Itrongay (Itrongahy), Mahasoa Est, Betroka District, Anosy, Madagascari

Regional Level Types
Itrongay (Itrongahy)	Area
Mahasoa Est	Commune
Betroka District	District
Anosy	Region
Madagascar	Country

The designation Itrongay refers to an area famous for yellow (due to Fe³⁺ substituting for Al³⁺) gemmy "orthoclase" (which actually are sanidine, see below), but also gemmy crystals of diopside and zircon.

In 1922 when the famous French mineralogist Lacroix wrote his monumental book on Malagasy minerals, the ancient village called Itrongay was situated on the path between the towns of Betroka and Benenitra in southern Madagascar. It was located near the village which today is called Tongay, south of the river Hazofotsy. Itrongay was at that time an important mineralogical centre in the area. But today you will not find the name "Itrongay" on contemporary maps - just the geological ones. The placename has erroneously been spelled as "Itrongahy" in some literature, but the correct is Itrongay. The word in the local Bara language is derived from the verb "mitrongay" meaning "to walk with a limp".

The famous gemmy sanidine (reported in earlier literature as orthoclase) is today found in a 40 km large area around what was the ancient village of Itrongay. Lacking local knowledge, minerals from this area and others close by are still labelled as Itrongay by mineral dealers. Some of the minerals found today originate from the area around Benono.

NOTE 1: Not all gemmy feldspar labelled Itrongay, originates from the Itrongay area. In the 1980s gemmy orthoclase was also found at Ampandrandava (Pezzotta, 1999). It is also reported from other localities in the Betroka district as well. One locality that produced several specimens in the 1990s was Andangovato.
Another is Beapompo II workings.
Not all the minerals referred to in the literature as coming from Itrongay have been found in the same geological setting as the sanidine. And some of the minerals found at the beginning of the 20th century were also found directly on the ground.
Lacroix (1922) states that the diopside and sanidine originate from a pegmatite. Pezzotta (1999) observes in a new site in the same area that the yellow gemmy sanidine occurs in pegmatite derived feldspar veins cutting marbles.

NOTE 2: X-ray studies of a bright yellow to colorless feldspar collected in 2001 at one of the localities around Itrongay and other samples showed it to be high to low sanidine, not orthoclase (Simmons and Falster 2002). Further investigations by Ackermann et al. (2005) on the cation distribution and ordering of Si, Al, Fe on the tetragonal (T1) site of a K-feldspar from Itrongay, showed it to be a monoclinic low sanidine (for more details see the paper at http://www.escholarship.org/uc/item/20f968sx ).

See also the discussion under http://www.mindat.org/mesg-7-162698.html and
http://www.mindat.org/mesg-7-219075.html

Geochronology

Mineralization age: Cambrian : 490 ± 20 Ma

Important note: This table is based only on rock and mineral ages recorded on mindat.org for this locality and is not necessarily a complete representation of the geochronology, but does give an indication of possible mineralization events relevant to this locality. As more age information is added this table may expand in the future. A break in the table simply indicates a lack of data entered here, not necessarily a break in the geologic sequence. Grey background entries are from different, related, localities.

Geologic Time				Rocks, Minerals and Events
Phanerozoic
	Paleozoic
		Cambrian
			Furongian	ⓘ Zircon 490 ± 20 Ma

Other Regions, Features and Areas containing this locality

References