Mica book

Most mineralogy class collections around the world have a specimen like this: a book of mica, typically the specific mineral muscovite.

Mica minerals are sheet silicates. These minerals have strong chemical bonds holding them together and extending out in 2 directions and weak chemical bonds in the third direction, causing the formation of a single dominant cleavage plane surface.

Literally, mica minerals stack up like sheets in a book. They can be peeled off one sheet at a time and large enough sheets can even be used industrially to cover open gaps.

This mica book comes from a pegmatite deposit in the Russian Ural Mountains. Pegmatites are the last dregs of the crystallization of large magma bodies. When magma bodies crystallize, they form common minerals like quartz and feldspar, but components like water and some minor elements don’t go into those minerals and just stay in this last-gasp fluid. Those fluids are hot and mineral loaded, allowing them to grow very large mineral grains and providing some of our most impressive mineral samples. This mica book has grown along with the mineral topaz from the same fluid.

-JBB

Image credit: http://commons.wikimedia.org/wiki/File:Topaz-Muscovite-189213.jpg

Read more: http://geology.com/rocks/pegmatite.shtml

Check out the cleavage lines on the crystalline hematite

Wow check this one out! It has the classic amphibole cleavage angle - watch for the 2 planes that are 56° apart, and the color is the classic green that chromium creates when it is substituted into a mineral. It’s like 3 of my mineralogy classes in a single crystal!

Slate and bedding

This is one piece of a slate, a rock that cleaves or breaks into nearly flat pieces. Slates develop this fracture pattern through metamorphism. The rock started off as a fine-grained sedimentary rock such as a mudstone. It was caught up in a mountain building event where the rocks were folded, compressed, heated, and squeezed. In the process, new minerals began growing, including mica minerals that form impressively flat surfaces. The newly grown micas are so small you can’t see them with your eye, but because they organize their flat surfaces perpendicular to the direction the rock is squeezed, they control how the rock fractures. Basically, as the rock was squeezed, it grew a bunch of new, flat minerals in response, and now the rock breaks along the weak plane in those flat minerals.

The light and dark pattern is a remnant of the original bedding in the rock. Metamorphism doesn’t care about original sedimentary features in the rock; the rock gets squeezed and heated in patterns that cut right across bedding. 2 beds with different composition, such as a siltstone and a claystone, will lead to different rock compositions when metamorphosed. In high grade metamorphic rocks there is often no trace of original bedding left, but here the rock has only been lightly metamorphosed, so bedding can still be seen as a pattern cross-cutting the foliation.

-JBB

Image credit: James St. John https://flic.kr/p/rMhDzj_ _

Ooh so shiny...

This is an unbelievable garnet.

Triplite

First described after a discovery in Western France in 1813 this rare phosphate mineral forms in the last phases of cooling granitic magmas that frequently form large crystals known as pegmatites. It was named after the Greek for three fold as it has perfect cleavage in 3 directions, said property being a tendency for a crystal to split (including diamond) along lines of weakness induced by the geometry of the crystal structure. The weak planes occur where there are less atomic bonds holding the atoms together. It incorporates manganese and iron in its formula, hence the rosy hue that is characteristic of Mn as a light absorbing chromatophore (colour inducing element) that leaves the non absorbed residual wavelengths as the colour that our eyes perceive.

Along with rosy brown, yellow and dark reddish are other common colours and material that has been altered geochemically is often black, and its Mohs hardness is 5.5, enough to allow material like this to be faceted for collectors. Being incredibly rare in gem quality (and very rare full stop) such pieces command high prices. It is found in various places in the USA, the Shigar Valley in Pakistan (where this 1.2 x 1 x 0.9 cm crystal was found), China, Germany, Finland and the Kalahari manganese Field in Namibia.

Loz Image credit: Rob Lavinsky/iRocks.com

https://www.mindat.org/min-4021.html http://bit.ly/2o07LLy

Why is obsidian so useful for tools?

The ancient inhabitants of many continents knew the properties of obsidian. This black volcanic glass was a key component in tools and hunting weapons; arrowheads and shards from their production are found all over the world and trading paths between different civilizations can even be tracked using obsidian.

The reason why this rock was so useful comes from the structure of the rock. Obsidian isn’t a mineral, by definition. Minerals have a defined structure that repeats over and over again. Obsidian is what we’d call a “glass”. A mineral growing from lava needs time to grow; atoms need time to move together and form a defined structure. If lava cools off too quickly, it can instead have all its atoms locked into whatever format sat there when the magma was molten, a state we call a glass.

