Garnet, Spinel and Zircon

GARNET DEPOSITS Garnet comprises a complex group of minerals

having the general formula R3

M2

(SiO4

), where R=Ca, Mg, Fe2 , or Mn and M= Al, Fe3+ , or Cr . One group of garnets, ca

lled Pyralspites

, are named according to the dominant R cation present: Mg is Pyrope ; Fe2+ is

Almandine , Mn is Spessartine . The domi nant M cations in these garnets is Al, wi

th some Fe3+ usually present. A second group of calcic garnets (R=Ca), called

Ugrandites

, are named according to the dominant M cation present: Cr is Uvarovite ; Al is G

rossular ; and Fe3+ is Andradite. Natural garnets are rarely pure (e.g. natural al

mandines usually contain variable amo unts of Ca, Mg and Fe3+ ) and as a result

these names apply strictly only to ideal ized ("end member") garnet compositio

ns. Nevertheless, mineralogists apply t hese names according to which one mo

st closely matches the composition of t he garnet

One of the more common rock-forming minerals in medium to high temperature, aluminous metamorphic rocks. Formed by contact or regional metamorphism of shales and limestones. Contact metamorphism of limestones is main source of gem quality, ugrandite series garnets. Those garnets formed by the metamorphism of shales, though common, are usually highly included and are not of gem grade. Most almandine and rhodolite is of this origin, however. Garnets rich in pyrope are found in kimberlite (the source rock for diamond) and some high temperature and pressure metavolcanic rocks. Gem spessartine is known only from pegmatites. Currently the most prolific production of gem garnet is from East Africa. Major discoveries in the 1970's and 1980's of rhodolite, tsavorite, and malaia garnet have resulted in the resurgent popularity of this gem mineral.

Almandine

Dark red to brownish or purplish red; the most common of all garnets. Most on the market come from mica schist and alluvial deposits in India (Jaipur area). Other sources are Sri Lanka, Brazil, Idaho (some 4-rayed star stones), New York, N. Carolina and Alaska.

Almandine, Idaho

"Pyrope”

Dark red (blackish-red) Mg-rich garnet; most contain a component of almandine. "Bohemian" garnets, popular in late 1800's are "pyrope". Pyrope are found in conglomerates, volcanic breccia, tuffs and alluvial deposits. Color-change pyrope from Norway and Tanzania show a change from violet in sunlight to red in tungsten light.

Main localities for pyrope are Czechoslovakia (Bohemia; since 1500 AD); South Africa; Otteroy, Norway; Tanzania, and Arizona. Also localities in Australia (New South Wales, Anakie), Myanmar, Argentina, and Brazil.

Pyrope Garnet Victorian HairpiecePhoto by Chip Clark (http://nmnhwww.si.edu/minsci/images/gallery/28.htm)

Rhodolite

Original material from Macon Co., N. Carolina has a distinctive purplish red ("rhododendron red") color with almandine to pyrope ratio of 2:1. Best are a lovely violet or purplish-pink, reminiscent of fine pink sapphire or rubellite. Some Tanzanian rhodolite shows a color change from bluish green in daylight to the more typical purple red in incandescent light. East Africa now main source (Tanzania, Kenya); also Madagascar (color change blue to burgundy is of this type?), Sri Lanka (noted for higher priced, pure violet red stones), Zimbabwe, and India.

Rhodolite: These have been mined in Tanzania since the 1900s.Their colour ranges from pink to purplish red and they are extremely popular.http://www.gemstones.com/amulet_bin/menu/gems/chapter15.html?Tg3zKReKIeg

Spessartine

Lovely orange, yellow, or flame red color, usually small. Color can resemble some hessonite garnets, with which they are sometimes confused. Rare, main sources are (were) at Amelia, Virginia, San Diego Co., California (Little Three Mine), and Brazil. "Mandarin" garnet is spessartine from Namibia that has gained popularity in the past decade. The exceptional orange color commands considerably higher prices than other spessartine. Others include Idaho, Norway, Pakistan, Madagascar, Myanmar, Sri Lanka and New South Wales, Australia.

The name "Little Three Mine" refers to the three men who discovered the site in the early 1900's. Most of the gems found on the site were discovered by Louis Spaulding, Jr. in the mid-1970's.

