전기석(드라바이트) Tourmaline (dravite)

원산지 : Soklich Open Cut, Yinnietharra Station, Pilbara Region, Western Australia.

 

This huge single crystal of dravite weighs over 1 kilograms. It has the proper brown color and vitreous luster of dravite, and while this tourmaline is transparent, you can see no further into the interior than a millimeter or two, due to the many internal fractures and inclusions. Taken as a whole, the crystal has very good form, with three-sided pyramidal terminations at both ends, and with 9 faces on the sides (3 major faces with minor faces at each corner). While the dravite is essentially undamaged, it is intergrown with a large amount of mica and quartz (everything white in the photographs), and the faces are all stepped in a complex pattern, yielding many minor crystal faces and intermediate terminations. The mica is all damaged, much of it broken off at the surface of the dravite. This is a great cabinet specimen, due to its size and shape.

 

THE MINERAL DRAVITE (Tourmaline)

• Chemistry: NaMg3(Al,Fe)6Si6O18(BO3)3(OH)4, complex Sodium Magnesium Iron Boro-Aluminium Silicate
• Class: Silicates
• Subclass: Cyclosilicates
• Group: Tourmalines
• Uses: mineral collection specimens

Dravite is a little known species of the Tourmaline Group. Two other tourmalines are Schorl (black, iron rich) and Elbaite (various colors and sometimes cut for gems, lithium rich). Dravite will sometimes produce rather large well shaped crystals that are important specimens in a rock hound's collection.

 

PHYSICAL CHARACTERISTICS:

• Color is light brown to dark brown.
• Crystal Habit is typically elongated three sided prisms. The terminations can be either a simple to complex trigonal pyramid or a flat basal face. The prism faces are striated lengthwise. In cross section, all tourmalines will appear predominantly triangular in shape with some crystals showing a hexagon. Doubly terminated crystals are hemimorphic meaning that the two ends of the crystal are not exactly alike. Massive forms can also be found.
• Transparency: crystals are translucent to opaque.
• Crystal System is Trigonal; 3m
• Hardness is 7 - 7.5
• Specific Gravity is 3.2+ (slightly heavier than average)
• Cleavage is absent although there is basal parting.
• Fracture is uneven to conchoidal.
• Luster is vitreous.
• Associated Minerals include those associated with metamorphic rocks such as micas, feldspars and quartz.
• Other Characteristics: crystals are brittle and refractive indices = 1.62 and 1.68 .
• Notable Occurances include brazil and some other South American countries, and Africa.
• Best Field Indicators are crystal habit, triangular cross-section, color and hardness.