This is from a friend:
"The bluestone we find in stores around here is a coarse sandstone with a porosity of 6 to 8 percent. A sandstone is a sedimentary rock containing any kind of mineral grain and rock fragment. Most commonly they are quartz rich because quartz is a tough mineral that resists weathering better other common minerals. In the case of the bluestone--which comes from upstate New York--there are lots of small rock fragments too, and we give it a special name--graywacke (from the German). The grains in the local bluestone are weakly cemented together, which accounts for the porosity and hence the water-absorbing capacity. Beware using the term bluestone in other parts of the US or other countries. The term refers to the color, not the rock type. Where I grew up in Australia bluestone was applied to a grey-blue colored limestone.
Slate is a metamorphic rock. Slates start life as mudstones, meaning they are clay-rich, and on heating and squeezing the clays react and form micas. The micas align so the flat sheets along which they readily break are perpendicular to the direction of maximum pressure. That is why slates break--we say cleave--along planar surfaces--they break parallel to the cleavage directions of the tiny mica flakes.
Metamorphism and the growth of new minerals greatly reduces porosity, which is why slates don't absorb water."
Best,
Leland
-----Original Message-----
From: The listserv where the buildings do the talking [mailto:[log in to unmask]] On Behalf Of Larry Simpson
Sent: Tuesday, November 11, 2008 4:05 PM
To: [log in to unmask]
Subject: Re: [BP] New (maybe) Thread
From a geologic point of view, slate is usually more compressed as well as finer grained. Surface tension is a factor of actual microscopic surface, so sandstone being more bumpy has more surface.
Larry2
---- [log in to unmask] wrote:
> John,
>
> When I split a piece of slate, and split a piece of bluestone (I have a
> place three miles from the Helderberg quarries - the stuff is all over) I can
> pour water on the slate and it will be absorbed at a much lesser rate than on
> the bluestone. When we are judging wether a slate roof requires replacement, the
> water repellency (surface tension) of the existing is a factor. Is it a
> metamorphic characteristic or binder/aggregate property that causes the
> difference?
>
> Tw
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