[Rockhounds] Popular Mineralogy [Ad]

[Sicree, Andrew, Ph.D.]

Please pass this information along to mineral club editors
or other club officers you might know:

For club editors and club presidents:

Having trouble getting your newsletter out?  Need more
material to print?  Want something new to keep your
members interested?

Try the Popular Mineralogy newsletter supplement service.

The idea is that, for a 12-month subscription,
I provide each month 4-pages of camera-ready newsletter
articles that your club or mineral society can use to
supplement their newsletter.  The supplement is also
available electronically as a WORD file or a .rtf file.
Clubs can use what they want (one article or all articles)
in their newsletter or on their web page.

Below is an example of one article that has appeared
in Popular Mineralogy.

A 12-month subscription is only $69.99 or less than
$6 per month (that is only $1.50 per page of original
content!)

The articles are written by a professional
Ph.D. mineralogist (me) and are original and accurate.

Please contact me, sending me your postal address,
and I'll send you a sample Popular Mineralogy
and details about subscribing.

Thanks,

Andrew Sicree, Ph.D.
Professional Mineralogist
P. O. Box 10664
State College PA 16805
(814) 867-6263
sicree at verizon.net


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A Different Type of Tiger


     Shimmering tiger's-eye is a popular semi-precious gemstone.  Almost all
mineral collectors are familiar with the fibrous, golden-brown material.
Most systematic collectors typically have a piece or two with the label
"quartz, variety tiger's-eye" in their collections.  Museum displays label
it with the chemical formula of silica, SiO2.

     In antiquity, lapidaries prized this stone.  Although discovery of
abundant sources of tiger's-eye in South Africa in the 1870's sent prices
for good material plummeting, it is still highly popular with stone-cutters
and kids.

     Until recently, tiger's-eye was labeled as a pseudomorph after
asbestos.  After all, with its fibrous appearance, it does look like
asbestos.  In the standard explanation crocidolite (a variety of the mineral
riebeckite which occurs as asbestos, and is found as veins in rocks) was
replaced by silica on an atom-for-atom basis so that the original fibrous
habit was preserved.

     But Peter Heaney and Donald Fisher, geoscientists at Pennsylvania State
University in State College, PA, re-examined the tiger's-eye story.  Using
the optical microscope and the transmission electron microscope, they
discovered that tiger's-eye is made up of quartz encasing minute crocidolite
fibers.  In other words, the crocidolite is still present.

     Observations by Heaney and Fisher indicate that, as cracks in the host
rocks opened and widened due to tectonic stresses on the rocks, both quartz
and crocidolite were deposited in the veins.  Crocidolite and quartz
nucleated (i.e., the crystals started to grow) on opposite sides of the
fractures and grew toward each other, sealing the fractures and creating the
intergrown quartz/crocidolite mixture we call tiger's-eye.

     The brown color of tiger's-eye appears to be due to the later addition
of small crystals of iron oxide minerals (goethite, hematite, etc.?).  The
zig-zag growth patterns seen in tiger's-eye are due to the reopening of the
cracks as the host rocks shifted under tectonic stresses.  Shifts in the
host rock's orientation lead to a change in the growth direction of the
tiger's-eye mixture, thus the kinks or bends seen in most specimens.

- copyright 2007 Andrew A. Sicree