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more about silicates

Amorphous silica is noncrystallline and deposited as a gel with water content.  The most obvious example is opal, which comes in many varieties.  Common opal, which is pale colored, precious opal with its play of color, and fire opal which is red or yellow, also with a play of color.  Opal wood is petrified wood with opal as the petrifying material.  Tripolite is opalized diatoms (small critters) which forms a powder used for polishing, and hyalite which is opal crusts on rocks.

Another example of amorphous silica is silica glass, formed when lightning strikes quartz (fulgarites) , or when quartz is struck by a meteorite.  It's quite rare  because the temperature required to melt quartz is quite high. 

Gem varieties of silica are opal and quartz crystals in various colors:  rock crystal, rose quartz, amethyst, smoky quartz, milky quartz and citrine.  Sagenitic quartz has inclusions such as rutiles (rutilated quartz) and quartz pseudomorphs are minerals in which the structure has been replaced by quartz.  An example is tigereye, in which the original asbestos fibers  are visible.

 Cristabolite is  the form of silica that remains stable at high temperatures.  Quartz turns to cristabolite at 1055 C and the change is permanent. Cristabolite is often seen in obsidian as the "snowflaek" pattern.  Similar to cristabolite is tridymite, also found in high temperature minerals.

Posted by oh5/ojhoff at 3:29 PM CST
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frame silicates

The frame silicates are exciting for the average rockhound.  It is these minerals that brought a lot of people into the business/hobby.

The silica group of frame silicates includes free silica in large crystals, cryptocrystalline varieties (grains can only be seen in a microscope), and amorphous silica containing water.

Quartz is everywhere.  Its uses are many:  firebrick, gemstones, electronics, and it is the principal component of glass.  Most people are familiar with the hexagonal structure of a quartz crystal.  Cryptocrystalline quartz starts out as amorphous gel and the water settles out, leaving colorful banded and patternes materials that have been given separate names.

Chert is the duller, more opaque variety of cryptocrystalline quartz.  Jasper is one of its subcategories.  Jasper can be any color but is usually brightly colored and patterned.  Flint is a black variety of chert.  Aventurine is chert with mica particles that make is sparkly.  Cherts are useful for making stone tools such as arrowheads, due to their conchoidal fracture.

Chalcedony is the more translucent variety of cryptocrystalline quartz.  Gem varieties of onyx are agate, onyx, sardonyx, tigereye, carnelian and chrysoprase.

Posted by oh5/ojhoff at 11:26 AM CST
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clay minerals

The clay minerals are commonly thought of as any rock or soil material consisting of small particles that can be molded when wet.  Technically, the term for them is  hydrous aluminium phillosylicates.  Clays are a major constituent of soils.  There are many groups, such as kaolinite, montmorillonite/smectite, Illlite and chlorite.  There are numerous subcategories, which is a subject unto itself. 

Uses for clay run into the thousands:  ceramics, fuller's earth, meerschaum, fire bricks, carbonless paper, bricks, tile, pottery, filtering, as a filler in plastics and rubber and so on.  

I remember an old Girl Scouts book I had  once that explained how to extract clay from soil, make it into useful objects, and fire it.   



Posted by oh5/ojhoff at 6:27 PM CST
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mica group

The mica group of sheet silicates is kinda confusing.  There are two basic types, muscovite and biotite.  So here goes:

Muscovite is the commonest mica.  Only feldspar and quartz are more abundant in the earth's crust.  It is formed in granites and other silica-rich rocks, and in metamorphic rocks.  It is also a component of shales.  There is a chromium bearing muscovite called fuchsite that is attractive as a specimen.  

There are many more varieties of biotite than muscovite and many have been given separate names.  Biotites contain much iron and are found in igneous rocks, granites, gabbros, lavas and many metamorphic rocks.  Some of the varieties are haughtonite, siderophyllite, manganaophyllite, wodanite, titanobiotite, and calciobiotite.  

Phlogopite is essentially a biotite with more magnesium than iron.  It typically shows an asterism in transmitted.  Phlogopite too is divided into several varieties:  paragonite, margarite, prehnite, lepidolite, and glauconite.

 I am back in the deep freeze and the cats are back in the house.

Posted by oh5/ojhoff at 9:25 AM CST
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talc, pyrophillite

Talc is one of the more useful sheet silicates.  It's used as talcum powder, in ceramics and for insulation.  Soapstone and steatite are impure talc formations used as sinks and countertops in chemical labs.  They are acid resistant.

Pyrophillite is almost identical to talc and looks quite a bit the same.  It is more rare but has the same uses as talc.  Pyrophillite has a structural relationship to mica.  Both talc and pyrophillite are common in metamorphic areas such as New England.

I spent the past two or three days working on the computer and watching TV, MLK observances, the inauguration, and so on.  



