Southwest Greenland. In among the even older rocks is a huge intrusive igneous complex formed more than a billion years ago. Exhumed by erosion to form the dramatic topography of fjords and glacial terrains and the subject of geological scrutiny, these rocks are indeed complex; many different varieties reflect a series of pulses of intrusion and the compositional evolution of the fluids in the original magma chamber, not to mention their chemical interactions with the rocks into which they were intruded, intrusions into intrusions. Exposed not only to geologists but to weathering, the rocks of the Ilímaussaq complex are rotting and disintegrating, shedding their detritus into scree (or talus) slopes along the flanks of the cliffs and, ultimately, into rivers and fjords. Amongst this detritus are, of course, sand-sized grains reflecting the sizes of the original minerals, new-born sands. And, in amongst the many wonderful samples sent to me not long ago by a very generous arenophile colleague in the Netherlands, was a sample of these sands from the area of Kvarnefjeld, a modest mountain within the intrusive complex not far from the town of Narsaq. This area, and the overall location are shown below:
The entire Ilímaussaq complex is made up of exotic rocks with exotic names. Overall, these rocks contain little or no quartz and are thus relatively low in silica and referred to as alkaline; commonly, the dominant minerals are feldspathoids, resembling feldspars but with a different structure and much less silica. In terms of their broad family background, they belong to the group of alkaline igneous rocks termed syenites, but many of their specific compositions and minerals are exotic. There’s an excellent and quite detailed summary of the geology and mineralogy in the Geology of Greenland Survey Bulletin from 2001, and the rock types shown on the geological map include lujavrite, arfvedsonite lujavrite, kakortokite, naujaite, sodalite foyaite, and pulaskite. Give a petrologist a slight variation in composition and off they run conjuring up entirely new and fantastic names, challenges to remember or even spell. In fairness, of course, many of these names derive from the locality at which the rock type is first identified so a degree of cultural and linguistic tolerance is in order, but even so….
Exotic rocks contain exotic minerals, and there are plenty of those to be found in the Ilímaussaq complex, the type locality – the definitive location – for many of them. In among the rock fragment grains in the sand at the top of this post, and embedded in some of those rock fragments, are diamond-shaped minerals: these are the mineral naujakasite, a sodium-iron-manganese-aluminium silicate with the formula Na6 (Fe+2,Mn+2) Al4 Si8 O26. The original specimens came from a small boulder collected in 1897, but the mineral was tentatively identified as a variety of the mica-like mineral, chlorite. It wasn’t until 1933 that it was recognised as a new mineral and was named after the place where it was found. It later became clear that naujakasite occurred widely through rocks of the complex, particularly in the lujavrites, but for a long time this was its only known occurrence in the world. But in 1998 it was found in the Lovozero alkaline complex of Russia’s Kola Peninsula – and today these remain the only two known locations of this mineral. A full description of both can be found in the same Geology of Greenland Survey Bulletin linked above.
Naujakasite itself is of interest for its rarity, but it has no particular value or use. Not so with the rock-type in which it’s found, lujavrite. This is an important source of uranium and, of even greater interest today, rare earth elements. These comprise the fifteen lanthanides and yttrium, and were named rare earths after their original identification in the 18th century in rare minerals. These elements have a wide variety of clever and critical roles in all kinds of high-tech applications and were for a long time extracted from placer sand deposits around the world. Today, however, the market is dominated by China, the minerals extracted from hard-rock ore bodies in Mongolia. Given the economic importance of these elements, this is creating momentum for the search for alternative supplies: the Ilímaussaq complex is one focus of this effort; an example of this, with a good geological description, can be found in the recent bulletin by Greenland Minerals and Energy Ltd. Here’s an example of the occurrence of a black lujavrite from that publication – shedding its sandy detritus into the fjord below.
Exotic rocks, exotic minerals, exotic names, and exotic locations – many thanks, Carla, my arenophile friend, for providing the source of this Sunday sand story.
Rather fascinating as ever. A Quick question. Unsure how short the answer. I assume that I understand (to some degree) what happens when organic matter rots. I associate rotting with the work of living agents such as bacteria, molds, fungi, etc. All sorts of buggy things.
What happens when rocks rot? I would guess that the answer would vary with type of minerals involved, the climate and other environmental factors. Dissolution? Chemical reactions causing change in structure and consequential mechanical integrity (and solubility, etc.)?
Or are biological processes sometimes (or always) involved?
I've heard of rocks rotting before but just realized that I have no idea what it means.
Thanks.
W.
Posted by: Walter | November 14, 2010 at 05:36 PM
Hi Walter - you make a good point, and here's a reasonably short answer. Yes, "rotting" is generally associated with organic materials, but I'm exercising a little poetic license here (not that I pretend to be a poet)and using the term rather broadly. Because, in many ways, rocks do rot - it's primarily a chemical process of breakdown, caused by the atmosphere, water - and organic processes. Any mineral that formed below the surface of the earth under higher temperatures and pressures will, once it's exposed at the surface, be out of equilibrium, unstable. The greater the difference between surface temperature and pressure and the conditions of original formation, the more unstable it will be.
In, for example, an igneous rock, say a granite, the feldspars and micas are extremely unstable and will succumb first to chemical attack, dissolving and disintegrating. The relatively stable quartz grains will then be left without support, and, like old teeth, drop out to form quartz sand grains. We know that microbes can penetrate deep into rock via cracks and fissures and their biochemical activity accelerates the whole process.
You can walk up to an outcrop of weathered granite, once a hard and solid rock, and simply scrape it off into your hand - it has lost its integrity and original character, and, I think it's fair to say, rotted....
Posted by: Sandglass | November 14, 2010 at 08:38 PM
In the short term (for geology), this loss of stability has serious consequences for buildings. Somewhere I saw an architectural conservator for Venice quoted as lamenting the absence of treatments for "diseases of stone."
"Competent rock" is a commonplace, a lovely Disneyish image of stone dutifully and skillfully holding itself up. Deep time reveals process analogies among animal, vegetable, mineral, as speeding up earthquake vibrations makes them sound like whale song.
Posted by: Richard Bready | May 22, 2011 at 10:59 AM