ENSTATITE FROM YARROW CREEK
NEAR GLEN INNES NSW
In my first years of gem hunting in Yarrow Creek I
occasionally came up with pieces of a mineral which looked like black spinel
but differed in that they showed a green to orange-brown colour when held up to
the sun. The best specimens were transparent and gemmy.
As I had done before with specimens of pink
sapphire, I sent a few samples to Hylda Bracewell, the highly regarded
gemmologist then living in Torrington NSW. Hylda concluded that they were
enstatite with some examples approaching hypersthene in their properties.
These minerals are members of the pyroxene family,
which are commonly found in basic to ultrabasic igneous rocks and some
metamorphic rocks. Their chemical formula can be summarised as (AB)₂Si₂0₆,
where A and B are metals. In the family of which enstatite and hypersthene are
part, the metals are magnesium (Mg) and iron (Fe). Pure enstatite contains no
iron, only magnesium, hypersthene contains both iron and magnesium.
An important question to be answered is this: where does this mineral come from? Not from the local granites, which
appear to contain no pyroxene minerals. Not from the same volcanic source as corundum, zircon and spinel, since it is not found with them in most local creeks. Perhaps from the gabbro dyke which
outcrops just upstream from Garnet Corner, some other dyke yet to be
identified, or from xenoliths in the granite.
These minerals are not especially hard or dense,
but hard and dense enough to end up in the sieve centre along with the sapphire,
zircon, garnet and spinel which make up the bulk of the concentrate. Enstatite
has a hardness of 5.5 on Mohs’ Scale and a specific gravity of 3.2 – 3.3. They
can be cut as gems but need to be protected from undue wear. The colours are
not particularly attractive and difficult to see in a set stone.
Despite its shortcoming as a gemstone, a piece of rich
green enstatite is a good find and many amateur cutters would like to work on a
Yarrow Creek stone if they could find one.
Why not check out my You Tube channel here where you can view gem hunting, mining and general geology videos.
https://johnsbluemountainsblog.blogspot.com/2013/12/links-to-all-blog-entries-and-relevant.html All Blue Mountains blogs and videos
All New England and other Geology blogs and videos
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| Facetted enstatite 1.5 carats |
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