PHYSICAL PROPERTIES:
Chemistry: (K,Na)Ca4Si8O20(F,OH)
- 8H2O
Composition: Hydrated Potassium Calcium Sodium Silicate Fluoride
Hydroxide.
Class: Silicates
Subclass: Phyllosilicates
Group: Apophyllite
Crystal system: tetragonal
Color: clear, white, green, yellow, pink,
violet and rarely brown.
Fracture: uneven
Hardness: 4.5 - 5
Specific gravity: 2.3 - 2.4
Refractive Index:
Pleochroism:
Luster: vitreous to pearly
Streak: white
Cleavage: perfect in one direction (basal)
Transparency: transparent to translucent
Associated Minerals: calcite, cavansite,
gyrolite,
heulandite, idocrase, kinoite, natrolite, prehnite,
quartz, stilbite and many of the other zeolites.
COMPOSITION:
The Apophyllite group is layered with alternating silicate sheets
and the potassium,
sodium, calcium, fluorine and water layers. But unlike other phyllosilicates,
the Apophyllite Group silicate sheets are composed of inter connected
four and eight member rings. This is what gives these minerals their
typical tetragonal or pseudo-tetragonal symmetry. Only Apophyllite Group
members have this unique inter connected four and eight member ring
structure. It seems apophyllite is not a mineral, but rather a general
term for a group of
three official minerals that are similar in their chemistry and physical
properties. And a
fourth mineral Carletonite.
ENVIRONMENT:
Apophyllite specimens are found in ancient lava and basalt
flows and are usually associated with zeolites. The crystals
grow in cavities, called amygdules or vesicles, that were formed
by air bubbles when the rock was molten. Apophyllite is also found in
the voids in the contact metamorphic zone limestones that surround intrusive
rocks.
CRYSTAL DESCRIPTION:
Almost always as well formed individual crystals, varying from the common,
short prismatic habit that has four sided prisms (with square cross
sections) truncated with either a steep
four sided pyramid or a pinacoid termination or both. Simple, blunt,
square prisms develop
if the pyramids are missing. Rarely are the prisms missing, but if they
are, crystals could also appear octahedral because of the four sided
pyramids. The faces of the pyramids do not line
up with the prism faces but with their edges, therefore the pyramid
faces have four edges and appear diamond shaped instead of triangular
like the pyramid faces of quartz. Rare tabular hydroxyapophyllite crystals
are also known.
TESTS:
Fuses easily, bubbling and swelling to a white vesicular enamel. The
depths of the mass fluoresce weak greenish white after heating.
DISTINGUISHING CHARACTERISTICS:
Normally colorless or white. The different (pearly)
luster on the basal face distinguishes apophyllite from any cubic mineral
it might otherwise suggest. The typical square pyramid
and prism combination with the two lusters is easy to recognize. In
case of doubt, the basal cleavage makes the identity certain. Distinguished
from stilbite and heulandite by lesser swelling, quicker melting with
boiling, and the usual fluorescence after heating
LOCALITIES:
The traprocks of Paterson, New Jersey, and the Bay of Fundy. Produce
the typical, well
formed white crystals. By far Apophyllite's most impressive
color is the pastel green color found in theDeccan Traps (ancient
basalt flows) in India. These are most prominent near Poona. Mont Saint-Hilaire,
Canada, has the rare blue Carletonite. There are many other locations
here in the USA. Such as Arizona, Michigan, Oregon; Pennsylvania and
North Carolina. Other localities include Japan, Britain, Many Nordic
countries and Brazil.
USES:
Some crystals of apophyllite are cut as gemstones. But most crystals
are used by collectors
as mineral specimens.
FACTS & HISTORY:
Apophyllite, whose name roughly means "to leaf apart" in Greek, is
a mineral classic. It was given its name because crystals tend to peel
or flake apart when they are heated due to the loss of water molecules.
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