[ Hello All - Before Rock Net went off-line early last year, a number of the regulars had started doing sort of "book reports" on various minerals and mineral localities for the gang here. I had started working on a write-up on quartz and its bewildering array of varieties and sub-varieties - but the site went down before I got a chance to post it. I went hunting for the draft in the guts of my electronic beast lately, but couldn't find it. So I started from scratch, trying to recreate it. I'm not so sure how successful I've been at that; but what follows is at least a reasonable facimile of the original - with some changes and recent additions that I've made on purpose.]
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..........QUARTZ..........

Quartz is considered to be the single most common species of mineral found in the crust of our planet. There are very few mineral environments where it isn't found, and when rocks not containing it are found you can bet that rocks containing quartz won't be too far away.

Chemically, quartz is SiO2 - silicon dioxide. The silicon and oxygen in quartz are arranged in a pyramid fashion, with four atoms of oxygen forming the points of the pyramid and a single atom of silicon located in the center - sort of the mummy king entombed within. The "SiO4 tetrahedra," as it is called, is the basic building block of all Silicate minerals - what makes them Silicates. The SiO4 tetrahedra in quartz are linked to one another in a framework pattern which repeats over and over again throughout the mass of any given specimen - millions upon millions of times in an "average" hand-sized sample. The interlinked framework pattern is the hallmark of the Tectosilicate Superclass of Silicate minerals, of which quartz is a member.

In quartz the organization of the silicate tetrahedra produces Trigonal crystals, typically six-sided prisms with pyramidal terminations - the simple quartz crystal shape we are all familiar with. Even in a massive chunk of typical milky quartz which doesn't show any crystal faces the crystalline structure is still there at the atomic scale - one doesn't necessarily have to have an actual crystal of a mineral in order for it to be crystalline: We may not see the crystalline nature of the mineral, but it's still there...

Quartz forms in a wide variety of environments in a variety of geologic settings. The outward appearance of any given sample of the mineral is controlled by the conditions in which it forms - particularly temperature, pressure, and the precence of other elements, with temperature perhaps being more influential than anything else. Thus we find huge crystals that formed in the high temperature and pressure environment of a hot igneous pegmatite, and microcrystalline material that formed at nearly surface conditions. It is this wide range of conditions under which quartz will form which gives us the wide range of varieties and subvarieties that we find.

Quartz varieties are divided into two main groups: *Macrocrystalline* and *Microcrystalline.*

*Macrocrystalline* quartz forms crystals that are visible to the naked eye, and includes varieties such as smoky quartz, milky quartz, amethyst, citrine, and colorless rock crystal. Occasionally we see hybrids of these color-based varieties, such as "amatrine", in which two or more of the colors are combined in a single crystal. As noted above, one doesn't have to see crystal faces in a macrocrystalline sample of quartz for it to be macrocrystalline: A chunk of milky quartz (or smoky, or amethyst, etc...) broken out of a vein of it is as macrocrystalline as the crystals of milky quartz in a plate of crystals. In fact, the rose quartz variety - which is macrocrystalline - is only rarely found as good crystals, and those are usually rather small. The important thing to remember is that macrocrystalline doesn't necessarily mean visible crystals - it just means that the internal structure is repeated over *relatively long* distances.

In *Microcrystalline* varieties and subvarieties of quartz the internal structure is still there - all those silicate tetrahedra linked up in that framework pattern - but the pattern is not repeated over long enough distances to be able to produce outwardly visible crystals; instead the pattern is broken into smaller "bites" - so small that it takes extreme magnification to find it. And when you do find it, you find that it is found in the forms of grains and fibers; so microcrystalline quartz can be divided into two groups: *Granular* and *Fibrous.*

In *granular" quartz the crystalline structure is found within small - usually microscopic - grains of the material. The repetitive framework organization of the silicate tetrahedra is found in each grain, but the grains are oriented at different angles to one another, so the crystalline pattern in each is not lined up with the crystalline pattern of its neighbors. The structure is bounded by the surface of each grain. Perhaps a good analogy is a "butcher block" counter made of maple versus a solid maple board: In a solid maple board the pattern of the wood is repeated throughout the length, width, and thickness of the board - while in the butcher block counter the pattern is contained within each block of maple, with the blocks, arranged at angles to one another so that the pattern isn't seen across the entire surface. It's broken up. The solid maple board, then, is our macrocrytalline quartz, while the butcher block counter is our microcrystalline quartz.

