THE MINERAL ZOISITE

  • Chemistry: Ca2Al3(SiO4)3(OH), Calcium Aluminum Silicate Hydroxide.
  • Class: Silicates
  • Subclass: Sorosilicates
  • Group: Epidote
  • Uses: mineral specimens, ornamental stone and gemstone.
  • Specimens
Zoisite, like all epidote minerals, is a structurally complex mineral having both single silicate tetrahedrons, SiO4, and double silicate tetrahedrons, Si2O7. The formula of zoisite could be expressed in a such a way so as to reflect this organization; Ca2AlOAl2(SiO4)(Si2O7)(OH).

Zoisite has been known for nearly two centuries as as a sometimes ornamental stone of limited distribution. Only in 1967 was the blue gemstone variety found in Tanzania. The variety was named Tanzanite and was a surprise to minerologists and gemologists alike in that it had come from a very ungemstone-like mineral. The blue-lavender color of tanzanite is unique and sets it apart from the other gemstones.

Besides tanzanite, zoisite has produced other attractive specimens that are of interest to collectors. A pink variety called thulite is usually massive and used for beads and cabochons. A brilliant green variety is associated with medium grade rubies and is quite popular as an ornamental stone. The red rubies are often distorted and irregularly spread throughout the sea of massive green zoisite. It is one of the most colorful of ornamental stones and competes well with the popular pink tourmaline and lavender lepidolite of California.

PHYSICAL CHARACTERISTICS:

  • Color is variable but well known as green, blue to violet and pink to reddish in color, Also grey, white or brown.
  • Luster is vitreous.
  • Transparency crystals are transparent to translucent.
  • Crystal System orthorhombic; 2/m 2/m 2/m
  • Crystal Habits include long, somewhat prismatic or tabular crystals with a typically dominant pinacoid that the crystal is often flattened against. The terminations are usually poorly developed. Also massive or granular.
  • Cleavage good in one direction lengthwise.
  • Fracture is uneven to conchoidal.
  • Hardness is 6 - 7
  • Specific Gravity is approximately 3.3
  • Streak is white.
  • Other Characteristics: striated crystals common, lengthwise. Also strongly pleochroic with an index of refraction of 1.68 - 1.72.
  • Associated Minerals include calcite, biotite, hornblende, quartz, corundum, andradite garnets and other metamorphic minerals.
  • Notable Occurances include Tanga, Tanzania; Ducktown, Tennessee, USA; Switzerland, India and Austria.
  • Best Field Indicators only one direction of cleavage, associations, color, pleochroism and hardness.
ZOISITE specimens:
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ZOISITE specimen zoi-1
$ 110.00
Dims: 1.1 x 0.8 x 0.5" (2.8 x 2.0 x 1.3 cm)
Wt: 8.4 g w/ base
Alchuri, above Shigar, Baltistan, Northern Areas, Pakistan
This is the first example of green Zoisite crystals that I have ever seen! This small thumbnail specimen consists of several heavily intergrown crystals, almost all of which are damaged or broken. One of the longest crystals, however, is intact and measures over 1" (2.5 cm) in length. It shows only very minor damage on its termination and has good orthorhombic prismatic form, with well-defined edges and clean faces. All of the crystals have a dull green color with a hint of gray and a noticeable violet tinge, and range from translucent to transparent and very clear (most are transparent at their terminations and become cloudy at their prism lengths). There is no host rock present.
no photo
zoi-1 ($110.00)
Alchuri, above Shigar, Baltistan, Northern Areas, Pakistan
ZOISITE specimen zoi-2
$ 25.00
Dims: 1.12x0.91x0.44" (2.84x2.31x1.11cm)
Wt: 0.19oz (5.3g)
West Gate, Churchill County, Nevada, USA
This is a specimen of thulite, the pink variety of zoisite. While most of the specimen displays a massive form, there are areas of crystalization, and a high-power loupe does reveal a few good crystals. Also, while the pale pink thulite is dominent, the other colors (colorless, very pale green, and blue) appear to have the same form, and I believe that these are other varieties of zoisite, too. The specimen is significantly more pale in color than the image as seen on this computer, although the image looked perfect on the "camera" computer. Of course, it may look different on yours, as well.
no photo
zoi-2 ($ 25.00)
West Gate, Churchill County, Nevada, USA
ZOISITE specimen zoi-3
$ 25.00
Dims: 0.93x0.93x0.53" (2.37x2.35x1.34cm)
Wt: 0.35oz (9.9g)
Westgate, Churchill County, Nevada, USA
While there are other minerals present (notably quartz), the bulk of this specimen is granular thulite, the pink variety of zoisite. The individual crystals are too small to examine, even with my loupe.
no photo
zoi-3 ($ 25.00)
Westgate, Churchill County, Nevada, USA
 

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