Jerrygibbsite

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Jerrygibbsite (Mn,Zn)9(SiO4)4(OH)2 is an uncommon and relatively unknown mineral. It is believed to be a member of the leucophoenicite family, but this is unconfirmed and membership of the humite group is also possible. Jerrygibbsite was originally discovered by Pete J. Dunn in 1984 and named after a professor at Virginia State University and Virginia Polytechnic Institute, Professor Gerald V. Gibbs. It is a very rare mineral of which there are only five known samples, three of which having been discovered in Franklin, New Jersey, United States in 1984 and two in Namibia's Otjozondjupa region (by Pete Dunn), in 1988. Jerrygibbsite is closely related to both the leucophoenite family and the humite group by both crystal structure and chemical composition. It is always found with these two minerals, and is most likely a polymorph between leucophoenicite and the humite group mineral, sonolite.

Physical properties of Jerrygibbsite include violet-pink color, light pink streak, a hardness of 5.5, and a density of 4.00 g/cm3. Crystals tend to be transparent and orthorhombic with an imperfect cleavage along the {001} plane.

Contents

[edit] Introduction

The mineral jerrygibbsite was discovered in 1984 by Pete Dunn while conducting an x-ray spectroscopy of a sample previously assumed to be leucophoenicite. A new diffraction pattern unlike anything seen before led to the discovery of this new mineral. All samples found of jerrygibbsite are impure. All are incorporated within leucophoenicite, many by mixed layering, and tend to be found with many manganese humites such as sonolite. Physical properties are similar to those of leucophoenicite and sonolite, including hardness, coloring, and density. Because there are only five known samples of jerrygibbsite, there are very few pure specimens to analyze.

[edit] Composition

The formula for jerrygibbsite is (Mn,Zn)9(SiO4)4(OH)2, although it often contains impurities of Fe, Mg, Ca, or H2O (Webmineral-Jerrygibbsite). The idealized formula is Mn9(SiO4)4(OH)2 which is the same ideal formula as sonolite, a member of the humite group (Dunn). The humite group consists of many manganese and magnesium rich species of the base formula (SiO4)1-4(OH)2. This group of minerals forms a morphotropic series with the mineral olivine and brucite (the humite group). Every specimen found of jerrygibbsite has contained the mineral zincite (Zn,Mn)O, also of which most known samples have been found at the Franklin Mines (Dunn). This inclusion of zincite to jerrygibbsite gives us the empirical formula Mn8.5Zn0.5(SiO4)4(OH)2. Sonolite tends to have fluorine in solution with the (OH)2 group, leading to a nonideal formula of Mn9(SiO4)4(OH,F)2. Jerrygibbsite has been found to be dimorphous with sonolite (Dunn). The physical properties of sonolite and jerrygibbsite have some similarities, but in general they are quite different. Sonolite tends to be a dark brown to red crystal, while jerrygibbsite is generally violet-pink. Jerrygibbsite has a density of 4.00 g/cm³while Sonolite’s is less, but still close, at 3.82. The hardness of the two minerals is the same, at 5.5, about the hardness of a knife blade. Sonolite’s crystals are monoclinic with a space group P21, while jerrygibbsite forms orthorhombic crystals of space group Pbnm, with Z=4.(Dunn) The structure of jerrygibbsite is derived from that of sonolite by unit cell twinning, where the glide plane operation relates (OH,F)-coordinated octahedral, as opposed to those with oxygen ligands (Yau).

