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| The polished slice of garnet plagioclase biotite |
While my friend
David and I were talking, waiting for Evensong to start one Sunday, I commented
that it would take a fair bit of heat and pressure to turn the limestone structure
of the cathedral we were at into marble. He nearly fell off his seat in
astonishment as he asked me whether one type of rock could change into another!
The short answer is yes. A slightly
longer answer is that it can, but what becomes what depends on factors
including the elements and minerals in the original rock and the heat and
pressures they are subjected to. Rocks
that have been altered in this way are referred to as metamorphic.
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I first came
across Oxford University’s Museum of Natural History (OUMNH, @morethanadodo on
social media) during Lockdown in 2020. You don’t need me to tell you about how
so many things were stopped in their tracks by Covid and, like me, you’ve
probably marvelled at how science communication has changed in the last five
years. Much of the relevant tech has
been shaped by the need for remote working and video calling. Before 2020, both local groups’ talks and full-scale
conferences were pretty much all in-person events. During and after that same year, Zoom (other
platforms are available) came into its own, and OUMNH hosted a major series of fascinating
talks in their ‘First animals’ series. What
I could see of the Museum onscreen looked fascinating, and visiting it was
rapidly added to my bucket list.
I wasn’t
disappointed when I got there. I’d had a
really good journey in, bumping into one of my friends at our local station and
passing the time of day with other people on the journey, and the weather was
kind. I walked in the Museum’s front
door, and I noticed two things straightaway – the light from the glass roof and
an American black bear (sadly no longer animate) with a ‘please touch’ notice
next to it. You know what? Its fur was incredibly soft!!
There were
so many highlights for that day, so many amazing things to see… There was a Tribrachidium,
a fossil from the Ediacaran biota – I hadn’t realised just how small they
were. One of the things I gain most, I
think, from museum visits is a sense of the scale of organisms. Hummingbirds are tiny, bat skeletons are so
delicate, hatchetfish are much smaller than I’d realised, and at the other end
of the scale, giant ground sloths has incredibly robust skeletons.
I [metaphorically!]
drooled over the mineral specimens and the gemstones and, having recently read Rebecca
Wragg Sykes’ Kindred: Neanderthal Life, Love, Death and Art, I was
fascinated by the Neanderthal tools on display.
They were stunning!
There were fossils
enough for me to pore over for hours, and the very structure of the building
was a geological smorgasbord. The rocks
and minerals, though, the rocks and minerals. I spent several hours looking at
them. As I was looking at the one in
photo above, I thought of the conversation I’d had with David and I thought
that he’d had loved to have seen this specimen.
It’s a beautifully polished slice of garnet plagioclase biotite from India
that had been metamorphosed from a sedimentary rock. The garnets, a variety known as almandine,
are the ‘glacé cherries’, the plagioclase [a variety of feldspar] is the white
mineral that looks rather like icing, and the darker minerals are the biotite,
a variety of mica (that doesn’t look like any food I’ve ever seen).
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