Geology of Gems banner

Lab Exercise 5 - Gems From Metamorphic Rocks

Purpose: Identifying the settings where gems occur in metamorphic rocks.

 
Identification Table for Metamorphic Rocks

In the table below identify the numbered metamorphic samples. Give the characteristic that helped you name each stone and list criteria that will allow you to recognize future examples. If certain tests such as hardness or reaction to acid are useful, note it.
  Texture Mineralogy Rock Name
foliated
(linear
texture)

Very finely crystalline; may have slight sheen; more compact than shale Fine clay minerals are just beginning to enlarge above their size in shale Slate
Visible crystals of mica make the rock shiny; garnet knots may interrupt the layers of mica Mostly muscovite or biotite which may flake of easily; some garnet, feldspar, and quartz Schist
Dark and light (white or pink) banding; bands may be discontinuous or straight through Mostly quartz, feldspar, and dark minerals; may contain garnet Gneiss
Dark and light (white or pink) banding; bands may be discontinuous or straight through Extremely similar to regular gneiss. Less distinct banding Granite Gneiss
       
massive
texture -
lacks
foliation
Usually fine-grained or sandy texture; Very hard; will scratch glass Almost pure quartz. Pink or white colors are common but may be stained with other colors Quartzite
Large sugary crystal randomly oriented. May be hard or crumbly Typically calcite or dolomite. Should react with acid. May have other minerals. Pure pieces will not scratch glass Marble
Massive texture of fiberous crystals that wrap together. May show splintery fracture Contains greenish or dark serpentine or jade minerals; may also have feldspar or other non-fibrous minerals mixed in Serpentine/Jade
     

Sample Number
Sample Characteristics
Rock Name















1) Ruby Crystal in Schist (Be careful, the sample is not that strong!)

Karnataka (including Mysore, India). Another important ruby source is in India. The classic Karnataka Indian locales produce mainly opaque, low-grade star rubies; recently better material has been reported from Orissa, but as of 1994, India mainly supplies lower grate materials as in
your example. Probably from a schist or gneiss sample. Your sample appears to be in one of these two matrixes, but the small specimen makes the rock matrix hard to identify.

A) Was this rock be produced by contact or regional metamorphism?

B) Is the sample opaque or translucent? What is its shape and size? Look up ruby crystals for shapes (hint look at number of sides).

C) Is there any structure on the crystal surface? Is there a shape on the surface? A texture?

D) This is not a star ruby. What is a star ruby? Illustrate the shape.

E) Compared to the crystal shape, how should the crystal be cut to create a star? Try to illustrate.

2) Ruby in Zoisite from Tanzania, Africa

A metamorphic rock known as anyolite consists of green zoisite with black amphibole and ruby crystals. The rubies as a whole are not of gem quality, but nevertheless their color provides a striking contrast to the green zoisite, and greatly enhances the decorative pieces that are carved from the rock. As well, small sections of some larger crystals are big enough to facet (cut into) melee (a term often used for diamond size).

Both the ruby and the green zoisite are colored by chromium. The chromian zoisite-ruby are the result of very high-grade metamorphism of anorthosite (a mafic rock), an intrusive igneous rock dominated by Ca-rich plagioclase feldspar. The chromium (Cr) in the zoisite and the corundum (ruby = red corundum colored by chromium) is thought to be derived from metamorphic alteration of chromite crystals (FeCr2O4 - iron chromium oxide) in the original anorthosite unit. The anyolite is hosted in gneisses exposed in the Mozambique Collision Belt, an ancient, north-south trending, tectonic collision zone in eastern Africa. That is an area affected by plate convergence (continent to continent collision). It dates to the Pan-African Orogeny (Neoproterozoic), during which the ancient continents of West Gondwana (~modern-day South America & Africa) and East Gondwana (~modern-day India-Australia-Antarctica) collided, forming the long-lived, small supercontinent Gondwana.(James St. John, 2012). Ruby, zoisite, and black amphibole show a little foliation (schistosity; like schist). Besides the foliation, the collision of the plates may have continued after formation of the ruby that appears to rather fractured.

A) Describe the contrasting colors. Why or why isn't this attractive?

B) Can you see fractures in the rubies?

C) How well formed are the rubies. What shape is their outline?

D) Is ruby likely to show foliation well? Can the crystals be elongate? Could they be parallel to each other in the rock. Look for a slab of ruby zoisite on eBay and see if you can find foliation.

E) Would the difference in hardness between ruby and zoisite be a difficulty in carving this stone? Why?

