FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Hi,
I did not find on this forum a topic dedicated to Madagascar. Have I searched badly?
It seems to me that this huge island needs to be illustrated by some pictures.

I am now proposing an aquamarine.

[Bob Harman]

I'll add this to ROGER's Madagascar thread. The island of Madagascar is indeed a mineral rich area. One of its more famous minerals is CELESTINE from the remote coastal site of Sakoany. Discovered here in 1967, in a coastal sandy-marly matrix, pits are hand dug. At low tide and at depths of several meters into the coastal marls, these geodes are encountered. The geodes range from several cm in size to more than 30 cm in size. Many contain hi quality collectible crystals of 2 basic habits. One celestine crystal habit is the blocky one as shown in my 2 examples. More recently mined are the spear point or pig-tooth habit crystals. Recently, it seems, that production of these examples has decreased.

My larger example was acquired at the 2003 Tucson show. It is 29 cm x 19 cm and weighs about 20 kg. Of most note is the extremely rare occurrence of a 9.5 cm doubly terminated celestine crystal. Just behind this area is a perpendicular 4+ cm gemmy crystal of celestine. This whole area has been suggested to be culled from the rest of the specimen, but I have resisted. Don't ask about $$$ value, but it is more than considerable. BOB

[Roger Warin]

Another one from Madagascar.

[Roger Warin]

Hi,
Some specimens of corundum from the Zazafotsy deposit in the south of the Madagascar island.
More specifically, Bevoaroy, near Ambatrazo, Fianantsoa Province.
The corundum has the crystalline symmetry of calcite. The pure corundum (aluminum oxide) is white. The presence of metal impurities brings the color (Cr, Fe, TI, V ...), giving sapphires and rubies, not often gems.

[Roger Warin]

Two colors in the same crystal.
At first, there was chrome. When this cation was exhausted there was a change of color.

[Roger Warin]

If Al3+ is replaced by Cr3+, corundum turns red.
The amount of Cr3+ intensifies the color. When it is weak, the ruby is pink. The more Cr3+ there is, the more the ruby is red. For a concentration of 1 to 2% (weight), the red becomes deep and takes a tip of blue color. This is the ruby called "pigeon blood" (typical from Kashmir).
The presence of Fe3+ disturbs these definitions. If the ratio Fe2O3 / Cr2O3 is greater than 1, the corundum takes the name of sapphire. We can thus have red sapphires that are not rubies.
When Iron and Titanium are present, sapphires turn blue. Vanadium and magnesium can also be involved. In fact other electronic mechanisms can also occur giving sapphires of all colors, even red (charge transfers).
The Padparadscha sapphire has a very special red hue. This is sometimes enhanced by heating.

The photo shows a rather rare specimen of a corundum hexagonal dipyramid ending in a ruby gemstone, with several modifying facets. The sketch gives indices of the faces.

[Gagat Minerals]

[Roger Warin]

Yes that’s correct. Excuse my big distraction.
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Fixed. Thanks Gagat Minerals!