Another mineral, to the tune of another...

We recently shared a photo showing a mineral that had replaced another during the geological journey through time that those particular rocks were subjected to (see http://on.fb.me/1yoNeiF). It is an example of a pseudomorph, a mineral that progressively takes the place of another, right down to preserving the crystal shape of the original, despite forming in its usual crystal system. Here is another example, of the copper lead arsenate bayldonite (see http://on.fb.me/14NwiEF) standing in for bladed crystals of the lead arsenate mimetite (already formed in turn by the oxidation of primary sulphide minerals into a secondary one, seehttp://on.fb.me/1zo1vvI) Some fluids passed through and their chemistry allowed them to add copper to the mimetite, changing it into one of its cousins. The specimen comes was mined from the upper levels of the Tsumeb mine in Namibia sometime before the first world war, and measures 3.8 x 3.6 x 2.7 cm.

Loz

Image credit: Rob Lavinsky/iRocks.com

One mineral, to the tune of another

The world we live is ever changing, including the rocks and soils beneath our feet. Metamorphism of many kinds is transforming minerals into ones more appropriate to their pressure/temperature environment. Similarly exposed deep crustal and mantle sourced volcanic rocks are being chemically weathered towards their stable form at the surface, ending up as clay minerals. Nearer to the surface, as anoxic rocks that are gradually being exhumed by erosion encounter oxygenated and carbonated groundwater on its journeying beneath the ground, minerals are changing. Most metal rich rocks start life as anoxic sulphide minerals, spat out of a cooling granite or a deep sea vent system on a oceanic spreading ridge. As they near the surface and meet the waters, they turn into oxides and carbonates. The usual sequence is oxygenation first, carbonation second, and the specimen in the photo illustrated the process perfectly. The copper sulphides first turned into beautiful stubby prisms of the visually stunning red oxide cuprite, also known as ruby copper (seehttp://on.fb.me/1AjbRrW andhttp://on.fb.me/1Cbe5f9). Sometime further than the line, the cuprite transformed into the more familiar blue carbonate azurite. If the process carried on some more, the latter would change in turn into its green cousin malachite. We recently shared a stunning specimen of the latter transformation athttp://on.fb.me/1CbfPFa.

The beauty in the photo measures 1.4x1.3x0.8 cm, and comes from the Rhone Alpes region of france.

The world beneath our feet is one of endless slow and fast changes. Bacteria have been found as deep as we've been. Plates and rocks are moving and grinding at very scale, elements are being distilled out of the mantle and crust by many kinds of magma and the surface skim of life is revealing ever more complex layers of its interaction with minerals, rocks, air, tectonics and oceans.

What a wonderful world is ours, and how fortunate are we to live in a golden age for geoscience.....

Loz

Image credit: Joe Budd/Rob Lavinsky/iRocks.com

Calcite cube Calcite forms a bewildering away of crystal shapes, most commonly bladed or in a distorted rhombus. Sometimes though it doesn't form in one of its own shapes, but replaces crystals of another mineral, a process known as pseudomorphism. It occurs when the chemical conditions change, for example when heated mineralised waters pass through rock. The chemistry determines whether they will dissolve a mineral and leave another in its place or not. In the case of this beautiful white cube measuring 5.5x5.6x3.8cm, I'm not sure what it replaced. Loz Image credit: Rob Lavinsky/iRocks.com

Quite a process What you have in the accompanying photo is an example of a peculiar type of pseudomorphism. Most cases involve one mineral being replaced by another in a straightforward manner as passing fluids of appropriate chemistry leach the elements of one away while precipitating another set in their place so that the second mineral takes on the crystal; shape of the first rather than its own usual habit. Here the gold was first precipitated in a vug (a cavity) filled with crystals of Quartz and Ankerite and a subsequent event dissolved the two minerals leaving behind a gold nugget imprinted with their distinctive crystal shapes. Such epimorphs involve the hollow cast of the original minerals being filled and surrounded by the new mineral, and in this case the bonus joy is its lovely golden nature. Loz Image credit: Kevin Ward and The Mineral Gallery

Malachite pseudomorphing gypsum After minerals have crystallised, it can happen that new element laden waters pass through the same rocks, leaching some elements and leaving behind others depending on their chemistry, acidity, salinity and element content. Here copper laden waters passed through an evaporite sequence of gypsum, where long gone seas and lagoons repeatedly evaporated leaving behind their dissolved salts. We recently shared a similar photo where the mineral chrysocolla had replaced gypsum as well at http://tinyurl.com/q39gy6v Loz Image credit: Labyrinth Minerals

Chrysocolla pseudomorph Pseudomorphs are crystals of a mineral that have replaced another, often as the geochemical or temperature conditions change in hot hydrothermal fluids moving deep within the Earth or as these solutions permeate different rocks from the ones that they originated in. In this case, beautiful crystals of the hydrated copper silicate chrysocolla (see http://tinyurl.com/qz8dvsx) have replaced crystals of the evaporite mineral (resulting from evaporation of lakes or inland seas in arid environments) gypsum. Loz Image credit: Jake Slagle

Beautiful replacement

After minerals first form it is not uncommon for a change in geological conditions such as fluids of different chemistry percolating through to change them into other related minerals, picking up elements and taking them elsewhere or precipitating others brought from other areas that they picked up earlier. This often results in one mineral replacing another, and while massive minerals often don't reveal what has happened to the naked eye ones that have crystallised into distinctive crystal shapes (known as a mineral's habit) can retain it while being totally replaced by one that normally has a very different form. In this geochemically unusual specimen, barrel like crystals of the lead phosphate Pyromorphite (see http://bit.ly/2DicURy) have been replaced by the lead sulphide Galena (see http://bit.ly/2fLDz1E).

