Microscopy of Garnet Stone from Scotland

The following photos are microscopic images using Zeiss IM microscope of Garnet Paragneiss (Precambrian) from Loch Duich (Glenelg Inlier), Scotland.

This slide was obtained with number others from ebay second hand.

Information on Garnet from Wikipedia:

Properties: Garnet species are found in many colors including red, orange, yellow, green, purple, brown, blue, black, pink, and colorless, with reddish shades most common.

A sample showing the deep red color garnet can exhibit. Garnet species’ light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral’s luster is categorized as vitreous (glass-like) or resinous (amber-like).

Crystal structure: Garnets are nesosilicates having the general formula X3Y2(Si O4)3. The X site is usually occupied by divalent cations (Ca, Mg, Fe, Mn)2+ and the Y site by trivalent cations (Al, Fe, Cr)3+ in an octahedral/tetrahedral framework with [SiO4]4− occupying the tetrahedra. Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. (Note: the word “trapezohedron” as used here and in most mineral texts refers to the shape called a Deltoidal icositetrahedron in solid geometry.) They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed (conchoidal).

Hardness: Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.5 to 7.5. The harder species like almandine are often used for abrasive purposes.

Magnetics used in garnet series identification: For gem identification purposes, a pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones commonly used in the jewelry trade. Magnetic susceptibility measurements in conjunction with refractive index can be used to distinguish garnet species and varieties, and determine the composition of garnets in terms of percentages of end-member species within an individual gem.

I tried a neodycromium magnet on the slide and it did not appear ot be magnetically attracted – but then I do not know what proportion of this rock is garnet (possibly just the crystals within the matrix) and it is thin, both of which might explain the failure for magnetic attraction.

Andy

x4 objective:

 

x10 objective:

x32 objective:

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