In *fibrous* quartz the crystalline structure is found within elongated fibers of the material - strings of 3-dimensional SiO4 tetrahedra, rather than grains. As in granular varieties, each fiber contains the pattern and the pattern is bounded by the surface of the fibers - the structure doesn't repeat in an aligned pattern across the fibers. Typically, the fibers in fibrous varieties are arranged parallel to one another, but it is also possible for them to be tangled. While poring over the technical literature to read up for this report I looked for specific mention of tangled versus parallel patterns, but couldn't find any - just the bare mention that either was possible. So perhaps this is a question that is still ripe for research. (Or perhaps I simply didn't dig deep enough? :~} )

At this point in the report we can now divide quartz into macrocrystalline and microcrystalline groups, and divide the microcrystalline group further into fibrous and granular subgroups. And we know that the varieties which form visible crystals all fall into the macrocrystalline group - amethyst, citrine, etc... But what about the many varieties (or subvarieties) of microcrystalline materials? Which of them are fibrous, and which granular? - And how do we tell them apart, if we can?

To start with, all fibrous varieties fall under the general name "chalcedony." So if a sample of quartz is shown to contain fibers of the mineral, it is a chalcedony. Alas, it isn't so simple with granular varieties; it would be nice of we could say that all of them fall under a single name - such as jasper - but that's not the case. To make matters worse, there are hybrids - materials containing both fibrous and granular material... - But let's stick to the chalcedonies for the moment - get the fibrous varieties dealt with.

Brown to grey chalcedonies with no distinct patterns apparent in them are simply known as "chalcedony." If it is medium to dark brown it is called "sard." Sard grades into red "carnelian" - with no good dividing line between them. Some might call a sample towards the middle a "sard" while others might still condsider it to be a "carnelian." It's rather subjective - not really "mineralogy," more "art." :~} "Chrysoprase" is a green chalcedony, the green caused by nickel oxide in the material as an "impurity." "Heliotrope" (aka "bloodstone") is a hybrid - a green chalcedony with spots of red jasper (a granular variety) in it.

Note that none of the above mentioned varieties contain any distinct patterns of layers or bands of different colors. They may be mottled, may contain inclusions (as in heliotrope), and may have gradations in color over a distance, but they do not have distinct zones of color. The next distinction made in the fibrous varities is that of layered or banded colors - onyxes and agates.

In "onyx" the color banding is in flat, parallel, zones. In "sardonyx" - a subvariety of onyx - the layers are alternating white and black ones. Other onyxes may be any colors that might be found.

"Agate" is defined as chalcedony which has concentric bands of color - the bands being more-or-less circles around a central point. But in reality agates are a rather complex group of materials - with the banding often difficult to decipher as being "concentric." We see zig-zag patterns, and wavy ones, to name a couple. And the supposedly circular/concentric nature of the banding can be pretty hard to see - sometimes even impossible in small samples. Nor is the definition rigid in it's requirements regarding "circular" - more often than not the banding is laid out in ovals rather than actual circles. Actual circles are probably quite rare. But, by-and-large, the banding IS *concentric* - given a big enough sample we can see how the colored patterns more-or-less circle around a central point.