I call this specimen unusual because the more normal version has an oxidised mineral replacing a reduced one (see http://bit.ly/1I4XWKt for an explanation of these terms) as groundwater percolates a metal sulphide deposit rather than hte other way around as we find here. In this case deeper oxygen poor fluids from the Earth's depths more likely rose and altered for a second time a deposit that originated as reduced and was then oxidised only to be reduced again, showing that this cycle can repeat several times for the same mineral during the vagaries of geological time. This rare specimen was mined in the Brittany region of France sometime between 150 and 250 years ago when it was an active iron mine and measures 3.6 x 2.3 x 2.0 cm. We shared an example of this mineral frozen in the act of turning into another at http://bit.ly/2Hpn19R.

Loz

Image credit: Rob Lavinsky/iRocks.com

Once red, then green, now blue

A stunning pair of crystals reveals some of the complex changes that happen in ore deposits as they get uncovered by erosion and their primary minerals (usually dull grey or silvery sulphides) are gradually altered into a succession of secondary ones as the ore is percolated by circulating waters of varied chemistry that move through all the surface layers of our planet. The 1.4 x 1.3 x 0.8 cm piece from France started as the copper oxide mineral Cuprite (see http://bit.ly/1Cbe5f9 and http://bit.ly/1AjbRrW) as oxygenated fluids first altered it, followed by a succession of copper carbonates. It first turned into green Malachite (with some remnants still visible) before changing again into blue Azurite. The final product retains the shape and crystal habit of the original mineral, a process named after the Greek for fake shape and known as pseudomorphosis.

Loz

Image credit: Joe Budd/Rob Lavinsky/iRocks.com

An unusual pseudomorph

It is common for one mineral to fake the shape of another since geological processes (usually involving hot fluids as they cycle in giant cells through the crust heated by depth or magma) frequently rearrange the geochemistry of rocks. Here we have one oxide of manganese (Hausmannite) replacing another (Manganite) taking on its distinctive crystal shape as opposed to its usual pseudo octahedral habit (ie looking like it crystallises in the cubic system like diamonds or fluorite, but actually doing so in another system, tetragonal in this case, but mimicking the shape of another). Our culprit varies from opaque brown through grey to black, and is relatively soft (5.5 on Mohs hardness scale) and was named in 1827 after a professor of mineralogy at Gottingen University who first described it, being renamed in his honour a couple of decades later by a colleague. Its type locality (where first discovered) is in the Thuringia region of Germany, where the 4.0 x 3.2 x 2.9 cm specimen in the photos comes from. It is also found in Arkansas, Sweden, the Ural Mountains and the Kalahari Manganese Field of South Africa and Namibia. Its typical form is as black granular aggregates, occurring in hydrothermal veins and metamorphosed manganese deposits, in this case replacing the original Manganite in a classic piece mined in the 19th century.

Loz

Image credit: Rob Lavinsky/iRocks.com

https://www.mindat.org/min-1832.html http://www.galleries.com/Hausmannite http://bit.ly/2zuGd5p

One, replacing seven...

Mineral hardness is measured in a variety of ways, the best known and the most rough and ready being Mohs hardness scale, which compares the relative hardness of ten minerals by the simple expedient of playing the game of who scratches who. It is not a linear scale, with the steps between 8 and 9 (topaz and corundum) and 9/10 (corundum/diamond) each being orders of magnitude greater than the previous ones, but it does serve for basic field tests. In this beautiful 5.7 x 3.8 x 3.1 cm specimen from Bavaria, the softest mineral has replaced one of the hardest, providing a nice excuse for a digression into the clay mineral talc.

We are all familiar with this clay, most of us were liberally sprayed with it in powder form as babies, and many of us continue to do so on a regular basis throughout life. Its colour ranges from the well known white to grey or green, and in massive form its metamorphic variant has a greasy feel and is known as the carving material called soapstone. The name dates from antiquity, and comes from ancient Persian.

It is formed by the chemical weathering of magnesium rich volcanic minerals such as pyroxene and olivine in the presence of water and CO2 or by a metamorphic reaction between the magnesium rich limestone known as dolomite and silica rich rocks, often mediated by the hot mineralised fluids of intruding granites spewing into the surrounding limestones, forming complex mineral assemblages known as skarns. It can also form at the deep pressures encountered at the keel of continents and in rocks that have been dragged deep into subduction zones and spat back out.

As well as talcum powder it has many uses, including whitening paper, a filler in plastics, ceramics (in the body and in fluxes or glazes) and paints, as a lubricant, i rubber, and varied industrial and laboratory uses that take advantage of its resistance to heat, electricity and chemicals. The top mines are in France, China, South Korea, India, United States, Finland and Brazil. Here talc has replaced a crystal of quartz in a process known as pseudomorphism (literally translating as faking the shape).

Loz

Image credit: Rob Lavinsky/iRocks.com http://www.minerals.net/mineral/talc.aspx https://www.mindat.org/min-3875.html http://www.galleries.com/talc http://bit.ly/2kKgFth http://geology.com/minerals/talc.shtml

Mineral seaweed.

This stunning specimen of malachite resembles a marine boulder encrusted with greenery, which can almost be seen to undulate in a gentle marine current. The fronds are pseudomorphs, the replacement of one mineral in its original crystal shape by another, in this case azurite (another copper carbonate mineral). The green bubbles are secondary malachite, showing a habit called botryoidal.

The specimen is from Brazil, and measures 10x8x5 Cm.

Loz

Image credit: Rob Lavinsky/irocks.com