[edit] Geologic occurrence

Jerrygibbsite has been found only in the Franklin Mines in Franklin, New Jersey, and in the Kombat Mines in Namibia (Dunn, Webmineral-Jerrygibbsite). These mines are curious because many minerals have been found only in these two localities. Most of the minerals in the humite group have been found only here, as well as leucophoenicite. The above mentioned zincite, a leading ore of zinc, has also only been found at these sites (Webmineral- Zincite). Of the original three samples, one was previously incorrectly labeled leucophoenicite, so it is possible that other samples exist, but are simply incorrectly labeled. It is interesting that in the Sterling Mines, very near the Franklin Mines, neither jerrygibbsite nor leucophoenicite are known to occur, suggesting very localized conditions needed for formation of these phases.(Dunn) However, of the five known samples, four of them are texturally different, suggesting they occurred somewhat spatially distributed in the Franklin orebody.(Franklin-Sterling) So it is strange that there have been no findings of either jerrygibbsite or leucophoenicite in the nearby Sterling Mines. Jerrygibbsite is not found as a pure specimen, Jerrygibbsite has been found to occur in contact with willemite, zincite, and sonolite in a very colorful and uncommon assemblage (Dunn). Jerrygibbsite typically occurs as a massive mineral in interlocking anhedral crystals, up to 0.5x2.0 mm, which display a typical metamorphic texture (Dunn). This typical metamorphic texture only applies to the original three specimens, of which two were identical. Subsequent finds from the Namibia mines were of two different textures. The only real way to positively determine presence of jerrygibbsite is by x-ray or chemical data. Many jerrygibbsite specimens are possibly labeled incorrectly, and many other minerals appear that they could be jerrygibbsite, by a few defining characteristics such as the opaque lamellae, but contain no jerrygibbsite (Dunn). Pete Dunn had thought at one point that they might have identified an incorrectly identified sample of jerrygibbsite labeled as leucophoenicite because the lamellar structure was similar to that of jerrygibbsite, but in further tests, it was labeled correctly in the collection.

[edit] Physical properties

Jerrygibbsite, in pure form, is a violet-pink mineral with a light pink streak. It has a calculated density of 4.045, and a tested density of 4.00 g/cm³, agreeing favorably, since measurements used to test density have few significant figures. It has a hardness of about 5.5, that of a knife blade (Dunn). The general luster is vitreous, or shiny. Crystals are generally transparent to translucent. Crystals are not luminescent or fluorescent (Dunn). Jerrygibbsite forms orthorhombic crystals with an imperfect cleavage along the {001} plane, which can be seen by opaque lamellae alternating with the transparent jerrygibbsite. Optically, jerrygibbsite is negative biaxial with 2V = 72˚ and a maximum birefringence of 0.017. (Dunn) In thin section, jerrygibbsite appears light pink. The crystal structure described by Kato is the equivalent of a unit-cell-twinned sonolite in which the cells are related by a b/4 glide plane (Dunn).

[edit] Related species

In the article by Pete Dunn, jerrygibbsite is presented as a member of the leucophoenicite group. Leucophoenicite has the chemical formula Mn7(SiO4)3(OH)2 which varies from that of jerrygibbsite by only 2 atoms of manganese. Jerrygibbsite and leucophoenicite are both characterized as being fluorine free, but the manganese humites have significant fluorine content (Yau). It has many of the same physical properties of jerrygibbsite also. The color is pink, and it is optically birefringent negative. Leucophoenicite is slightly higher, ranging from 5.5-6.0, and the crystals are monoclinic as opposed to orthorhombic. Most leucophoenicite is glassy with no evident textural features. It is noted in Dunn’s article that no known leucophoenicite approaches end member composition. All specimens found contain some Zn and/or Ca replacing Mn. The article determines that jerrygibbsite is a probable member of this group as opposed to the humite group of which sonolite is a member because during x-ray powder diffraction]], the d-values obtained are very similar to those of leucophoenicite. It is noted, however, that attempts to obtain high quality powder diffraction patterns were unsuccessful due to contaminating phases such as leucophoenicite (Dunn) and other members of the Mn humite minerals. There have been no pure samples of jerrygibbsite found, and attempts thus far of coming up with a pure powder sample have been unsuccessful. X-ray powder diffraction has therefore been calculated by assuming an impurity of leucophoenicite and working around them, because leucophoenicite is a mineral that has been found in pure form, so the d-spacings are known from prior x-ray powder tests. Optically, in thin section, jerrygibbsite is characterized by having parallel lamellar features, approximately 1000 angstroms (100 nanometers) in width randomly distributed through the crystal, which was otherwise homogenous (Dunn).

