One woman's quest for geological understanding!

Rock/Mineral/Fossil of the weekCategory Archives

Foraminifera under the microscope – A learning experience

Tweet  Today has been a day for joyful microscope experimentation! I recently received a Thin Section (TS) / Slide of Holocene Foraminifera from the Philippines (3357 feet).  Not my area of expertise at all, but very pretty.  And after years of seeing studies where foraminifera were used as proxies for climate change in geological history, and spending so much time incorporating these into my project course and personal research, it seemed a great idea to actually look at some under… Continue reading »

“Tuning fork” graptolites

graptolites final

This is a slab of slate with many specimens of the “tuning fork” graptolites, Didymograptus murchisoni.

Graptolites are very imported index fossils in the zoneation of the British Ordovician.
These are from the Llanvin Series, Abereiddy Bay, Dyfed, South Wales.

Fossil of the week: “Horn Coral”

Heliophyllum (or “Horn Coral”) is an extinct Genus of Coral that existed predominantly in the
Devonian (408-360 million years ago).
This well preserved specimen was found in the beechwood limestone (Clark County, Indiana, USA)

This picture of the Heliophyllum horn coral shows the beaded pattern along its septa lines.

Heliophyllum (Rugosa / Horn Coral) Heliophyllum (Rugosa / Horn Coral)

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Check out new website: 3D fossils – official launch 22/8/13

The GB3D Type Fossils Online project is in the process of developing a database of fossils found in the UK, including links to photographs and a selection of 3D digital models!



This is one of my favourite pieces – especially because it also contains  garnet, black tourmaline and mica!  It was a gift from my mother a few years ago, so no idea where it was collected…

  • Chemical Formula: Al2SiO5
  • Crystal symmetry: Triclinic
  • Crystal habit: Columnar; fibrous; bladed
  • Crystal system:Triclinic pinacoidal
  • Twinning: Lamellar on {100}
  • Cleavage: [100] perfect [010] imperfect with 79° angle between
  • Fracture: Splintery
  • Tenacity: Brittle
  • Mohs scale hardness: 4.5-5 parallel to one axis, 6.5-7 perpendicular to that axis
  • Luster: Vitreous to pearly
  • Streak: White
  • Pleochroism: Trichroic, colorless to pale blue to blue

Kyanite is one of  which are useful indicators of the conditions of metamorphism in metamorphic rocks.   (A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals.)

The Al2SiO5 polymorphs are useful index minerals because they are very common in aluminous metamorphic rocks.  The presence of one of the polymorphs gives a broad indication of the pressure-temperature regime that the metamorphic rock attained.

Kyanite is formed at high-pressures and low-temperatures.

Kyanite Kyanite

(The coexistence of two polymorphs in a rock provides more specific pressure-temperature information because they are restricted to a line in pressure-temperature space.  The coexistence of all three polymorphs defines a specific pressure and temperature of metamorphism.)