holes, micro-holes, and macro-holes. There are also hybrids. Basalts without holes are not as common as those with holes so G684 is unusual in this respect.
The Australian basalts tend to be a combination of micro-hole with patches of macro-hole. They generally have a high degree of vesicularity (about 10-30%) which must be allowed for in construction. A high number of small vesicles means that it is porous and therefore has the behaviour resembling a sponge. It is well-known through years of practice that increasing the thickness of the basalt panel or paver provides the necessary additional rigidity, yet time and time again the construction industry specifies thin elongate panels in the ignorant belief that “stone is stone”, i.e. that basalt behaves like any other stone, such as black granite. And typically the stone is to blame!
The Chinese basalt G 684 is a strong, dense stone of negligible porosity and natural radiation. Globally it is one of the most produced and widely exported stones. Because of this wide distribution, good physical properties and cheap price it has been placed into numerous applications by architects, builders, tilers and stone companies. But lack of knowledge of this stone in terms of reactivity and colour change, and no guidelines from the suppliers, has resulted in this stone inevitably being placed into inappropriate applications. This has seen many disappointments because of the change from a dark stone to an unevenly light stone. Laboratories do not assist in providing this additional important information.
Dr. Hans-Dieter Hensel