Monday, December 24, 2007
I
trust
that
most
people
noticed
the
spectacular
image
on
the
inside
cover
of
this
issue
of
Wat
On
Earth.
I
must
admit
that
when
I
received
this
photograph
together
with
several
others
about
a
year
ago
from
a
friend
in
the
United
States
my
initial
reaction
was
…
“Well,
someone
has
been
busy
with
Photoshop!”
However,
a
little
checking
on
the
web
revealed
that
these
are
genuine
photographs
and
they
tell
a
very
unusual
tale
of
an
interesting
locality
in
Mexico.
Now
I
(like
many
of
you)
have
found
individual
selenite
crystals
before.
In
fact
selenite
and
its
more
massive
and
shiny
siblings
(gypsum
and
satin
spar)
were
some
of
my
favourite
mineral
finds
as
a
child
growing
up
in
the
vicinity
of
sequences
of
Triassic
marls
in
South
Wales.
On
my
Saturday
afternoon
walks
back
from
the
National
Museum
of
Wales
in
Cardiff,
my
coastal
route
took
me
along
the
cliffs
at
Penarth
where
nodules
of
gypsum
were
falling
out
of
the
face,
and,
if
I
was
really
lucky,
I
could
find
veins
of
Satin
Spar.
Later
in
my
geological
career
I
found
even
larger
gypsum
crystals
developed
in
mudflows
derived
from
Jurassic
and
Tertiary
shales
in
the
Midlands
and
Eastern
England
(Figures
1
and
2)
and
finally
saw
some
really
large
crystals
exhibited
in
Tuscon
(Figure
3).
However,
all
of
them
pale
in
comparison
to
the
specimens
found
at
the
Naica
Mine
in
Chihuahua,
Mexico.
Figure 1: A Selenite (CaSO4 .2H2O) crystal.
The
Naica
mine
is
situated
at
27°
50'
N,
105°
29'
W;
approximately
470
km
south
of
El
Paso,
Texas.
Located
in
carbonate
rocks
the
mine
has
been
exploited
for
galena,
sphalerite
and
silver
within
skarns
in
the
Cretaceous
limestones
of
the
region.
High
temperature
(55ºC)
calcium
carbonate
and
calcium
sulphate-charged
groundwater
have
been
brought
into
the
limestone
along
two
large
fault
systems
known
as
the
Gibraltar
and
Naica
Faults.
The
hydrothermal
waters,
likely
emanating
from,
or
passing
near,
regional
magma
bodies
are
responsible
for
the
deposition
of
the
calcite
and
gypsum
minerals
—
selenite
and
anhydrite
—
as
well
as
the
economic
ore
bodies.
Figure 2: Selenite crystals from central England.
The
Naica
Mine
was
first
exploited
in
1794
by
prospectors
who
discovered
silver.
By
the
late
1800s
it
was
being
examined
as
a
source
of
lead
and
zinc
ores
and
the
mine
was
formally
opened
in
1900.
The
Mine
was
closed
in
1922,
reopened
in
1935
and
was
taken
over
by
the
Penoles
Group,
the
current
owners,
in
1961.
The
Naica
Mine’s
large
gypsum
and
calcite
crystals
(The
Cave
of
the
Swords)
have
been
known
since
1910
(Foshag
1927)
and
another
similar
occurrence
at
the
Potosi
Mine
in
Santa
Eulalia,
near
Chihuahua,
about
100
km
north
of
Naica,
was
discovered
in
1912
.
In
1910
selenite
crystals
between
one
and
two
metres
long
were
recorded
from
the
Maravilla
property
(now
within
the
Naica
Mine)
and
the
adjacent
Lepanto
property
at
depths
of
about
120m.
It
was
assumed
by
Foshag
(1927)
that
the
cavities
that
housed
the
crystals
were
created
by
surface
water
dissolution
and
also
owe
their
origin
to
the
oxidation
of
sulphide
ores.
He
was
not
able
to
resolve
how
the
crystals
that
are
now
found
in
air-filled
cavities
were
formed.
Figure 3: A group of Selenite “swords” about 1 m long in the Tucson Desert Museum
More
recent
and
far
deeper
mining
exploitation
set
the
stage
for
the
latest
discoveries
within
the
mine
at
the
300m
(1,000
feet)
level.
In
1999
the
Penoles
Group
calculated
that
regional
water
levels
had
been
sufficiently
lowered
to
allow
penetration
of
the
Naica
Fault.
On
December
4th
1999
Juan
and
Pedro
Sanchez
were
drilling
a
new
tunnel
that
broke
through
into
a
steam-filled
cavity
that
houses
the
giant
selenite
blades
in
the
Cave
of
the
Crystals
(illustrated
on
the
inside
front
cover).
The
crystals
were
truly
spectacular
with
grey
white
and
translucent
forms
more
that
10m
long
and
up
to
2m
in
diameter.
These
are
amongst
the
largest
crystals
ever
recorded.
Thanks
to
the
forethought
of
the
mining
engineer,
Roberto
Gonzales,
action
was
swiftly
taken
to
minimize
any
damage
to
this
natural
wonder.
Figure 4: A person exploring the Naica Mine with giant crystals in foreground.
The
“Cave
of
the
Crystals”
is
now
protected
by
an
iron
gate
to
prevent
vandalism
and
looting
of
the
crystals
although
the
ambient
conditions
inside
the
cavern
(60°C
and
100%
humidity)
almost
provide
a
sufficient
natural
deterrent!
The
cave
has
been
visited
by
a
number
of
photographers
and
cave
exploration
groups
and
attempts
are
reportedly
being
made
by
the
Penoles
Group
to
open
the
cave
to
the
public
through
installation
of
electrical
lighting
and
air
conditioning.
Perhaps
more
on
this
later.
Reference:
Foshag,
W.F.,
1927.
The
Selenite
Caves
of
Naica,
Mexico.
Amer.
Min,
V.
12,
pp.
252
–
256.
Alan
V.
Morgan.