Metamorphic Gemstones

[1dave]

(Under Construction)

Geologists recognize three kinds of rocks: Igneous, Sedimentary, and Metamorphic.

When rockhounding in Metamorphic Areas, keep a sharp eye out for special Gemstones!

ANY preexisting rock may be distorted and changed by earth’s pressure and/or heat into a new rock called a metamorphic rock.

Metamorphosis literally means “a change in form.”

Metamorphic rocks are not melted as they form -that would be igneous, but heat and pressure can force amazing crystal structure changes in minerals.

So far I am aware of Low, Medium, and High grades; Burial, Compression, Contact, Continental Collision, Dynamic, High Strain, Hydrothermal, Pneumatic, Regional, and Shock Metamorphosis.
Each has it's unique reaction on rocks in it's domain.

"Meta-" is often placed in front of rock names to indicate mild metamorphism when the original form is still easily identified: metabasalt, Metarhyolite, Metagranite. But when "meta" is placed in front of a a metamorphic rock,- like "metaquartzite" - you know it got a double whammy!









And then there are "Facies" for example, formed from basalt:
en.wikipedia.org/wiki/Metamorphism

Temperature - - - Pressure - - - - - - Facies
Low - - - - - - - - - - Low - - - - - - - - - Zeolite
Lower Moderate - - Lower Moderate - - Prehnite-Pumpellyite
Moderate to High - - Low - - - - - - - - - Hornfels
Low to Moderate - - Moderate to High - Blueschist
Moderate → High - Moderate - - - - - -  Greenschist→Amphibolite→Granulite
Moderate to High -  High - - - - - - - - - Eclogite

The most common types of Metamorphic rocks are:



1. Gneiss - Less than 50% thin layers. Most commonly found, course grained, roughly foliated. Nearly all traces of the original structures such as layers, ripple marks and fossils have been altered. Essentially distorted quartz and feldspar layers separated by mica. Usually formed from igneous rocks like granite, but may form from sedimentary rocks as well. Banded-gneiss, lenticular-gneiss, biotite-gneiss, hornblend-gneiss, granite-gneiss, diorite-gneiss, etc.


2. Mica-schist (Split) - More than 50% thin layers. Second most commonly found, predominate mica "books" with quartz. As gneiss, formed from igneous and sedimentary rocks, but retains much of the original structures in evenly layered sheets. Frequently with garnet, staurolite, kyanite, epidote, hornblend, etc.


3. Quartzite - Common and widely distributed. Formed from sandstone, compact, breaks with uneven, splintery, or conchoidal fracture.


4. Slate - Common. formed from shale, Slaty cleavage may not be parallel to their original bedding planes.


5. Various schists - Formed from various ferromagnesian minerals. Talc-schist, chlorite-schist, amphibolite-schist, hornblende-schist.


6. Marble - Formed from limestone or dolomite.


7. Anthracite - Formed from coal


In Special Facies with Special Minerals, Very Special Gemstones are formed!

(Temporary Chart until I can make the real one)

[1dave]

Part 1 - Low Heat and Low Pressure

Zeolites (boiling Stones)
Zeolites are all around us, but seldom recognized, even though they are in great demand.


Canadian Mining in the News!
www.collegegreenmag.com/unique-mineral-removes-contaminants-from-air-and-water


Canadian Mining Company Inc. Sun-Group Phase-1

The term zeolite was coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon heating stilbite, large amounts of steam issued from the adsorbed water. He called the material zeolite from the Greek zéo-, meaning "to boil" and líthos, meaning "stone".

Zeolites are the result of very low grade metamorphism typically forming in the cavities of volcanic rocks. Some form in subtle amounts of heat and pressure and can just barely be called metamorphic, while others are found in obviously metamorphic regimes.


2.3Cowlesite.jpg
Cowlesite in Basalt Cavity

The zeolites are a family of about 45 glassy minerals analogous to feldspar containing hydrated aluminum silicates of calcium or sodium or potassium; presently broken down into seven compositional and structural subfamilies. They are framework silicates consisting of interlocking tetrahedrons of SiO4 and AlO4 with the ratio (Si +Al)/O equal to ½.

Industrially speaking, the term zeolite includes natural silicate zeolites, synthetic materials and phosphate minerals that have a zeolite like structure. The complexity of this combined group is extensive with over 120 structural variations and more are being discovered or made every year.


ZeoliteStructureC.jpg

Unlike most tectosilicates, zeolites have large vacant channels or cages in their structures that allow space for large cations such as sodium, magnesium, potassium, barium and calcium and even relatively large molecules and cation groups such as water, ammonia, carbonate ions and nitrate ions.. The large channels explain the consistent low specific gravity of these minerals.