With small pieces of onyx or agate it can be hard to tell which you have: Is that seemingly flat layered chalcedony a piece of onyx? - Or a piece of agate taken from the flattest section of an oval chunk? Sometimes you just have to take it on faith that the label is accurate - or close your eyes and pick if you are trying to stick a label on something that doesn't have one. :~}

The "granular* group includes jasper, prase, flint and chert - the last two more by tradition than by scientific rigor, as they are actually rocks, not the mineral quartz... Strictly speaking, the variety known as "jasper" is a red granular quartz - with "red" being the key word, and being due to hematite inclusions. But over the years the definition has been broadened to include both blue and yellow materials. A "classical mineralogist" would not call these other colored materials jasper; but with the quartz varieties and subvarities we have really left the bounds of rigorous mineralogy and entered the world of art: We needn't be too picky. :~} "Prase" is a dull green colored material, basically a green jasper, and it occurs with red jasper - so we can, for all intents and purposes, say it is a green subvariety of jasper.

"Flint" and "chert" are somewhat problematic. Not only are they technically rocks - not just quartz - but there is no clear distinction between them. My copies of Dana's Manual of Mineralogy say: "Flint and chert resemble each other and there is no sharp distinction between them. Dark, siliceous, nodules, usually found in chalk, are called flint; whereas lighter-colored bedded deposits are called chert." And we typically use the color - dark or light - to make the distinction; which works fine until you get a light colored flint or a dark colored chert... Perhaps a better distinction is the environment it is found in: If it is as nodules in chalk or chalky limestones, it's flint; but if it's found as beds between layers of limestones (or chalks?) then its chert. Of course if you didin't see the rock it came out of - and whether it was as a nodule or as a chunk out of a bed - how are you going to know?

Then there is the very inclusion of these two materials as varieties of quartz to deal with. As I noted above, flint and chert are - technically speaking - rocks, not just the mineral quartz. A microscopic examination of them shows them to contain at least small amounts of other minerals - typicaly calcite, dolomite, and perhaps one of the Mica Group species, such as phlogopite or muscovite. Still, they are basically what are called "monomineral rocks" - composed largely of a single mineral, in this case quartz. And they are granular. Also, so far as how they "work" in the hands of a lapidary or flint knapper, they have all the characteristics of a granular quartz material. So traditionally they are treated as varieties of quartz by rockhounds, and even many mineralogists - it's just the sedimentary petrologists who really know better - and, now, you... :~}

As noted above, there are hybrids containing both fibrous and granular varieties. I mentioned heliotrope as one that is largely chalcedony, but containing spots of red jasper. There are also quite a few jasp-agate and ag-jasper combinations - which shorthand you use depending on which of the two is predominant. (What you call it if they are present in more-or-less equal amounts is up to you! :~} )

Then there is the matter of included quartz "varieites." Such as rutilated quartz and aventurine. Both of these are basically macrocrystaline quartz containing rutile needles (rutilated quartz) or hematite or a green chromium mica (aventurine). When you stop to think about it, almost all colored quartzes are included quartz - the color being imparted by whatever impurities are present. Smoky quartz and rose quartz being the exceptions, their colors being due mainly to irradiation causing dislocations in the structure that reflect light differently than quartz which has not been subject to radiation. But the colors seen in chalcedonies, onxyes, and agates are all basically due to inclusions of other elements or minerals. The same holds true for granular varieties - jaspers, prase, and flints and cherts.

Someone is bound to say at this point (if they haven't said it earlier...): "But what about things like the classic moss agate from India? That's a chalcedony with a mottled pattern of inclusions and no concentric banding; but it is still called an 'agate'." This leads us into a discussion of *misnomers* - the incorrect use of terminology. The indian moss agate isn't, in fact, a true agate. It is a chalcedony containing moss-like inclusions. Technically speaking, it is a "moss chalcedony." In fact, there are quite a few materials called moss agates which would be better described as moss chalcedonies.

In the past few years we have been seeing "crystals of prase" talked about - and offered for sale. This is another misnomer. Prase, by definition, is a microcrystalline, granular, variety of quartz - it doesn't (can't...) form crystals. The material in question is actually a macrocrystaline quartz - milky quartz, I believe - which is included with something that gives it the same or a similar dull green color that we see in prase. But it is not prase - can't be, given the definition of prase. Another nickname is need for this critter...