3) Gore Mountain Garnet
The garnet deposit at Gore mountain is in th Precambrian rocks of the Grenville Series. Located 110 km (70 miles) north of Albany (See Figure 7), it has been mined for abrasive garnet for such uses as sand paper and emery cloth, and for sandblasting, etc. Abrasives must be hard and durable, and the garnet can have a hardness up to around 8 on Mohs’ scale. The garnets seem to owe their origin to the mobilization of magnesium, iron, aluminum, and silica, chemical formula Fe3Al2(SiO4)3.

They are mined in an open pit, the original pit on Gore Mountain is now a tourist attraction, but
an adjacent property on Ruby Mount produces 1000 tons of garnet a month. One property of the garnet that is good for abrasive is how it breaks in chisel like shard. The garnet probably formed because of intrusions of gabbro, and other igneous rocks, but it is over a billion years old. A lot has happened in the region and plate tectonics may have regionally metamorphosed the region as well. Consider this later pressure metamorphism as you try to answer the following questions:

A) From the pieces of garnet you see, could large gems be faceted (cut)? Look carefully at the texture of the garnet? Explain why or why not?

B) Do you think interesting tiles or cabochons (smoothed over pieces could be made)?

C) What type of garnet is it? Guess based on the information given or look up? Is it very valuable as garnet goes? List its properties. Include some size of actual crystals.

D) Try and decide if this is regional or contact metamorphism. Any reason or reference?

E) What rocks seem to have been changed to make the garnets? Are they igneous?,
sedimentary? perhaps this has some bearing on your answer to D?
Map of New York showing the location of Gore Mountain
Figure 7. Location map showing Gore Mountain and the Franklin Furnace Zinc mine, New Jersey. This marble deposit makes Franklin the fluorescent mineral capital of the world.
4) The Franklin and Sterling Hill Mine and Fluorescent Minerals

Franklin Furnace, also known as the Franklin Mine, is a famous mineral location for rare zinc, iron, manganese minerals in old mines in Franklin and Ogdensburg (Sterling Hill), New Jersey, USA. These localities produced more species of minerals and more different fluorescent minerals than any other location. The famous mineral association (assemblage) from Franklin and Sterling Hill Mines includes the zinc minerals (Zn =symbol for zinc), willemite a green mineral (Zn2SiO4), zincite a red mineral (ZnO), and franklinite a black mineral (Zn,Fe)2O4. Willemite fluoresces bright green, zincite sometimes fluoresces yellow, but is not very fluorescent in most cases. Franklinite is non fluorescent and appears black and metallic. Calcite is the gangue mineral but fluoresces a bright red.

During the mid-to-late 19th century the furnace was the center of a large iron making operation and the name furnace refers to the one used to melt iron ore.

The ores (valuable minerals), particularly the element Zinc, are contained within the Franklin Marble. This was deposited as limestone in a Precambrian oceanic rift trough. It was metamorphosis during the Grenville orogeny, approximately 1 billion years ago creating the marble. During the Ice age glaciers strewed trains of ore-bearing boulders for miles to the south, in places creating deposits large enough to be worked profitably.

In the area of the Franklin and Sterling Hill mines at least 357 different minerals occur; this is approximately 10% of the minerals known to science. Thirty-five of these minerals are only found here and ninety-one of the minerals are fluorescent.

Franklin and Sterling Hill are located in the Reading Prong of the New England physiographic province. It is believed ocean sediments and volcanics were formed in a divergent (rift valley type) ocean setting and deeply buried and strongly metamorphosed to the sillimanite grade (high grade up to about 800° C; hot enough to start melting!) later in the Precambrian. Ages for this metamorphism event seem to range from 800 to 1150 million years ago.

The rocks were then affected by mountain building and convergence (subduction; see Figure 3), which folded and recrystallized them. Then they were then uplifted from the late Precambrian through early Cambrian (approximately 500 million years ago), their uplifted surface was extensively eroded. Then they were re-buried and exposed again during modern times.

Warning! UV light can burn your eyes! Do not do anything without your instructor!
Fluorescent minerals from Franklin, New Jersey
Common minerals of the Franklin mineral district, New Jersey. Top image is in white light. Bottom image is under short-wave ultraviolet light. Minerals are: willemnite (green), calcite (red) and franklinite (black).
A) Set up the the fluorescent mineral viewing cabinet with your instructors help.

B) Look at the rocks without UV outside the cabinet. Try to find 1) Franklinite, 2) Wilemite, and 3) Zincite. The calcite should be the surrounding white to grey rock. Explain the mineralsto the class.

C) Test for calcite with acid. Is there some? Show to class.

D) What is the overall rock type minus the ore minerals?

E) Look at the map and the web to describe how important the Franklin Marble and the mine deposits at Franklin and Sterling Hill were in the past. Give a little history to the class.
 