A glass has no defined, long-term structure, so it doesn’t break into crystal faces. This property means glasses are strong in all directions and when broken they will have what we call “conchoidal fractures”. This is different from crystals; they tend to break along fracture or “cleavage” plains controlled by the arrangement of the atoms. You can see the remnant of those fractures in the rippled breaks at the edge of this stone tool artifact; the fractures formed at a single point and widened as they broke outwards.

A skilled worker using obsidian can create a series of conchoidal fractures around the edge that bring the rock to an extremely sharp point. The angle of the tip won’t be limited by the natural crystal shape; instead the spear tip can be made both strong and sharp.

Obsidian is generally made out of high silica, rhyolitic lava. These high silica lavas are very viscous and therefore crystals don’t grow rapidly on them, making obsidian formation easy. Different obsidian compositions and structures do behave differently during processing, so some obsidian sources were highly prized and rocks that match in chemistry were traded across thousands of kilometers, covering entire continents.

-JBB

Image credit: John Atherton (Creative Commons): https://www.flickr.com/photos/gbaku/1287124990/

Read more: http://www.texasbeyondhistory.net/st-plains/prehistory/images/distant.html http://volcano.oregonstate.edu/obsidian http://www.public.asu.edu/~mesmith9/1-CompleteSet/MES-10-TradeEncyc.pdf http://www.fieldmuseum.org/node/4766

Topaz crystal surface and cleavage.

One of the characteristics of topaz is its perfect cleavage, which has nothing to do with the usual meaning we associate with this word, though it's a constant of corny geological jokes of the 'geologists make the bedrock' type. In minerals cleavage is a consequence of crystal structure and the regular arrangement of atoms in a lattice. In some crystals this arrangement has lines of weakness, where there are fewer bonds holding the atoms together and along which the crystal can easily break or be separated. Those of you who have played with a 'book' of mica crystals will have seen how it splits easily along parallel cleavage planes like pages in a book.

Cleavage is used to identify minerals, as its presence or absence will narrow the possibilities. it is described in terms of ease of splitting (called difficulty) and quality of split (eg perfect smooth faces like topaz or mica, or poor). If it is present, one then has to check if there is more than one smooth broken surface, and in which directions and angles they intersect to try and identify the crystal, obviously in conjunction with other properties like colour, lustre (how strongly it reflects light) and hardness. One may even have to try and reconstruct the crystal shape in a battered specimen.

Even diamond has cleavage, perfect in four directions, parallel to the faces of an octahedral crystal, a property used to break larger diamonds into smaller ones for faceting. A notch is cut in the crystal with another diamond, and a hammer and chisel used to split it. Needless to say this requires considerable planning and caution, a steady hand, and plenty of experience in squeezing value out of a rough stone, though nowadays CAD programs such as SARIN are used to supplement the experience of the cutters. When cleaving the largest diamond ever found, the Cullinan, the cleaver apparently fainted from nervous tension after striking a perfect blow that split it into the pieces that now grace the British crown jewels.

Topaz has one direction of perfect cleavage, which is parallel to the base and top of the crystal. Its crystals nearly always have flat bases, sometimes with steps, where pieces of the crystal have cleaved off. This photo shows some of these steps taken using a lighting technique called Nomarski differential interference contrast, that plays with wavelengths of light in order to reveal features that would otherwise be hard to see clearly. Calcite in contrast has three, resulting in the perfectly sculpted rhombs one can see in any mineral shop.

Needless to say, this property also makes it very difficult to facet such crystals, and they have to be orientated in such a way that placing it on the diamond impregnated polishing wheel will not lift off whole layers of the crystal along the cleavage planes, leaving you with a pile of dust and chips and no faceted gem.

Similar to cleavage is parting, but it doesn't occur along planes related to the arrangement of atoms in the crystal, but along twinning planes (where two or more crystals with different orientations have grown together) or stress lines imposed by its geological history (being squished in a mountain building episode for example). Crystals with several cleavage directions are invariably tricky, and faceters often get a high failure rate.

Loz

Image credit, magnification 30x: John Koivula

http://www.minsocam.org/msa/collectors_corner/id/mineral_id_keyi6.htm

http://www.minerals.net/resource/property/cleavage_fracture_parting.aspx