Zeolites are characterized by their ability to lose and absorb water without damage to their crystal structures.
Zeolites have basically three different structural variations.

1. Framework structures where the crystals are more equant in dimensions, ie Analcime.


Analcine-JohnBetts.jpg
Analcime Photo by John Betts

2. Sheet-like structures where the crystals are flattened platy or tabular with usually good basal cleavages, ie heulandite.


Heulandite1.jpg
Heulandite

3. Chain-like structures whose minerals form needle-like prismatic crystals, ie natrolite.


Natrolite1.jpg
Natrolite

These variations make the zeolite group very diverse, crystal habit-wise. They are typically light in density, transparent to translucent, soft to moderately hard, and have similar origins.

The following after “Zeolite Occurrences of Utah” Fall of 1989 by Bryce T. Trip and Bea M. Mayas.

Most zeolites form in one or more of six geologic/hydrologic environments:
saline alkaline lakes, saline alkaline soil and land surfaces, deep sea sediments, open hydrologic systems, hydrothermal alteration zones and burial metamorphic occurrences (Hay, 1977).

Minerals that make up sedimentary rocks are of two principal types— detrital and authigenic.
Detrital minerals, such as grains of quartz and feldspar, survive weathering and are transported to the depositional site as clasts. Authigenic minerals, like calcite, halite, gypsum, and zeolites, form in situ within the depositional site in response to geochemical processes.


SalineLake-1.JPG

Saline alkaline lake deposits (also known as closed hydrologic system deposits) form where saline alkaline lake waters alter volcanic ash deposited into the lake, Saline alkaline lakes typically occur in arid climates in block faulted or rifted terrains surrounded by igneous or metamorphic bedrock without significant pyritic shales or evaporites (Surdam, 1977). The combination of saline, alkaline waters and highly reactive volcanic material seems to be very favorable for zeolite growth. Zeolites in these deposits may be traced horizontally for several kilometers and are often laterally zoned according to the salinity gradient in the original lake water, Mineral zonation from the shoreline toward the center of the lake consists of unaltered glass; a zone of one or more of the following zeolites: chabazite, erionite, phillipsite, mordenite or clinoptilolite; an analcime zone and finally, at the center of the lake, a K-feldspar zone (Sheppard and Gude, 1969). Mineral zonation may not be apparent in rift valley settings because the long, narrow geometry of the valley compresses the mineral zones. Nonetheless, rift valley settings contain some of the purest, mono-mineralic concentrations of commercial zeolites known. High-grade beds are usually a few centimeters to a few meters thick, Saline alkaline soil and land surface deposits form through the interaction of alkaline brines with tuffaceous and non-
tuffaceous sediments such as claystones. Meteoric water percolating through sediments enriched in sodium carbonate-bicarbonate (by evapo-transpiration) transform sediments to zeolites.
Phillipsite, chabazite, natrolite and analcime are known to occur in these types of deposits (Hay, 1977).

Deep sea sediment deposits may form through the low-temperature alteration of volcanic glass, biogenic silica or poorly crystalline clays by pore water, although the exact mode of formation and required precursor minerals is still a matter of conjecture (Bass, 1976; Boles, 1977a). Major zeolites formed in deep sea sediments are phillipsite and clinoptilolite; analcime is a minor component and erionite, natrolite and mordenite occur only occasionally. Zeolites can constitute as much as 80% of some deep sea drilling cores. The type of zeolite encountered varies with the age of the sediments; phillipsite is more common in late Tertiary and younger sediments; clinoptilolite is most common in early Tertiary and Late Cretaceous sediments while analcime becomes more prevalent in Early Cretaceous sediments. Authigenic clays often occur with the zeolites (Hay, 1977).

Open hydrologic system deposits form by percolation of meteoric water through nonmarine or shallow marine tuffaceous sediments. These occurrences can be several hundred meters thick and be traced laterally for tens of kilometers. These deposits show a vertical mineral zonation with zones consisting, in descending order, of volcanic glass and clay minerals; zeolites such as clinoptilolite, mordenite, and minor phillipsite; and finally, analcime and/or potassium feldspar.
Contacts between zones are either horizontal or slightly inclined and cut bedding planes (Hay and Sheppard, 1977).


2.4Dachiardite.jpg

Hydrothermal alteration zone deposits occur in areas of hydrothermal activity where hot fluid alters rock along its migration route. Zeolites are zoned according to the temperature of the altering fluid in the hydrothermal system. Clinoptilolite and mordenite occur in cooler, shallower parts of the system while analcime, heulandite, laumontite and wairakite occur in deeper, hotter areas of the system (Hay, 1977).