Posted by oh5/ojhoff at 1:27 PM CST
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no ambition

I can't seem to motivate myself when it's this cold.  The weather man promised a warm up today but it was still 45 below according to my thermometer, which is always at least 10 degrees colder.  Either the thermometer is wrong or I am in a pocket of cold.  I use the computer in a heated porch but the windows on the south side leave a little to be desired so I go to bed early.  The next subject to be considered is talc but I will leave that to another day.

Right now  I'm going to fix a cup of hot carob.  The day seems right to make soup.  Somewhere I have a Tanzanian recipe and it is very very good.  Calls for coconut milk which maybe isn't good for you but when I was on Blogit, one of my fellow bloggers indicated that despite the drawbacks of tropical oils, there is something in coconut that contributes to human health.  Moderation is the key, I think.

Posted by oh5/ojhoff at 10:19 AM CST
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serpentine group

The serpentine group of sheet silicates is called that because most of the examples have a green, patterned appearance.  The recognized varieties are antigorite, lizardite, and chrysotile.  It is very difficult to tell them apart without using x-ray and miscroscope equipment.

 It is not known why the serpentine varieties differ in appearance.  Reasons may be impurities, pressure and temperature when they were formed, minerals present, the water content, and so on.

Fibrous serpentine is used to make asbestos fabric and making fire brick.

Posted by oh5/ojhoff at 1:00 PM CST
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Posted by oh5/ojhoff at 9:43 PM CST
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sheet silicates

Sheet silicates exhibit one directional cleavage.  This is because the bonding between the sheets is weak.  These minerals have a greasy feel and lubicating qualities.  An example of weak bonding is mica,in which the sheets separate easily.

 The kaolinite group includes kaolinite and three others with similar chemical composition but different structure.  The other three are dickite (funny name) , nacrite, and halloysite.  Kaolinite is important for use in pharmaceuticals, firebrick, china, and as a whitening agent.

I joined an online wire wrap group today.  I wonder if I will learn anything.



Posted by oh5/ojhoff at 10:37 PM CST
Updated: 01/15/09 12:42 PM CST
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double chain silicates

The double chain silicates are formed so that many ionic substitutions are possible, therefore the chemical varieties are endless.  All double chain silicates are in the amphibole group.  These contain water and are formed at lower temperatures than olivines and pyroxenes.  There are over thirty varieties.  Some of the more well known are actinolite, of which nephrite jade is largely composed, and hornblende, found in basaltic lavas in mountainous areas of the world.  Some very large rocks are mostly hornblende. 

 A bit warmer today.  I may get back to the photographing of materials from Ely or whatever I can find.

Posted by oh5/ojhoff at 11:11 AM CST
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the bad month of December

Between being sick, and snowed in, I've barely let the house this month.  Now I have to venture out and fill a rock order.  Fortunately, my worst cold in years might be going the other way.  On the dark side, it was thirty below this morning. 

The next group of minerals to be considered are the single chain silicates, also known as pyroxenes.  This group includes several well known gemstones. 

 Here are some of the single chain silicates:  diopside, enstatite, hedbergite, aigote. ju[ersthene, aegerite, johannsenite, rhodonite, jadeite, wollastonite, spodumene, chrysocolla.  How many do you recognize?Quite a few, I would imagine.  This is a major group of rock forming minerals.

Posted by oh5/ojhoff at 10:10 AM CST
Updated: 12/28/08 10:22 AM CST
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Christmas shoppin

Christmas is not a big season for my rock business.  It seems such a short time ago I was making this same remark :)  A few days ago I went with my brother to the science center and retrieved our fluorescents and made brave promises to get involved in the center as far as earth sciences are concerned.  Then we had a handful of charter school students visit and showed them the rock equipment and explained a few of the processes we are able to do.  It was, unfortunately, the coldest day of the year thus far.  I still have hopes of making plaster casts of fossils and of growing crystals so I can provide a more interesting hands on experience for such groups in the future.

 The season of peace and good will and good cheer isn't starting out all that well.  A gun battle in India, overzealous bargain hunters trample a store worker to death, shots fired in a toy store....

Maybe Im stupid but I still think most people are good.  Let's try to make a better world. 



Posted by oh5/ojhoff at 10:39 AM CST
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ring silicates

The ring silicates are complex in structure.  I struggle to understand them and wouldnt begin to try an explanation.  But the minerals that fall under this category are well known for their beauty:  tourmaline and beryl. 

 Tourmalines are found in pegmatites and are often treasured as gemstones, in particular the ones of which the chemical structure has changed during the formation, resulting in many colors.  I sold green tourmaline points in my shop for a while and they were tremendously popular.

Beryl in its transparent form is highly valued as emerald, aquamarine, and heliodor.  Its typical crystal form is a six sided prism.

Cordierite has the same structure as beryl but a different chemical composition.  It resembles quartz.  It doesn't fracture easily during heating and cooling and is therefore valuable as a refractory.

 Miserable cold and rainy today.  

