June Mineral of the Month

'Garnet Group'

Garnet is named from the word 'grantum', meaning pomegranate, for its resemblance to the fruit's seeds.

The Garnet mineral group is compromised of isomorphous minerals identical in crystal structure and in many cases difficult to determine one from another. The members of this group intermingle with one another, within limits.

The most common Garnet members are Pyrope-Mg₃Al₂Si₃O₁₂ Alamandine-Fe²⁺₃Al₂Si₃O_{12 Spessartine-}Mn₃Al₂Si₃O_{12 Andradite-}Ca₃Fe³⁺₂Si₃O_{12 Grossular-}Ca₃Al₂Si₃O_{12 and
Uvarovite-}Ca₃Cr₂Si₃O₁₂

There are seven lesser known members of the Garnet family known as Knorringite, Majorite, Calderite, Goldmanite, Schorlomite, Kimzeyite & Morimotoite. There are also two hydro garnet members know as Hibschite and Katoite. These nine members are extremely rare and very little is actually known about them.

Garnet crystals are dodecahedral or trapezohedral in form. They usually form as rounded grains and come in a variety of colors depending on the individual variety. They can also form in massive, granular or compact habits

Garnets are a member of the silicates group. They have a hardness of 6-1/2 to 7-1/2 on the Mohs scale. Garnets have no cleavage and and un-even to conchoidal fracture. It's specific gravity is 4.1 to 4.3. Garnet has a white streak and is transparent to opaque and has a vitreous luster.

Garnets form in a variety of ultrabasic igneous rocks, associated serpentines & metamorphosed rocks such as schist.

Garnet fuses fairly easily and is virtually insoluble in acids. Garnet is one of the only gemstones on the planet that is not heat treated or irradiated or undergoes any other type of man made process in order to enhance the quality of the finished gemstone.

May 2009 Mineral of the Month

'TOURMALINE GROUP'

From the Cingalese word"tourmali," this was the name given to colored gem 'zircons' found on the island of Sri Lanka (Ceylon). It wasn't until the early 1800s that it was discovered that some of these "zircons" arriving in European gem centers from the far east were actually a previously un-described mineral.

The trigonal crystals formed by this group are often vertically striated. These crystals may be rounded triangular in cross-section. It also forms in massive and compact habits. Seven distinct species make up the Tourmaline group. They are Elbaite (multi hued), Schorl (black), Buergerite and Dravite (brown), Rubellite (pink), Chromedravite (green) and Uvite (black, brown, yellowy green). Crystals are often pink at one end and green at the other and may be of considerable size. Tourmalines form in granites and pegmatites as well as in some metamorphic rocks. They may be found with a wide array of minerals including beryl, zircon, quartz and feldspar.

Tourmaline is a member of the Silicates group. It has a colorless streak, is transparent to opaque and has a vitreous luster. It has a very distinct cleavage and an uneven to conchoidal fracture. Tourmaline has a hardness of 7 to 7-1/2 on theMohs scale. It's complicated chemical compsoition is as follows: Na(Mg.Fe,Li.Mn.Al)3Al6(BO3)3Si6.O18(OH.F)4

This group is insoluble in acids. The darker minerals tend to fuse with more difficulty than the red and green varieties.