2.9 Ferrierite.jpg
Ferrierite Sample, Lovelock Nevada - Trabits Group: Project Photographs


2.9 Ferrierite Digging.jpg
Ferrierite Outcrop Sample, Lovelock Nevada - Trabits Group: Project Photographs

Burial diagenetic deposits (also known as low-grade metamorphic deposits) originate by authigenic growth of zeolites due to increased temperatures associated with burial in sedimentary basins. Zeolites and other authigenic minerals in these deposits show vertical zonation with increasing alteration corresponding to increasing depth of burial. Volcanic glass or calcic plagioclase is a precursor to the zeolite.
Clinoptilolite, heulandite, analcime and laumontite predominate although in Tertiary and younger rocks mordenite, erionite, chabazite, and phillipsite are significant. Together, zeolites may comprise up to 25 percent or more of the rock volume in non-tuffaceous, metamorphosed rocks and much higher proportions in altered tuffs. Associated minerals include authigenic quartz, albite, adularia, calcite, phyllosilicates and occasionally sphene, prehnite and pumpellyite (Boles, 1977b).

Successful zeolite prospecting since the 1950s has centered on altered Tertiary volcaniclastic deposits in the western United States. While geologists have identified clays, feldspars,
feldspathoids and gels as parent materials for zeolites, volcanic glass shards are the main precursor for authigenic zeolite formation. Chemicals in ground water or surface water alter glass
to zeolites in a short period of time, forming zeolite beds.

These zeolite beds can be white, yellow, grey,green or brown. They tend to be more resistant than surrounding unaltered tuff, often forming overhanging ledges in drainages. The zeolite beds do not necessarily have to be very thick to be of commercial interest. For example, the Bowie deposit, an unusually pure chabazite deposit in Arizona, is mined despite being only six inches thick and oyerlain by up ro 30 feet or over-burden. Zeolitically altered tuffs may also contain glass shards, quartz, clays, K-fe|dspar, opal, chalcedony, calcite, cristobalite, evaporates, mafic minerals, searlesite, fluorite, dawsonite, and other authigenic minerals.

Zeolites transform to other minerals under weathering, hydrothermal alteration or metamorphic conditions.Some examples:
Silica-rich volcanic rocks commonly progresses from: Clay ➫ quartz ➫ mordenite–heulandite ➫ epistilbite ➫ stilbite ➫ thomsonite–mesolite-scolecite ➫ chabazite ➫ calcite.

The sequence of silica-poor volcanic rocks commonly progresses from:Cowlesite ➫ levyne–offretite ➫ analcime ➫ thomsonite–mesolite-scolecite ➫ chabazite ➫ calcite.

Some zeolite students have divided them into 7 families,


01-ZeoliteFamilies.JPG

but Strunz has recently divided them into 7 groups:

A : Zeolites with T5-O10 Units – The Fibrous Zeolites
B : Chains of single connected 4-membered rings
C : Chains of doubly-connected 4-membered rings
D : Chains of 6-membered rings – tabular zeolites
E : Chains of T10-O20 Tetrahedra
F : Other Rare Zeolites
G : Unclassified zeolites

I've re-arranged and combined them.

Group 1 = Chains of single connected 4-membered rings, the Leucite Group.


Leucite-002.jpg
Leucite Crystals


Note that Leucite has a crystal transition point that changes it's structure recognized in some other members of the group.


Z1-Group-2.jpg


Z1-Group-Images.jpg


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Apatite - Ca5(PO4)3F (fluorapatite) and Ca5(PO4)3Cl (chlorapatite) - forms under a wide variety of conditions and is found in igneous, metamorphic, and sedimentary rocks. The most important deposits of apatite are in sedimentary rocks formed in marine and lacustrine environments where phosphatic organic debris accumulated and was mineralized during diagenesis (change to sedimentary rock). Apatite occasionally occurs as well-formed hexagonal crystals in hydrothermal veins and pegmatite pockets.

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Calcite - CaCO3- A "ubiquitous mineral" - one that is found everywhere throughout the world in sedimentary, metamorphic, and igneous rocks. At low temperatures may form Vaterite that belongs to the hexagonal crystal system. It converts to trigonal calcite at 37 °C, or orthorhombic Aragonite at ~60 °C).

[1dave]

Part 2 - Very Low Grade M: 150-300^oC

Mud rock turns to Slate, Basalt turns to Chlorite Schist.

