The Feldspar
group of minerals (calcium, sodium, or potassium aluminum
silicates) is the most abundant building block in the Earth’s crust.
It is commonly found in granite, pegmatite, & other igneous rocks.
Clay (shale & slate) is formed from decomposed Feldspar.
Like the Garnet group, the Feldspar group, also, splits into two
distinct sub-groups:
The Sodium Potassium “Alkali” Feldspar – (Sodium/Potassium
Aluminum Silicates).
Orthoclase
Sanidine
Adularia
Microcline (Amazonite)
The Sodium Calcium “Plagioclase” Feldspar – (Sodium/Calcium
Aluminum Silicates).
Oligoclase
Sunstone
Peristerite - Labradorite
Bytownite
Andesine
Anorthite
Between the two sub-groups is Albite – (Sodium Aluminum
Silicate).
The varieties in each subgroup are caused by the percentage of
(Alkali Feldspar) Sodium & Potassium ions, or (Plagioclase Feldspar)
Sodium & Calcium ions, within the mineral’s molecules. Also, it is
common to have two different varieties of Feldspar intertwined in a
single crystal. This intertwining of varieties can produce the
visual effects we see in Feldspar gemstones.
All Feldspar Gems share the following characteristics:
Mohs’ hardness: 6~6 ½ Crystal system: Prismatic & Tabular
Cleavage: Good to poor Fracture: Conchoidal.
Transparency: Transparent to translucent, opaque. Luster: Glassy,
pearly
Listed below, are the major varieties of Gem Feldspars, along
with their specific descriptions and histories.
The Sodium Potassium “Alkali” Feldspars – (Sodium/Potassium Aluminum
Silicates).
Adularia –
Moonstone
Moonstone
got its’ name from the “moon like’, shimmering white to blue sheen
that flashes across its’ surface. This gemstone has been valued
since, at least, the 14th century BC when the Egyptian pharaohs wore
Moonstones as protective amulets.
Moonstones are found in a variety of pale shades from beige, green,
peach, brown, and white. Uncut, these gemstones can be rather dull,
but when cut by a master cutter, who, before cutting, aligns the
crystal axis exactly at the zenith of the stone, produces a
wonderful cabochon that shows off the Moonstones’ exciting sheen (adularescence).
Moonstones’ adularescence is caused by light being refracted and
scattered when it hits the microscopic lamellar structures (an
intertwining of two different forms of Feldspar) within the crystal.
“Cats’ Eye” and multi-rayed “star” Moonstones are found in India.
The wonderful “Rainbow” Moonstone, with its’ blue sheen comes from
Sri Lanka. Moonstones’ classic discovery point is in the Adula
massif in the Swiss Alps. Adula is the origin of “Adularia” and “adularescence”.
Other deposits are found in Brazil, Myanmar, Australia, and North
America. In the Los Angeles (California) area, I believe that the
beach suburb of Redondo Beach was originally called “Moonstone
Beach” because (before the beach was artificially covered with sand)
there was an abundance of moonstones along the pebble-strewn
seashore. This is also true for a few other California beaches.
Historically, the gemstone
Moonstone
has been associated with the Moon and love. Currently, Moonstone is
associated with the month of June (the wedding month). It is said
that prosperity and good fortune are promised to those born in this
month. Throughout the world, Moonstone is the birthstone for June,
but here in the USA we also add Pearls and Alexandrite as Junes’
birthstone. Many cultures claim Moonstone to have mystical and
magical powers. In India, it is a bringer of sweet and beautiful
dreams. To Arabs, it is a symbol of fertility. In the USA,
Moonstones are the official state stones for the states of Florida
and New York.
In general,
Moonstone
is cut into cabochons, but it is also cut into cameos, spheres, and
beads. A century ago Moonstones were very popular, in particular,
when the famous French Master Goldsmith, Rene Lalique and his
buddies used the gem, extensively, in their jewelry. Moonstones, in
particular the Rainbow Moonstone with its’ blue flash, are seeing a
renaissance of popularity.
I told you about its’ most famous member, Moonstone. I find it
interesting, that while investigating
Moonstone,
I have found many conflicting statements as to which form(s) of
Feldspar are included. At this point, I have determined the soft
pastel
Moonstones to be Adularia and Sanidine, and
the “Rainbow Moonstone” to be Oligoclase (“Plagioclase”
subgroup).
As a reminder, lets’ review these two Feldspar sub-groups.
The Sodium Potassium “Alkali” Feldspar – (Sodium/Potassium
Aluminum Silicates).
Orthoclase
Sanidine (Moonstone)
Adularia (Moonstone)
Microcline (Amazonite)
The Sodium Calcium “Plagioclase” Feldspar – (Sodium/Calcium Aluminum
Silicates).
Oligoclase (Rainbow Moonstone)
Sunstone
Peristerite – (Labradorite)
Bytownite
Andesine
Anorthite
Between the two sub-groups is Albite – (Sodium Aluminum
Silicate).
The next important gem in the Sodium Potassium “Alkali” Feldspar
sub-group is Microcline. Amazonite is a form of Microcline
Feldspar.
Although Microcline Feldspar is found in the following
colors; colorless, white, yellowish, pinkish, and green, only the
light to dark green variety is considered to be Amazonite. There
seem to be two varieties of Amazonite. In one, the color is a
mottled soft blue-green to pale blue. The other, “Russian
Amazonite” (also found in other parts of the world), is a deeper
green and can have green or white striations.
The stories behind the name Amazonite vary. Some references
claim the name is ancient and the gem was supposedly connected with
the ancient female warriors (Amazons) of Mediterranean mythologies.