One difference between leucophoenicite and jerrygibbsite that should be noted is that leucophoenicite tends to contain Ca and/or Zn, which is believed to stabilize the mineral (Dunn). Jerrygibbsite is noted for its lack of any Ca bearing minerals (Franklin-Sterling). None of the five samples found of jerrygibbsite have contained any amount of calcium; however, jerrygibbsite has been found to be intimately ingrown with leucophoenicite.(Dunn) Both jerrygibbsite and leucophoenicite are interesting, however, because although they are both formed in close contact with many members of the humite group, of which many are rich in magnesium, both minerals display very low magnesium content. (Franklin-Sterling)

[edit] Biographic sketch

Gerald V. Gibbs, whom the mineral was named after has made “contributions to the science of mineralogy (that are) are immense and varied”. (Bloss) After completing his masters at the University of Tennessee, he went to Penn State and began to delve into mineralogy. His first major contribution was the identification of the mineral protoamphibole, which had originally been thought to be fluoranthophyllite. (Bloss) He spent much of his time devoted to the crystal structure of protoamphibole. His achievements spread out to include other minerals, and specialized in crystal systems. “Gibbs transformed the way mineralogists look at the crystal structures and properties of minerals. He devised computer models that accurately simulate the behavior and reproduce the properties of a number of important materials.”(Science Museum of Virginia) Much of his research has been on amphiboles, olivine, and quite heavily on the humite group. Doctor Gibbs was presented the honor of the Roebling Medal for the Mineralogical Society of America for 1987. Gerald has done much of his studies in the Franklin Mine, doing research on the crystal structures of the humite group minerals, specializing on norbergite, chondrodite, and humite. (Dunn)

[edit] Literature search

Very little recent activity has been noted about jerrygibbsite. Its most highly cited paper, with four citings is that of Yau, who tested mixed layering between leucophoenicite and jerrygibbsite. The second highest is that of Dunn, with three citings, who discovered the mineral in the first place. No recent literature or images are available.

[edit] References

  • Jerrygibbsite, A Rare Mineral Discovered at the Franklin Mines, New Jersey, 1984, and its Relation to the Leucophoenicite and Humite Group. Kira Norman University of Houston December 1, 2007
  • Bloss, Donald. Presentation of the Roebling Medal of the Mineralogical Society of America for 1987 to Gerald V. Gibbs. American Mineralogist, Volume 73, Pages 668-669, 1988.
  • Dunn, Pete J. Jerrygibbsite, a new polymorph of (Mn,Zn)9(SiO4)4(OH)2 from Franklin, New Jersey, with new data on leucophoenicite. American Mineralogist, Volume 69, pages 546-552, 1984. Franklin-Sterling.
  • Jerrygibbsite. Referenced November 4, 2007 from http://franklin-sterlinghill.com/dunn/ch15/jerrygibbsite.stm
  • The Humite Group of Minerals. Referenced November 4, 2007 from http://mineral.galleries.com/minerals/silicate/humite.htm
  • Mindat. Leucophoenicite. Referenced November 4, 2007 from http://www.mindat.org/min-2384.html
  • Science Museum of Virginia Referenced November 4, 2007 from http://www.smv.org/osi/recipients.asp Virginia's Outstanding Scientists and Virginia's Outstanding Industrialists
  • Webmineral- Zincite. Referenced November 4, 2007 from www.webmineral.com/data/zincite.shtml
  • Webmineral- Jerrygibbsite. Referenced November 4, 2007 from www.webmineral.com/data/jerrygibbsite.shtml
  • Yau, Yu-Chye, Donald R. Peacor. Jerrygibbsite-leucophoenicite mixed layering and general relations between the humite and leucophoenicite families. American Mineralogist, Volume 71, pages 985-988, 1986.
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