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Part 3 - Low Grade M: 300-450^oC

Slate becomes Phylite, Limestone becomes Marble, Garnet crystals begin to grow.

geology.com/minerals/garnet.shtml
Garnet is the name used for a large group of rock-forming minerals. These minerals share a common crystal structure and a generalized chemical composition of X3Y2(SiO4)3.
In that composition, "X" can be Ca, Mg, Fe2+ or Mn2+, and "Y" can be Al, Fe3+, Mn3+, V3+ or Cr3+.

These minerals are found throughout the world in metamorphic, igneous, and sedimentary rocks.



Most garnet found near Earth's surface forms when a sedimentary rock with a high aluminum content, such as shale, is subjected to heat and pressure intense enough to produce schist or gneiss.

Garnet is also found in the rocks of contact metamorphism, subsurface magma chambers, lava flows, deep-source volcanic eruptions, and the soils and sediments formed when garnet-bearing rocks are weathered and eroded.

From "Slave Labor Garnets" - they vary in magnetic susceptibility.



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Low Heat and High Pressure

Jadeite and Nephrite

[1dave]

Part 3 - Low Grade M: 300-450^oC

Slate becomes Phylite, Limestone becomes Marble, Garnet crystals begin to grow.

geology.com/minerals/garnet.shtml
Garnet is the name used for a large group of rock-forming minerals. These minerals share a common crystal structure and a generalized chemical composition of X3Y2(SiO4)3.
In that composition, "X" can be Ca, Mg, Fe2+ or Mn2+, and "Y" can be Al, Fe3+, Mn3+, V3+ or Cr3+.

These minerals are found throughout the world in metamorphic, igneous, and sedimentary rocks.



Most garnet found near Earth's surface forms when a sedimentary rock with a high aluminum content, such as shale, is subjected to heat and pressure intense enough to produce schist or gneiss.

Garnet is also found in the rocks of contact metamorphism, subsurface magma chambers, lava flows, deep-source volcanic eruptions, and the soils and sediments formed when garnet-bearing rocks are weathered and eroded.

From "Slave Labor Garnets" - they vary in magnetic susceptibility.



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Low Heat and High Pressure

Jadeite and Nephrite

[1dave]

Part 4 - Medium Grade M: 450-550^oC

Phylite becomes Schist, Chlorite Schist becomes Amphibole Gneiss, Sandstone becomes Quartzite.

Medium Heat and Medium Pressure



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Hydrothermal - Basalt becomes Serpentinite

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Pneumatic - Topaz, Tourmaline

[1dave]

Part 5 High Grade M: Above 550^oC
High Heat AND High Pressure:

Schist becomes Gneiss, Granite becomes Gneiss.


Emerald Aquamarine, Goshenite, Heliodor, Morganite, and Red Beryl[/font][/b] (from the Wah Wah Mountains west of Milford Utah.)
Beryl of various colors is found most commonly in granitic pegmatites, but also occurs in mica schists in the Ural Mountains, and metamorphosed limestone in Colombia.

en.wikipedia.org/wiki/Beryl
Beryl is a mineral composed of beryllium aluminium cyclosilicate with the chemical formula Be3Al2(SiO3)6.
Naturally occurring, hexagonal crystals of beryl can be up to several meters in size, but terminated crystals are relatively rare. Pure beryl is colorless, but it is frequently tinted by impurities; possible colors are green, blue, yellow, red (the rarest), and white.

www.minelinks.com/alluvial/emerald3.html
Since there is nothing to indicate in what part of the mine emeralds are likely to occur, the workers simply loosen blocks of rock from any part of the walls until a nest of emeralds, the presence of which is indicated by green quartz crystals, is met with.

The associated minerals are calcite, topaz, fluorspar, cassiterite, and mispickel.


Ruby and Sapphire
Corundum is a rock-forming mineral that is found in igneous, metamorphic, and sedimentary rocks. It is an aluminum oxide with a chemical composition of Al2O3 and a hexagonal crystal structure.

Zircon
Zircon is a zirconium silicate with a tetragonal crystal system, and also contains thorium and uranium.
It has a hardness of 8 out of 10 on the Mohs scale of mineral hardness, is tetragonal in structure, made of zirconium silicate, ZrSiO4.

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geology.com/articles/diamonds-from-coal/


1) Diamond Formation in Earth's Mantle, 2) Diamond Formation in Subduction Zones, 3) Diamond Formation at Impact Sites, 4) Formation in Space.

Diamond
The ultimate of metamorphic rocks. From soft black carbon to transparent hardest of all gemstones, formed miles deep in the earth's mantle, ejected through the most bizarre of all geologic processes - the carrot shaped diatreme.

Diamonds apparently pop up through miles of solid rock wrapped in "kimberlite" - like beach balls released underwater - WITHOUT following magma flows!