Captain Barnacle Pete Explores Kelmarsh

Granite Ahoy!

Captain Barnacle Pete and Kevin the parrot had searched countless cliffs, caves, and crumbling ruins for treasure, but this latest discovery seemed stranger than anything the captain had encountered before. While wandering beside an old granite memorial, Pete noticed sparkling pink crystals woven through the stone and proudly announced he had uncovered “frozen pirate magic trapped inside the rocks.” Kevin quickly informed him it was probably just geology again, though the captain refused to rule out curses entirely.

This granite has a pinkish appearance, therefore is rich in orthoclase feldspar. Hidden within the crystals are curious twinning patterns formed deep underground as the molten rock slowly cooled over immense spans of geological time. Now Captain Barnacle Pete invites brave EarthCache adventurers to become geological explorers, study the crystals hidden within the granite corner posts, and uncover which type of feldspar twinning lies concealed inside the stone.

Orthoclase Feldspar

Orthoclase feldspar, a key component of granite, often displays distinctive twinning patterns that reveal much about its crystallization and subsequent history. These twinning types are not just aesthetic features—they provide clues about the conditions under which the mineral formed and transformed.

Types of Twinning

• Carlsbad Twinning:
  •  The most straightforward and commonly recognized form is Carlsbad twinning. This occurs along the crystal’s c-axis and typically results in two crystal segments joined together in a simple, symmetrical fashion. Carlsbad twinning usually develops during the initial growth of the crystal from magma, so it’s considered a primary feature rather than a product of later alteration

• Albite Twinning:
  • In contrast, Albite twinning is more complex. It consists of fine, repeated lamellae aligned parallel to the a-axis. While albite twinning is characteristic of plagioclase feldspar, it can also appear in orthoclase, often as a result of exsolution or stress during cooling. Under a polarizing microscope, these lamellae create a striking striped effect.

• Pericline Twinning:

  • Closely related is Pericline twinning, which forms along the b-axis. Like albite twinning, it appears as delicate, repeated lamellae, but oriented differently. Pericline twinning is particularly significant because it often signals a slow transformation from monoclinic orthoclase to triclinic microcline as the granite cools over geological time.

• Cross-Hatched Pattern:

  • When albite and pericline twinning occur together, they produce the famous cross-hatched pattern known as microcline twinning. This intricate texture is a hallmark of feldspar that has undergone inversion from orthoclase to microcline at low temperatures, reflecting prolonged cooling and structural adjustment.

Tasks

• Describe the twinning and crystals whcih can be seen in one of the four corner posts?

• Please explain which kind of twinning this is? 

References

• Albite twin – Encyclopaedia Britannica - A clear, authoritative overview of feldspar twinning, explaining Carlsbad and albite twins, including how albite twinning appears as repeated parallel lamellae in crystals. [britannica.com].

• Crystal Twinning – Geosciences LibreTexts - A university-level explanation of crystal twinning in general, including how twin planes, axes, and symmetry operations work—useful for understanding the crystallographic basis behind feldspar twins. [geo.libretexts.org].

• Feldspar Twinning (UTA teaching resource PDF) - A very practical petrography-focused source describing Carlsbad, albite and pericline twins, including how they appear under the microscope and how they are used for identification. [bpb-us-e1....pmucdn.com].

• Alkali Feldspar – Alex Strekeisen Mineralogy - Explains twinning in different feldspar types found in granite, including Carlsbad twinning in orthoclase and cross-hatched (tartan) twinning in microcline, both typical of granitic rocks. [alexstrekeisen.it]. 

• Technical Guide to Feldspar Twinning Mechanisms -A more advanced reference explaining twinning as a crystallographic intergrowth, including formation mechanisms (growth, transformation, deformation) and the main twin laws (Carlsbad, albite, pericline). [pdf.benchchem.com].