Another common misnomer is "agatized wood" for petrified wood that has been turned to chalcedony. Here we can see concentric banding - but in this case it is not due to the deposition of fibrous quartz in concentric color bands, built up layer by layer as the chemical conditions vary. Rather what we are seeing is a banding that pre-existed - the growth rings of the trees - and which has been preserved as the silica fibers replaced the wood cells, cell-by-cell. While the two look the same, they aren't the same - different processes are involved. But it is a lot easier to say "agatized wood" than it is "chalcedonyized wood" - so "agatized wood" has emerged as the term of choice, even though it is actually a misnomer... Still, it doesn't hurt to keep in mind that the term is not really accurate - that a petrified tree trunk or limb isn't really a true agate.

Someone recently asked here at Rock Net how one tells jaspers and chalcedonies and agates apart: How do you know if what you have in your hand is one or the other? The above partially answers this question: Chalcedony and agate are distinguished by the lack or existence of concentric color banding. If it lacks banding, it is chalcedony, while if it has banding it is either agate or onyx. But telling chalcedonies apart from jaspers - distinguishing between the fibrous and granular varieties of quartz - is trickier. If it's banded, you know it has to be an agate or an onyx - a subvariety of chalcedony. But if it isn't banded, it's tougher to tell whether you have a chalcedony or a jasper (or even maybe a flint or a chert...) Since the distinction is primarily seen at the microscopic level - grains versus fibers - we are sort of stuck: Barring access to a powerful microscope, what can we do? - I think Jay Bates offered the best solution to this problem that I have heard so far: Chalcedonies tend to be translucent, while jaspers tend to be more opague. If you hold a slab or chunk of chalcedony up to a strong light, the light shines through; but when you hold a slab or chunk of jasper up to the same light the light doesn't shine through - or perhaps does, but only on the thinnest edges of a chunk or along the very edge of a slab. I think this is a good rule of thumb - that this difference in the ability of the two materials to transmit light through them is real, and can be used to make the distinction. But we should be mindful of the fact that jaspers will transmit light in thin enough pieces, or on the thinnest edges of chunks. Only the metallic minerals are truely opaque - won't allow light to pass through them in even the thinnest of splinters. So we have to use the rule judiciously.

Perhaps an equally tough chore is telling massive chunks of macrocrystalline quartz apart from chalcedony: How do you know if what you have is a chunk of - say - milky quartz, or a chunk of chalcedony? While macrocrystalline quartz can - and often does - exhibit the distinctive conchoidal fracture of quartz (which is not seen in chalcedony) it can also - and often does - just have a rough surface, the conchoidal fracture not readily apparent. Then the main difference between the two is their luster. Macrocrystalline quartz always has the vitreous - glassy - luster of the species. But chalcedony tends to have a more waxy luster. So it comes down to a question of whether the sample at hand looks vitreous like a chunk of glass, or duller like a chunk of wax. It takes a little practice to make this distinction - but not all that much. Look at a few pieces of massive milky quartz side-by-side with a few pieces of chalcedony, and you'll have it knocked in no time.

To recap, it is amazing how widely - and wildly - varied so simple a mineral can be. A little bit of simple crystalline SiO2 can range from crystal clear to jet black - or appear mottled or banded. It can sparkly like a diamond, or be a dull, lifeless, "blah" of color... It can be a magnificent crystal - or a wildly paisley riot of swirls and plumes in stunning colors. Just a little bit of simple SiO2 - the commonest mineral around. And yet a mineral of varied and by no means common colors. And all we can say in the end is "Thank you, Mother Nature. - Well done!"

:~}

KOR!

Alan Plante

Copyright (c) 2004 by the author.

From Alan Plante - July 01, 2004 at 12:01:03
Email: alsher[ ]ncia.net