5) Kyanite, a Strange Mineral/Gem. Be careful! The bladed crystals of kyanite are sharp!
Do as Homework
Kyanite crystals (Al2SiO5), found in association with staurolite, are found in the schists of St.
Gotthard, in the Tyrol Mountains of Switzerland. Other excellent specimens come from many
countries around the world, Brazil, India, Nepal, Kenya, in the USA in locations around
Massachusetts (including in central Vermont!) and North Carolina, etc. One of our samples in
quartz is from Minas Gerais, Brazil. The other with staurolite is in schist from Switzerland.
Though it can form in other environments, most kyanite is metamorphic in origin, and kyanite in
metamorphic rocks indicates higher pressures. Higher pressures suggest schist and gneiss
created by convergence.
This last specimen is for everyone to look at. Kyanite is an unusual mineral that forms elongated
crystals. It is blue approaching in fine specimens a blue almost as good as sapphire.
A) Is blue intrinsic to this gemstone, that is it must be blue because that is part of its chemistry?
B) Do you think kyanite is more likely to be in foliated, regionally metamorphosed rocks or in
contact metamorphic rocks.
C) Kyanite hardness is rather strange. Try and find out the strange thing about it and explain.
D) Would kyanite make a good gemstone (remembering quartz is always around and quite
hard)?

6) Plate Tectonics Map
A) Go to page 9, the map of the plates. Put arrows on the major plates showing their direction
of motion.
B) On the main plates mark plate edges that are 1) convergent (subducting) in Red and 2)
Divergent (ocean ridges) in Blue.
C) Remember that some plate edges are transform and slide past each other. Mark the San
Andreas fault. Some transform boundaries are harder to mark. Use Green for what you can
find that is transform in nature.

Tectonic plate map of the world (USGS)

References.

Metamorphic rocks, (http://www.columbia.edu/~vjd1/meta_rx.htm) Accessed, February 22,
2012. Used as Figure 2. Foliated rocks.
James St. John, Anyolite,
(www1.newark.ohio-state.edu/Professional/OSU/Faculty/jstjohn/Anyolite/Anyolite.htm)
accessed Feb 21, 2012.
Lobeck, A. K., 1932, Atlas of American Geology, the Geographical Press, Columbia University,
New York, 92 p.
Merguerian, C. and Moss, C. J., Newly Discovered Ophiolite Scrap in the Hartland Formation of
Midtown Manhattan
(http://dspace.sunyconnect.suny.edu/bitstream/handle/1951/47974/Merguerian-ophiolite.
pdf;jsessionid=9ABA6F9C035CBEA09356A872ABBF13BC?sequence=1) accessed, February
21, 2012.
Actinolite is produced by low-grade regional or contact metamorphism of magnesium carbonate, mafic, or ultramafic rocks; also in glaucophane bearing blueschists.

Benitoite occurs in natrolite veins cutting glaucophane schist in a serpentine body (San Benito Co., California, USA); in a magnesio-riebeckite-quartz phlogopite-albite dike cutting serpentinite

Diopside
- Typical of metamorphosed siliceous Ca, Mg-rich rocks of the pyroxene-hornfels or epidote-amphibolite facies; common in skarns, Ca, Mg-rich gneisses and schists, and some kimberlites and peridotites. Less common in alkalic olivine basalts and andesites.

Epidote - Most originate in low- to medium-grade metamorphic rocks.

Jade
(jadeite and nephrite) - Jadeite forms In high-pressure metamorphic rocks of the glaucophane facies; a component of eclogite. Nephrite (Greek - kidney) is a dense, fibrous aggregate variety of the tremolite-actinolite series that is tougher than jadeite. It can be all colours, often with a yellow tint. The most valuable is green.

Allanite is a member of the Epidote group. It is opaque, with a dark brown (sometimes virtually black) colour. Allanite contains the rare earth elements Yttrium and Lanthanum.

Pyrope occurs In ultramafic rocks, as peridotites, kimberlites, eclogites, serpentinites, and in "hornblende"-garnet-plagioclase rocks and anorthosites. Also in amphibole and biotite schists, and as a detrital mineral.

Almandine
is an iron alumina Garnet, of deep red color, inclining to purple.

Andalusite is named after the Spanish province Andalusia where transparent gem-quality crystals were first found. A result of contact metamorphism of argillaceous sediments, also in regionally metamorphosed schists. Rare in granites and pegmatites, which however afford the largest crystals. Detrital in some sandstones.

Bronzite is an iron-bearing variety of Enstatite from Austria. It has green-brown colour with bronze-like sub-metallic lustre. It was known long before Enstatite.

Brucite
occurs as a common alteration of periclase in marble and in low-temperature hydrothermal veins in metamorphic limestones and chlorite schists

Cassiterite
(tin ore) occurs in medium- to high-temperature hydrothermal veins.

 
   

Review Questions and Terminology

 

Click below to proceed to:
http://geologycafe.com/gems/labs/lab5.html
9/17/2014