Other references claim the gem was named for the Amazon River, a
place where Amazonite was found. I could not find a
definitive answer.
Historically, the deep green “Russian” Amazonite has been
used, throughout the Middle East and Asia Minor, as beads and
amulets and for décor, for thousands of years. I’ve seen beautiful
green Amazonite antiquities and beads from ancient Egypt and
Asia Minor that are over 5,000 years old. Currently, in Morocco and
North Africa, this same deep green gemstone (can be striated) is
called “Green Stone”, and it is cherished in the same way that
Turquoise is cherished by the American Indians.
The soft green-blue form of Amazonite is said to be a
“soothing stone”, and believe it or not, it is, also, claimed to
wake up lazy teenagers and give them energy. The deeper green
“Russian Amazonite” is said to be a “stone of courage”.
Amazonite is found in Colorado (USA), Russia, Brazil, East Africa,
Madagascar, etc.
Another gem Feldspar in this subgroup is Orthoclase. Although
Adularia (Moonstone) is a form of Orthoclase, when this
Champaign-colored Feldspar gem is clear (rare), it is faceted and
used in jewelry or held in rare gem collections.
Orthoclase is found in: Twentynine Palms, CA (USA) [“well formed
crystals & twin crystals up to 200mm across”]; Elba, Italy; Czech
Republic; Poland; Itrongay, Madagascar [“Yellow gemmy crystals up to
70mm across”]; Black Hills, South Dakota (USA) [“Gigantic crystals
up to tens of meters long”]; Germany; Switzerland; Sri Lanka.
The last gem in this subgroup is smokey-brown Sanidine. Like
Orthoclase, opaque gem Sanidine can be found as a
Moonstone, and when clear, this gem is faceted for jewelry and
rare gem collections.
Sanindine is found in: Germany [“Well formed crystals…up to
400mm in size”]; France; British Columbia, Canada; Bulgaria.
Let us explore the Gems in the Sodium/Calcium “Plagioclase” Feldspar
sub-group. The minerals in this sub-group are similar and primarily
differentiated by the percentage of Sodium vs. Calcium in their molecules. Most
of the
gemstones in this sub-group are known for their optical phenomena, and this
is produced by internal light interference caused by the intertwining of two, or
more, related feldspar minerals in their crystal lattice. Because of this, I
will mention each of the minerals in this sub-group, and go in depth in
describing the gemstones.
Below is a listing of the different minerals in the “Plagioclase”
sub-group. They are listed in the order of their Sodium/Calcium
percentages.
The Sodium Calcium “Plagioclase” Feldspar sub-group– (Sodium/Calcium
Aluminum Silicates).
Oligoclase (Rainbow Moonstone)
Andesine
Peristerite – (Labradorite)
Bytownite
Anorthite
Sunstone
Between the two sub-groups is Albite – (Sodium Aluminum Silicate).
The first mineral in this sub-group is Oligoclase. Oligoclase is
second, only, to Albite in its’ high percentage of sodium vs.
calcium (90-70% sodium to 10-30% calcium). As a gemstone it has
three different forms:
1) Gem Oligoclase - A clear, colorless to yellow crystal that is
usually cut into a faceted gemstone. Although these cut gems can be
used in jewelry, they are primarily kept in gemstone collections.
Gem Oligoclase is found in the Ural Mtns, Russia
2) Rainbow Moonstone – aka, Oligoclase Moonstone. This transparent
to translucent white gem with a blue shimmer floating across its’
surface is one of the most popular forms of Moonstone. The blue,
moonlike shimmer is created by the gems’ scale like internal
structure that causes light rays entering the stone to refract and
scatter into a blue shimmer
A century ago, during the Art Nouveau period, the French jewelry
artist, Rene Lalique and his contemporaries, would feature Rainbow
Moonstone in their wonderful creations. Today, these pieces of
classic jewelry are primarily found in museums and private
collections. The classic source is Sri Lanka, but it is also found
in the USA; Brazil; Australia; Myanmar; Madagascar
3) Sunstone – aka, Aventurine Feldspar. This opaque to translucent
to transparent, glitter filled, Oligoclase gem can have a reddish or
golden shimmer floating across its’ surface. Small, plate like,
inclusions of hematite or goethite produce the colorful “glitter”
that can define Sunstone. Light interference, caused by these
inclusions, produces the glitter colors – red and gold are common,
green and blue are more rare. Sunstone is usually cabochon cut, but
some of the most beautiful transparent stones are facet cut.
Sunstone is found in: Oregon, USA; Colorado, USA; India; Russia
4)
Peristerite – (Labradorite – Spectrolite). Labradorite has a
spectacular, metallic tinted, schiller effect known as
Labradorescence, floating across the surface of this colorless to
gray to brownish gemstone. The most common schiller colors are blues
and greens, but golds, reds, purples, and a full spectrum of colors
can be seen. Spectrolite is a most spectacular Labradorite from
Finland.
Labradorescence is produced by the interference of light. This
interference is caused by light bouncing off microscopic layers of
different forms of Plagioclase Feldspar within the gems’ crystal
lattice. Because labradorescence is produced by light bouncing off
of layers within the stone, this gem has a “grain” and must be
aligned before cutting, although, a cross-grain cut stone is, also,
quite beautiful.
Labradorite is found in Labrador, Canada; Ukraine; Madagascar;
Finland; USA; Mexico; Australia
Bytownite – This colorless to light pale yellow to champaign gem is
usually faceted. Cut Bytownite is both set in jewelry and held in
rare gemstone collections.
This gem is found around Bytown – (now Ottawa), Canada; Japan; USA,
(New Mexico – transparent red).
This completes the Feldspar family of gems and minerals.