Known locally as Telaga Tujuh, the Seven Wells Waterfall is named after the seven natural pools that have been formed at different levels by water streaming down Langkawi's second highest mountain, Mount Mat Cincang. It is arguably the most picturesque and best waterfall on the island.
Type of Earthcache: Mineralogy.
Potash Feldspar
The granite at the Seven Wells Waterfall, known as Sawar granite, is classified as a porphyritic biotite granite, localy sheared and at places tourmalinized. A rather obvious feature is the presence of potash feldspar (microcline, KAlSi3O8) phenocrysts (crystal intrusions) which tend to protrude from weathered surface of the granite outcrop. A feldspar is often referred to as 'potash' if it contains significantly more potassium than sodium (typically 2-5% Na2O).
The phenocrysts show a high degree of flow alignment. Likewise, xenoliths present within the granite also display a prefered orientation parallel to the flow.
Between the larger potash feldspar phenocrysts, two unusual pairs of twinned or bent crystals can be observed. Measuring about 7 cm in length, they appear to show twinning that reminds of the saw-horse penetration, often seen in minerals such as staurolite.
Another unusual feature of the feldspar here is that some are curved, forming open V-shaped crystals. The smooth curving of these crystals, along with evidence of sheared granite, suggest that these are likely bent, and not twinned, crystals.
How to claim this EarthCache?
Do not confuse the abundant potash feldspar phenocrysts with the odd patch of tourmaline quartz.
Send me the following;
1. The text "GC81GQ2 Seven Wells Waterfall - Potash Feldspar" on the first line.
2. The answers to the following questions;
- Describe the appearance (shape, colour, quantity) of the phenocrysts.
- What is the average length of the potash feldspar phenocrysts on site?
- Take a close-up photo of a bent or V-shaped crystal, with a coin alongside.*
(It would be a nice touch if you use a coin from your home country)
References
The contents of this page are an adaptation from "Occurrence of saw-horse penetration-twinned potash feldspar" by Khoo, T.T. and Kamarudin Salim, Jabatan Geologi, Universiti Malaya, Kuala Lumpur, published in Warta Geologi (Newsletter of the Geological Society of Malaysia), Vol. 9, No. 1, January-February 1984.
* Effective immediately from 10 June 2019, photo requirements are permitted on EarthCaches. This task is not optional, it is an addition to existing logging tasks! Logs that do not meet all requirements posed will no longer be accepted.
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