3http://www.roadtripamerica.com/places/little .htm

SPESSARTINE GARNET with Schorl and AlbiteMn3Al2(SiO4)3

Little Three mine, Ramona, San Diego County, California

The bright orang e spessartine ge

mstones from th e Hercules Dike at the Little Thre

e mine are regar ded as among th

e finest in the w orld. This is the best specimen k

nown from themine.

Size: 6 cm across

Name of recent vintage used for pinkish orange, reddish orange, yellowish orange, or golden pyrope-spessartine-almandine-mixed garnet from the Umba River valley bordering Tanzania and Kenya.

Best are said to have a pinkish or pure-orange color, but a browner orange resembling hessonite or spessartine is more common.

Malaia" (also "Malaya") Garnet

GrossularShades of yellow, pink, green and pale brown. Important varieties include Hessonite (cinnamon-stone), a brownish yellow, orange yellow, or brownish orange color, and Tsavorite (or Tsavolite), a bright, vivid lime green color. Tsavorite typically contains rounded inclusions of apatite and calcite. Tsavorite found only near the Kenya/Tanzania border, near Tsavo National Park, in a graphite schist. A similar lime-green grossular is found in the Umba Valley in Tanzania. Inclusions in stones from this locality are graphite and limonite-stained cracks. Gem green grossular garnet is also known from Pakistan. Most Hessonite from Sri Lanka; also from Brazil, Vermont, Italy, Mexico, Quebec.

Tsavorites have a bright yellow green to grass green colour and they cut gems in sizes less than 2 carats. However, the lime green tsavorites have cut stones exceeding 40 carats.

http://www.gemstones.com/amulet_bin/menu/gems/chapter15.html?Tg3zKReKIeg

Tsavorite Deposits (Kenya)

Taita Hills, near Tsavo National Park,

Tsavorite pocket (potatoes)Tsavorite Mine, Kenya

This photo represents t he total week's produc

tion of facetable Tsavo rite crystals

http://www.gemstones.com/adventure/?2bR7HWHsvR2

AndraditeOnly important variety is Demantoid, a very rare, brilliant yellow-green andradite. Has very high dispersion (0.057; higher than diamond) and high R.I.( 1.89); accounts for brilliance, also name (demant is Dutch for diamond). Hardness only 6.5, as compared to 7-7.5 for red garnets. Contains characteristic "horse tail" inclusions of a fibrous amphibole (byssolite) that are diagnostic for Russian demantoid. Finest demantoid from the Ural Mts., USSR, where it occurs in serpentinite. No present production. Gems of 4 carats or larger are extremely rare. Other sources are Italy (very small stones), Switzerland (Zermatt; little to no gem material), Zaire and Mexico (Piedra Parada; little to no gem material).

http://www.flamingo.ru/dem/dem.htm

DEMANTOID DEPOSITS OF RUSSIA

DemantoidsF F F F F F F F F F F F F F F F

http://www.gemresearch.ch/

certs/demantod/demant5.htm

://.stone.org/dema.

Typical chrysotile fibre inclusions in demantoid from the Ural Mountains.

Four Corners Area, ARIZONA, USA.

Pyrope Garnet Locality

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The Four Corners area of n ortheastern Arizona derive

s its name from the fact tha t there the state borders of

Arizona, New Mexico, Utah, and Colorado meet. Gem q

uality garnets have weathe red from the underlying int

rusive rock and can be foun d scattered throughout the

area. This location is on the Navajo reservation and is s

-trictly off limits to non trib al collectors.

Most of the garnets are smaller than a pea, are smooth, and rounded. Often they are ref

erred to as ant hill garnets because the tini est garnets are thrown out on the surface of the ground as ants excavate their nests. Th

e garnets are collected by the Navajo and so ld in bulk for the gem trade. Ninety percent of the material is the deep ruby red color of

pyrope garnet , but small quantities of rhodolite and spessartine garnets are also found . Most of the garnet is tumble polished, drill

ed and used as beads. Although the hue of t his pyrope is gorgeous, the garnet from this

locality is best suited for beads. That's beca use the saturation is so great as to make cut

stones over a carat or two in size too dark . Nevertheless, if properly cut, Four Corner

FFFFFF garnets are stunning gems.