Minerals Review Ruby Deposits: a review and Geological Classification Gaston Giuliani

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ruby deposits[01-22]

Figure 15. Gallium (Ga)–vanadium (V)–iron (Fe) ternary showing the distinction between natural and
synthetic rubies, modified from [67].
Sutherland et al. [73–75] used a Fe
2
O
3
/TiO
2
versus Cr
2
O
3
/Ga
2
O
3
diagram to decipher the metamorphic
versus magmatic origin of corundum. This diagram is commonly used for the separation of blue
sapphires from several deposits worldwide [76]. The corundum crystals termed “metamorphic” have
pastel (blue, pink, and orange) to red colors, and are rich in chromium and poor in gallium
(Cr
2
O
3
/Ga
2
O
3
ratio > 3). The corundum crystals defined as “magmatic” are xenocrysts of blue-green-
yellow (BGY) sapphires with a Cr
2
O
3
/Ga
2
O
3
ratio < 1. The diagram was applied to rubies and
sapphires related to alkali basalt from the provinces of Antananarivo (the Soamiakatra, Kianjanakanga,
and Ambatomainty deposits) and Toamasina (the Vatomandry and Andilamena deposits) in
Figure 15.
Gallium (Ga)–vanadium (V)–iron (Fe) ternary showing the distinction between natural and
synthetic rubies, modified from [
67
].
Sutherland et al. [
73
–
75
] used a Fe
2
O
3
/TiO
2
versus Cr
2
O
3
/Ga
2
O
3
diagram to decipher the metamorphic
versus magmatic origin of corundum. This diagram is commonly used for the separation of blue sapphires
from several deposits worldwide [
76
]. The corundum crystals termed “metamorphic” have pastel (blue,
pink, and orange) to red colors, and are rich in chromium and poor in gallium (Cr
2
O
3
/Ga
2
O
3
ratio
>
3). The corundum crystals defined as “magmatic” are xenocrysts of blue-green-yellow (BGY) sapphires
with a Cr
2
O
3
/Ga
2
O
3
ratio
< 1. The diagram was applied to rubies and sapphires related to alkali basalt
from the provinces of Antananarivo (the Soamiakatra, Kianjanakanga, and Ambatomainty deposits) and

Minerals 2020, 10, 597
15 of 83
Toamasina (the Vatomandry and Andilamena deposits) in Madagascar [
77
,
78
] (Figure
16
). Note that all of
the points related to rubies plot in the metamorphic field. They overlap data for rubies from Andilamena,
which is located in the north of the Vatomandry area, and where the corundum is hosted in different
types of rocks (viz. metamorphosed mafic and ultramafic rocks (M-UMR) and plumasites in M-UMR).
The majority of the sapphires plot in the magmatic domain. The distribution of sapphires from the
Vatomandry area is heterogeneous, and while most are located in the magmatic domain yet others are in
the metamorphic one. The latter corresponds to colorless, light to dark green, and light to purplish blue
sapphires with significant Cr
2
O
3
contents (150 ppm
< Cr
2
O
3
< 2100 ppm).
Madagascar [77,78] (Figure 16). Note that all of the points related to rubies plot in the metamorphic
field. They overlap data for rubies from Andilamena, which is located in the north of the Vatomandry
area, and where the corundum is hosted in different types of rocks (viz. metamorphosed mafic and
ultramafic rocks (M-UMR) and plumasites in M-UMR). The majority of the sapphires plot in the
magmatic domain. The distribution of sapphires from the Vatomandry area is heterogeneous, and
while most are located in the magmatic domain yet others are in the metamorphic one. The latter
corresponds to colorless, light to dark green, and light to purplish blue sapphires with significant
Cr
2
O
3
contents (150 ppm < Cr
2
O
3
< 2100 ppm).

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