I'm delighted to give the floor to Geoff Winkler, who's building a bluestone house in southern Victoria. He has direct experience with the various properties of stone at first hand... This is a very different kind of knowledge/obsession: very practical and intimate knowledge. Click through to the photos of his house.
Thanks to Geoff, and best wishes for his building project.
Thanks to Geoff, and best wishes for his building project.
BLUESTONE AIN’T BLUESTONE
Most Melbournians’ perception of bluestone has been gleaned from seeing it in its most common use as paving and building stone. Rock for this purpose was usually quarried from one of the many deep deposits over the western and northern parts of the greater metropolitan area.
In the thicker beds, where the lava flows cooled very slowly and uniformly, the contained gasses rose to the surface. This led to the texture of the basalt becoming very even and fine grained. Generally, this is regarded as the best quality stuff for the building industry, however, the range of variations in bluestone is actually quite diverse.
I have always had a great fondness of bluestone myself, so much so that in 1994 I embarked on the biggest undertaking of my life, building my own stone house at Bellbrae, in southern Victoria. I greatly underestimated the logistics involved and subsequently also set about building a saw to assist with the project. I have sourced the raw stone from many nearby localities and from as far afield as Stoneyford, near Camperdown.
When I’m sawing the raw stone into usable blocks, many different textures and features are brought to prominence. Some of these examples I share with you here. The muddy water produced in the process is also quite interesting and varies quite considerably. The spectrum runs from a “clean” deep blue/grey colour from the Warrion Hill stone, through rusty reds from high iron content stone sourced from Lovely Banks, to “dirty” browns from Mount Duneed and Winchelsea. By “clean”, I mean that the sediments tend settle very quickly, whereas with the “dirty” the water stays cloudy and mucky for quite a considerable time.
A Common feature in the finer grained bluestone is “Veining”. This is where bands of gas bubbles, or “Vesicles”, have become entrapped as successive layers of lava have been overlain.
Also quite common is what’s commonly known as “cat’s paw”. It can occur in a similar fashion to veining, where rounded groups of vesicles are entrapped, or when fragments of already solidified lava are melded into the mass, as in this case.
When the lava cools more rapidly, generally in the thinner and more erratic flows, or nearer to the surfaces on the thicker deposits, the entrapped gasses are unable to escape and remain in situ. In this state, the basalt is termed “Vesicular”, or as it’s known to more common people like myself, “Honeycombed”. This is the most abundant form of readily accessible bluestone in Victoria and is found right across the Western district and at least as far north as Bridgewater (on Loddon). The composition and textures vary quite considerably though. Despite it being not generally highly regarded, historically in the building industry, its appeal grew on me and I now much prefer it for a rock faced surface finish. I feel it has a much more “natural” look about it. This sample, from near Beeac and typical of the flows from Warrion hill, has a very even texture and little in the way of “impurities” and “fracturing”. This stone was used in a number of local buildings there and its only downfall, in the one case that I’m aware of, resulted from inadequate footings.
From a little further west at Mt Pollock, near Gnarwarre, this piece has small amounts of other minerals crystallizing inside the vesicles and forming small nodules. These commonly include quartz, calcite (calcium carbonate) or other minerals called zeolites. It is a similar phenomenon to that which occurs in the formation of “Geodes” found in sedimentary rock, where the crystallisation tends to occur evenly all around the void. In all the examples from within basalt that I have seen however, it tends to form from the bottom. This makes it easy to determine the original orientation of any basalt containing it.
This “Vein”, contained in a piece from a small outcrop near Winchelsea, was exposed when splitting the rock along the same plane. Most of the vesicles were nearly filled entirely with Quartz. When this occurs and the rock assumes a more solid mass, it is referred to as being ‘Amygdaloidal’.
Mt Porndon near Stoneyford produced this interesting example, the nicest I have yet come across. Where a wide, but shallow, fully enclosed “Vug” (void), was formed within the flow, the crystallizing Quartz formed within it has the appearance of a coral garden.
Other rocks can also found in Basalt as inclusions, these are formally termed “Xenolith(s)”. This is where pre-existing fragments have been incorporated into the molten lava. This sample contains, what appears to be, a lump of quartz.
The correct term used to describe the small white flecks in this sample, from the earlier flows surrounding Mount Porndon, is that it contains “plagioclase phenocrysts”. It sounds a bit brain numbing, but the word “plagioclase” refers to a form of ‘feldspar’, which is part of a group of minerals that make up as much as 60% of the Earth's crust. “Phenocryst” refers to their conspicuous crystal size, being distinctly larger than the grains of the host rock.
Basalt found on the flanks of Mount Duneed, south of Geelong is about the crankiest stone I have worked with. It is a paler grey colour and the vesicles, generally flattened, show no consistent orientation. They swirl in all directions and vary considerably in size up to some quite large voids. Foreign inclusions (Xenoliths) are many, the stone is more brittle and stress fracturing is common.
I didn’t set out to be too technical with the terms used when writing this, but as the subject is investigated further, I am finding it increasingly difficult not to do so. An almost unlimited number of variations exist that I have not seen myself as yet. One that I would particularly like to witness is the relatively large Olivine inclusions, apparently quite common in the lava flows from Mt. Shadwell, in the Mortlake area. I intend to continue documenting my observations and will hopefully be able to provide further updates.