Thursday, July 14, 2011

Staurolite Collection Field Trip to Marble, North Carolina

Staurolites crystals can be collected from the river next to the
pacesetter camp
The name Staurolite is derived from the Greek, stauros for cross and lithos for stone in reference to the common twinning. A special property of staurolite is that it often occurs twinned in a characteristic cross-shape.  Staurolite crystals has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: (Fe,Mg,Zn)2Al9(Si,Al)4O20(OH)4 With Iron, magnesium and zinc occur in variable ratios.   Macroscopically visible staurolite crystals are of prismatic shape. They are often larger than the surrounding minerals and are then called porphyroblasts. 

The middle of the river has plenty of
larger rocks.  They need to be
removed be
An official field trip of Gem and Mineral Society of Franklin, NC brought us to the Pacesetter camp in Marble, North Carolina to collect Staurolite.  Staurolite can be found in the river next to the camp.  The water was mostly shallow except one area where the water was become stagnant.  There are plenty of big rocks on middle of the river bed.  They need to be removed and get to the pebble and sandy layer.  Scoop up a shavelful of the gravel or sand and place them in the screenbo.  Allow smaller sand and pebbles to go through the screen and carried away by the river.  The Staurolite crystals are larger and retained on the screenbox (Instruction for making screenbox).

The 1/2 inch mesh screenbox

The staurolite crystals collected from the middle of the river are more rounded in shape because of the tumbling effect from the river while the staurolite crystals collected from the size of the river tends to be more angled.  Belows are the Staurolites collected from the field trip.
The twinned Staurolite crystals are the most saught after form.  I collecte each one of them in the field trip.  They are collected from the middle of the river so they are very short and rounded.

Twinned staurolite crystal crossed in ninety degree angle.

Twinned staurolite crystal crossed in sixty degree angle.

Most Staurolite Crystals are not twined

Staurolites under stereo microscope.  The color of Staurolite
can be any where from brownish to black.

In thin sections staurolite is commonly twinned and shows lower first order birefringence similar to quartz, with the twinning displaying optical continuity. It can be identified in metamorphic rocks by its swiss cheese appearance (with poikilitic quartz) and often mantled porphyroblastic character.  This will be my next project.

PS. After couple days since my first collection of pond water.  Most pond lives are mostly disappears.  They were either died or go into hibernation.  I am thinking it either lack of oxygen or energy.  Summer is the season for observing pond lives.  I will postpone the making of thin section of staurolites until the summer is ended.

1 comment:

  1. Great research studies.. with the help of microscope you could distinguish the difference on every rock types...