Posted by oh5/ojhoff at 10:40 AM CST
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The wind was brutally cold on Sunday.  Two parties in the barn were not held..on cancelled and the other postponed.  The house is cold.  A decision has been made to hold off on using electricity or propane for heat until Nov. 20.  Of course, that means a fire in the furnace which sometimes doesn't happen.  

As their name implies, the double tetrahedral silicates are complex in structure.  The melllilite group includes gehlenite, akermanite, and hardystonite.  All are commonly found in limestone.  Akermanite is relatively rare in nature but is found in industrial slag.  Hardystonite has been found only in Franklin, NJ.  All are dull in sppearance and to be honest, I have never knowingly seen any of them, nor do I care to.

Posted by oh5/ojhoff at 12:19 PM CDT
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subsaturate silicates

The easiest way to sum up the chemical composition of the subsaturate group is to say these have extra oxygen ions.  Examples are kyanite, staurolite, sillimanite, andalusite, and topaz.   I just recently acquired a sample of the kyanite and found it attractive with its underlying blue shimmer.  My specimen comes from North Carolina.  Staurolite is found in a range from New England to Georgia, also in Sweden, and often forms cruciform crystals that can be made into cross jewelry.  Sillimanite has the same composition as kyanite but a different structure and is a bit harder.  Andalusite also has the same chemical composition as kyanite and sillimanite.  It forms at lower temps and is sometimes faceted and used as a gem.  Everyone knows topaz.  It has a similar structure to other subsaturates but is otherwise unrelated.  Topaz can be found in a variety of colors.  The amber colored is the birthstone for November.

Posted by oh5/ojhoff at 2:46 PM CDT
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The chemical composition of a mineral can be expressed by a formula but said formula might not be exact.  It simply means that this or that named mineral refers to definite limits.  The variation between the two limits is called solid solution.  For example, the glassy, brittle, translucent/transparent mineral known as olivine varies from forsterite which is a pure magnesium silicate, to fayalite, which is a  pure iron silicate.  Both are uncommon but the variations between the two are very common.  There are also tephroite, roepperite,  knebelite, and  monticelllite.   A gem variety of olivine is  peridot, which is green,  and there are green sand beaches in Hawaii.

Posted by oh5/ojhoff at 3:34 PM CDT
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mineralogy ho hum

I've mislaid my mineralogy text and I am postitive that if I have any readers they probably aren't too unhappy.  I know that's some dull reading.  It's more interesting to hear about, for example, scams in the world of colored stones and who got by with what.  All the juicy details.   I am not a  purist but I am upfront about rocks that have been altered, if I know about it.  I wet down a dry slab now and then to show what it will look like polished.  But sometimes I don't.  Depends on what it looks like and how it strikes me.  I was reading about dealers who fiddle with their photos in adobe.  I have done this but not to deceive people.  It is that sometimes I see one thing with the naked eye and another in the photo, and I am trying to make the two match. 

It's a chilly day.  Maybe I should bake.  Grandma used to make oatmeal cookies that she called rocks.   

Posted by oh5/ojhoff at 1:22 PM CDT
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next topic

I don't have access to the mineralogy book I am studying, so I have given some thought to the next topic I might explore.  The word tektites popped into my head.  These are the small bodies of silicate glass that are believed to be of extraterrestrial origin.  Their similarity to terrestrial obsidian caused them to  be referred to as obsidianites.  But more evidence has come in that they did not originate on earth. 

I am photographing, pricing and posting the minerals I got in a trade.  My business is not concentrated on mineral specimens and it moves rather slowly so I think a majority of these will end up in my private collection.  

Fall is in the air, but it's still nice and warm.   

Posted by oh5/ojhoff at 10:19 AM CDT
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There are two series of garnets, the pyralspite and the ugrandite.  Within the two series are three varieties each, for a total of six.  Many individual specimens are a gradient between two types.   All have gemstone potential, are glassy, hard, and transparent to tranlucent.  Garnets come in all colors, includinga  recently discovered blue, but red and green are the most well known.  Garnets were formed under hydrothermal pressure and are found in pegmatities and granites. 


As the birthstone for January, the garnet is the first of my revised and enhanced informational videos I have done and is linked to the front page of my website. 

Posted by oh5/ojhoff at 6:13 PM CDT
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great trade
It has taken me a while to unpack the box I got from someone with whom I made a rock trade.  This person sent me a lot of intriguing stones, some of which I was unfamiliar with.  I was amazed to open a newspaper and napkin packet all done up with masking tape and discover a garnet almost the size of a baseball.  Plus two other garnets including one from Norway.  My next chapter in the mineralogy book deals with the garnet group of silicates so that works in well.  Was also pleased with a whale bone fossil from the Miocene and  specimen that includes both gold and silver, plus iron pyrite.  And, oh yes,  bluish  kyanite.   Samples too numerous to mention here.

Posted by oh5/ojhoff at 4:26 PM CDT
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