Introduction to Crystallography and Mineral Crystal Systems
by Mike and Darcy Howard
Part 9: Conclusion and Further Reading
Well, here we are. We've finished the 6 crystal systems
and we're feeling kind of smug with what we have learned. But listen here
readers, although we had some chalkboard discussions about the crystal
systems and their symmetry elements and classes, now we must leave the
ideal world of symmetrical crystal forms and enter the real world. The
world where the growth mechanisms and the environment of formation of a
crystal have as much to do with the expression of the crystal forms we
see as the molecular arrangement of atoms composing the mineral we are
Almost all minerals are crystalline (part of the definition of a mineral),
but only under special conditions do you form minerals with well expressed
crystal form(s). The mineral may grow from a melt as in igneous rocks,
from solids under elevated temperature and pressure, or in open crevices
or pockets from hot, warm, or cold mineral-laden fluids.
So, when you examine euhedral crystals of any mineral, please realize
that they are rather remarkable objects. Probably 99 % of all minerals
present in the earth's crust display no external crystalline form. A euhedral
crystal is a flower of the mineral world! Treat crystals with care and
respect because they probably survived millions of years before someone
discovered them. One careless moment and they may be destroyed forever.
But enough of our soapbox lecture on the need to understand the scarcity
of well-formed crystals! Throughout the series of articles, we have attempted
to show you the world of symmetry present in the crystalline world around
you. But symmetry is present everywhere. Symmetry is present in varying
degrees in the biological world. From simple bilateral symmetry (mirror
symmetry like left-right) to 5-fold symmetry not present in the crystalline
world. Members of the echinoderm family are beautiful examples of this
type of symmetry. Next time you look at a starfish, a fossil blastoid,
or a sea urchin spend some time examining its symmetry. You will recognize
elements of several principles you learned from studying minerals.
Symmetry or the intentional lack of symmetry governs many artistic works.
One of the most remarkable persons to use masterfully a variety of symmetry
elements was M. C. Escher. His works not only display many symmetry elements,
but he includes much of his personal theology hidden in his designs. An
excellent recent book, available by ordering from your local bookstore
is M. C. Escher: His Life and Complete Graphic Work, by M. C. Escher, L.
J. Locher (ed.), and F. Bool, published by Harry N. Abrams, November, 1992,
ISBN: 0810981130. You could also order this book from Amazon.com on the
internet. There is a CD-ROM of M. C. Escher's Works also available for
less than $50 through Amazon.com.
are many articles written in journals concerned with various aspects of
symmetry including an article in Scientific American on the symmetry
of the patterns of dimples on golf balls!
of the most interesting books we ever came across is Snow Crystals by W.
A. Bentley and W. J. Humphries.
This remarkable book contains 2,453 photographs of snow crystals (the
W. A. Bentley's hobby was the photography of snow crystals. After his
death, his collection of several thousand snow crystal and frost crystal
photographs were donated to the American Meteorological Society. The AMS
was charged with designating someone to put the collection of photographs
into a sensible order and overseeing their publication. W. J. Humphries
oversaw the work which involved several noted mineralogists of the day,
including S. B. Hendricks, H. E. Merwin, C. S. Ross, and W. T. Schaller.
These men classified the photographs into basic types of formation and
growth forms and worked up the descriptive portion of the book's text on
the crystallography of the Snow Crystal.
The presentation of so many illustrations of a single mineral (ice)
all from the same crystal system makes the book unique and gives the viewer
the opportunity to examine how the various methods of growth of a mineral
may affect a crystal's appearance, even though the symmetry elements remain
constant. You have probably heard the comment that no two snow flakes are
identical. You will believe it after examining the photographs in this
book! Every instructor teaching mineralogy should make this book available
to their class just to enlighten the students as to nature's infinite variety
on the same theme!
We do not know if it is still in print, but it was first published by
McGraw-Hill in 1931 and republished in 1962 by Dover Publications, New
York. The standard book number is 486-20287-9 and the Library of Congress
Card Catalog Number is 63-422 for the Dover edition.
I could rave on for a few additional pages, but it would do little good.
By now, you are either hooked on symmetry or sick of my discussion of it.
But never again will you look in a mirror and see just yourself. Instead
you will see an organism with bilateral symmetry. Symmetry is a way of
looking at the world around you and seeing order in the apparent randomness
of nature. It is truly a way to define order out of apparent chaos.
and chaos are popular topics these days, in mathematics and in philosophical
discussions concerning the general nature of life, but I assure you that
if you look around and are thinking of symmetry you will find it. It is
everywhere in the plant and animal world as well as the mineral kingdom.
Just stop, bend down, and look at common clover, a plant whose individual
leaves exhibit mirror plane symmetry, but whose 3-leaf arrangement displays
trigonal symmetry elements. Think about the symmetry of a 4-leafed clover.
Now you are not interested in finding one for good luck, but so you can
more thoroughly examine its symmetry!
Speaking of good luck, we wish all of you the best of it in your continuing
quest for knowledge. We hope we have brought you to a better understanding
and appreciation of the symmetry and beauty of the world around you! For
the effort involved in reading all this series, we salute you and you are
hereby deserving of the Mineralogical Order of the Crystal Sphere,
that one object of infinite symmetry. May the sphere ever remind you of
the infinite patience it takes to learn about Crystallography and Geometry.
Mike & Darcy Howard
Index to Crystallography and Mineral Crystal Systems
Table of Contents