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Accessory Rock Forming Minerals - Metamorphic Minerals

Previous classes are available at:


And as these classes are completed I'll be adding them and the illustrations to that site

The main metamorphic minerals (not already covered in earlier classes) are all silicates. Several of them form related groups with similar chemistries and/or structures.

The first of these groups are the Garnets:

Garnets all have the same crystal form - 20 sided "balls" and they are the only common minerals to have that shape. All are fairly hard minerals (6.5 - 7.5) and dense (3.5 -4.3) and all garnets fracture without any cleavage leaving many rough surfaces. The main metamorphic Garnets are:

Almandine: Fe3Al2(SiO4)3 - Almandine is a red to violet or brown color

Grossular: Ca3Al2(SiO4)3 - Grossular is clear when pure but more normally cinnamon with traces of iron (Hessonite) or green with traces of chrome (Tsavorite)

Andradite: Ca3Fe2(SiO4)3 - Andradite is normally brown to black but may be yellow (Topazolite) or green (Demantoid)

Spessartine: Mn3Al2(SiO4)3 - is the manganese rich garnet variety. It is a yellow to orange or brownish color. It is rare and associated manganiferous igneous or metamorphic deposits.

Clear colored stones are used in jewelry (they are all the January Birthstone) while poor materials are used as abrasives. Garnets form as the materials in the rocks undergo moderate heat and pressure and recrystalize redistributing the elements somewhat because of this the presence of garnets helps to identify the pressure and temperatures that the rocks have been subjected to. At lower temperatures and pressures only micas form and at higher T-P

other minerals form in addition to the garnets. In most cases the garnets found in a rock are not pure forms but rather mixtures of them. Garnets can be found in all mountain ranges but some localities are better than others for collectors. The mountains of NC, VA, NY, MA and CONN are among the best places. Garnet mountain NY is so rich that they mine the mountain for the garnet to make sandpaper. The clear purple Almandines of the Franklin, NC area are often called Rhodolite garnet and are fairly common along with the sapphires and rubies of the area.

any questions on the garnets?

{Z} no

{EW} you really covered it good dad.

We are going to move on to the alumino-silicates - these 3 minerals all have the same chemistry ( Al2SiO5) but different crystal forms. They form under different conditions of temperature and pressure and transform from one to the other as these conditions change during metamorphism. The 3 minerals are:

Andalusite - the low temperature-pressure form. Andalusite (H: 7.5, D: 3.2) forms square section columns, in many cases it forms stubby prisms that show a cross pattern called Chiastolite. Andalusite is used in refractories and to make acid resistant ceramics. Good xstals are found in Andulusia Spain (where the name comes from), Fresno California, Pennsylvania, Mass, and Brazil.

Kyanite - the middle to high pressure form. Kyanite forms long blue blades or tabular crystals these are hardness 6-7 across the blades but only 4-5 along them. The mineral is fairly dense (3.6) and it has a perfect cleavage. It is often a greyish white but the bladed form, density and hardness's make it easy to identify. While it is occasionally faceted most commercial deposits are like those in VA where they are mining a mountain rich in Kyanite to bake it and grind it up to use in making insulators and spark plugs. Other deposits are found in Switzerland (gem qual.) Italy, Austria, Brazil, Mass, Conn, and NC.

Sillimanite - the high temperature form Sillimanite (H 6-7, D: 3.2) is the last of the alumino- silicates. It forms needles that have a square x-section. It may occur as single needles or as fibrous masses known as fibrolite. It also often occurs as inclusions in other minerals. Gem quality crystals show a chatoyance in shades of blue and green. Its commercial use is as a component of high temperature refractories, and crucibles. It is found in Czechoslovakia, Italy, Austria, Germany, Brazil, SC, Burma, Sri Lanka and India; as well as in most high grade metamorphic regions of the Appalachians and Rockies.

So - squares, blades and needles in ascending temperature-Pressure regimes :)

any questions about these 3 minerals?

{EW} what did you mean dad by crucibles?

crucibles - containers for heating materials in a furnace

{EW} oh ok

the stuff inside melts but the container doesn't

* EW nods

{A} it always amazes me that such delicate crystals can form under such high pressures.

think of the pressures as forcing all the bad stuff out so the crystals can form.

Staurolite (Fe, Mg, Zn)2Al9Si4O23(OH) is hard (7-7.5) and dense (3.7) forming hexagonal columns or twins that form crosses at either right angles or 60 - 120 degrees. These are often called fairy crosses. Staurolite ( is a medium temperature - pressure mineral normally found with garnet and kyanite. The crosses are often used in jewelry but other than that it has no commercial uses. Staurolite is found as crosses in the soil in Virginia's Fairycross State Park. Other locations include Georgia, New Mexico, Germany, Italy, and France. It is often found in sand deposits downstream from the source rocks since it is so hard

Epidote (Ca2 (Al ,Fe)3Si3O12 (OH) (H: 6-7, D: 3.3 - 3.4) forms oval columns with 12 sides.It normally is found yellow to greyish masses its fairly fragile but can form beautiful xstals and be used for gemstones. It is found in contact metamorphic iron rich rocks - usually in metamorphosed basalts and such where it forms at lower temperatures and low pressures. Epidote is found California, Mass, Conn, Norway, Austria, Italy and many other areas where it is associated with greenschists.

Any questions on Staurolite or Epidote?

{Z} any commercial uses for epidote

Not really it can be used as a gem but it's fragile

{EW} is it just like for rock collectors?

Really yes its a common accessory in greenschist rocks and in low temperature metamorphosed basalts and such. epidote is found frequently as tiny grains but good crystals are considerably rarer.

{Z} wish we had some of these crystals in W. PA

{A} feel the same about Ohio :)

Well there may be some in central Penn and down south of you {A}. Garnets form the red streaks and grains in most sands and the alumino-silicates can be found in Pennsylvania, Kentucky, and Tennessee as well as Virginia and the Carolinas.

{A} yes, on the KY - Tn line there are some zinc/iron and a few other mines that I know of

Then there should be some Kyanite and garnets there and some staurolite.

{A} the problem is knowing where to look and more important getting the ok to look :)

{A} I did find some lil geodes in my brothers yard down there tho and started my niece's interest in rocks :)

Hmmm - check with the state geological survey - they may have a release with good mineral colleting sites

{A} oohh good idea

{A} never thought of that

Or at least a Geologic Map with descriptions of the rock types you might also check the USGS website and see if you can track down a map or report from them

{Z} if your looking for NC there is a fellow with it on disk 1000+ collecting sites, directions and current tele numbers.

sounds good I have the NC state bulletin with the sites also well that should have helped a bit *G*

{EW} right :)

{A} will give em a look DH ... you never know what you will find on the net.

Also checking with local rock shops and mineral clubs will get you connected with sites.

next week: the Economic Mineral Groups

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Website created: January 22, 1998
Website last updated: October 11, 1998