Magnesioriebeckite (Magnesio-riebeckite) is a sodium magnesium ferric-iron silicate hydroxide mineral of the amphibole group. The only reliable analysis of magnesioriebeckite is that given by Klein and Ito (1968). Although there is no reliable analysis of the blue felty material, Palache's (1935) recalculation of Bauer's analysis, deleting Zn, should be carefully evaluated. As shown herein, all the Franklin amphiboles from the orebody host significant quantities of zinc, and this one might also.
Magnesioriebeckite was first reported from Franklin by Palache (1928a) as crocidolite; it has not been reported from Sterling Hill. A subsequent description of different material, providing much of the data given here, was presented by Klein and Ito (1968).
The magnesioriebeckite described by Klein and Ito (1968) is dark green, with a density of 3.26 g/cm3. The bulk of the known magnesioriebeckite specimens, however, are from the occurrence reported by Palache (1935). In this assemblage, magnesioriebeckite is gray-blue with a felty texture and fibrous appearance and occurs in hand-sized masses.
The material described by Klein and Ito (1968) occurs as 4-cm dark green prismatic crystals associated with rhodonite, calcite, and franklinite.
The assemblage described as crocidolite by Palache (1935) was found on the Trotter Dump in 1906, and much was preserved; the mineral was probably subject to preferential retention by miners because of its attractive delicate blue color, fine willemite inclusions, and uncommon appearance. A note in Bauer's catalogue indicates that [some of] this material was found in 1920 on the 300 level.
Examination of a large number of such specimens permitted the recognition of an association with willemite and one with sphalerite and blue calcite. Some specimens have small corroded remants of aegirine which may have provided the Na, Fe3+, and Si for the formation of magnesioriebeckite, but aegirine is otherwise uncommon at Franklin. The in-situ relationships, as with most Franklin minerals, are obscure, and the available specimens do not permit a definitive interpretation. The assemblage is clearly a late-stage one; some specimens have serpentine present, and much of the magnesioriebeckite occurs, together with lennilenapeite, as a breccia-cement or late-stage coating on rounded fragments of willemite, calcite, and sphalerite. This assemblage, uncommon because of its high Mg and Fe3+ concentration, is also host to the more abundant occurrences of lennilenapeite and has provided rare and superb crystal groups of hematite. The fine elongate twisted quartz crystals illustrated by Palache (1926, 1935) are also from this assemblage. (Dunn, 1995)

 Location Found: Franklin
 Year Discovered: 1949
 Formula: ☐{Na2}{Mg3Fe23+}(Si8O22)(OH)2
 Essential Elements: Hydrogen, Iron, Magnesium, Oxygen, Silicon, Sodium
 All Elements in Formula: Hydrogen, Iron, Magnesium, Oxygen, Silicon, Sodium
 IMA Status: Approved
 To find out more about this mineral at minDat's website, follow this link   Magnesio-riebeckite

Dunn, Pete J. (1995). Franklin and Sterling Hill New Jersey: the world's most magnificent mineral deposits. Franklin, NJ.: The Franklin-Ogdensburg Mineralogical Society. p.456. 'Magnesioriebeckite'

Frondel, Clifford (1972). The minerals of Franklin and Sterling Hill, a checklist. NY.: John Willey & Sons. p.65

The Picking Table References
 PT Issue and PageDescription / Comment
View IssueV. 12, No. 2 - August 1971, pg. 7New Minerals for Franklin-Sterling Validated List - Magnesioriebeckite (small article)
View IssueV. 10, No. 1 - February 1969, pg. 11Harvard Research on Franklin Minerals - Magnesioriebeckite

Non-fluorescent blue magnesioriebeckite, gray sphalerite, calcite matrix from Franklin
Non-fluorescent blue magnesioriebeckite and gray sphalerite, on a calcite matrix from Franklin, New Jersey. Photo by JVF.

Magnesioriebeckite, sphalerite, lennilenapeite and minor calcie from Franklin, NJ
Magnesio-riebeckite (blue), sphalerite (golden brown), lennilenapeite (gray brown) and minor calcie (white) from Franklin, NJ. 4 7/8" x 3 1/4". From the collection of, and photo by Robert A. Boymistruk.

Magnesio-riebeckite and sphalerite from Franklin, NJ
Magnesio-riebeckite (blue) and sphalerite (gold to brown) from Franklin, NJ. 3 1/2" x 1 1/2. From the collection of, and photo by Robert A. Boymistruk.

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