Chinese basalt G684 - User beware
Introduction
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.
Stone source
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.
Photograph 1. Spectacular columns of G684 ankaramitic basalt in the largest government quarry. The face of
the quarry is about 100m away. Note two additional lava flows over this one (top right).
Nomenclature
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%.
Petrographic analysis
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.
Photograph 2. Typical textural view in polarized light showing most of the features of this rock. The coloured
crystals at the bottom are olivine, those across the top are calcic pyroxene and the grey striped crystals are
twinned plagioclase feldspar. Importantly, the dark greyish minerals in the centre are feldspathoids and zeolites
which are reactive to acidic substances. Scale: side of photograph is 1.6mm
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).
Modal analysis
olivine
plagioclase feldspar
calcic pyroxene
opaque minerals
apatite
interstitial material
15%
30%
35%
2%
<1%
18%
Technical characteristics
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.
Photograph 3. Polished tile of G684 variably stained by food products after 24 hours. Jars were placed onto the
food products to break the surface tension. The brightest (whitest) rings are tomato sauce (top) and balsamic
vinegar (far right). Rings around the centre are red and white wines and champagne.
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.
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.
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).
Photograph 4. Newly laid, flamed G684 that has
been enhanced to darken it then cleaned with acid
to remove it. Note the colour contrasts depending
on the strength of the acid and dwell time.
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!
Conclusion
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
Hensel Geosciences