HYDROXYLHERDERITE: ANSWERING ONE OF LIFE'S PERSISTENT QUESTIONS

I picked up a mineral in Tucson last year because: 1) it was a phosphate (one of my favorite groups); 2) I knew very little about locality or abundance, well actually nothing; 3) it was sort of a bland looking, did not have really well developed crystal faces and was like an orphan on a table of more spectacular and showy specimens; and 4) it was cheap in price.  So, off I went with my small specimen of herderite, rescued from its disgrace at the showy ball.  Herderite [CaBePO₄(F,OH)] is a calcium beryllium phosphate and is usually found in granite pegmatites rich in beryllium (for example beryl).  It was discovered many decades ago in Germany (Haidinger, 1828) and so I now had a little jewel that was no longer an orphan.  But wait, something is wrong!

According to the people-in-the-know, I almost certainly have a specimen of hydroxylherderite, also a calcium beryllium phosphate [CaBePO₄(OH,F)]. There is a subtle difference between the two minerals—note the end of the chemical formulae.  In hydroxylherderite the hydroxyl radical (OH) is greater than the fluorine component (F).  The opposite is true in herderite.  Could I tell the difference?  Probably not. Gatta and others (2014), in studying the herderite-hydroxylherderite in Oxford County, Maine, used the following instruments: single-crystal X-ray diffraction and neutron Laue diffraction, electron microprobe analysis in wavelength-dispersive mode, inductively coupled plasma-atomic emission spectrometry and polarized Raman spectroscopy.  Let’s see, my lab has a microscope or two, a loupe, a hardness kit, and a bottle of acid---no inductively coupled plasma-atomic emission spectrophotometer though (maybe for Christmas?).

Hydroxylherderite was given the name hydrohererite when it was described from localities around Paris, Maine (Penfield, 1894).  I am a little uncertain about how Penfield was able to distinguish his specimens as different from the original herderite—one of life’s persistent questions (see addendum).

Both minerals are quite similar in their physical properties with a hardness of around 5.0-5.5 (Mohs), poor cleavage, white, colorless, blue green, blue, purple/lavender (only hydroxylherderite), brown, light yellow (in color), translucent to transparent with a luster ranging from vitreous to waxy/greasy.  The streak is white.  Both belong to the Monoclinic Crystal System and crystals often are prismatic.

The "orphan,"  a hydroxylherderite prismatic crystal on lepidolite (a lithium-rich mica).  The lack of a "pink" color of the lepidolite evidently is due to a lack of manganese.  Length of exposed crystal is ~1.2 cm.

As one might suspect there is a solid solution series between herderite and hydroxylherderite; however, no mineral with an intermediate composition is named.  And again, one needs some rather sophisticated instrumentation to distinguish chemical composition.  MinDat notes that most specimens labeled herderite are actually hydroxylherderite and confirmed F-dominant specimens are presently known only from Brazil (two locations), Mogok (Burma), Yichun (China), and Namibia.  Evidently, even the type locality specimens (Germany) of herderite are now confirmed as hydroxylherderite.  The only way that I might be able to distinguish specimens is that the lavender/purple crystals from Brazil are hydroxylherderite.  In fact, my specimen is from the Virgem da Lapa, Minas Gerais, Brazil (Xanda Mine?), and shows some purple/lavender color. Hydroxylherderite from this area is composed of 53-64% (OH) as the end member (Dunn and others, 1979).

Another partial crystal of hydroxylherderite on the same specimen displaying the purple/lavender color common in Brazilian specimens.  Width of photomicrograph ~2.5 cm.

Both minerals are late-stage members of granite pegmatite hydrothermal mineralization (Dunn and others, 1979) and seem to be formed from the alteration of beryllonite [NaBePO₄] (Palache and Shannon, 1928), or beryl [Be₂Al₂(Si₆O₁₈)] (Yatsevich, 1935).  I thought perhaps hydroxylherderite might be widely present in the beryllium-rich pegmatites of the Black Hills, South Dakota; however, MinDat only lists a single locality, the famous Tiptop Mine near Custer.

REFERENCES CITED

Dunn, P.J., C.W. Wolfe, P.B. Leavens, and W.E. Wilson, 1979, Hydroxyl-herderite from Brazil and a guide to species nomenclature for the herderite/hydroxyl-herderite series: Mineralogical. Record, v. 10.

Gatta, G.D., S.D. Jacobsen, P. Vignloa, G.J. McIntyre, G. Guastella, L.F. Abate, 2014, Single-crystal neutron diffraction and Raman spectroscopic study of hydroxylherderite, CaBePO4(OH,F): Mineralogical Magazine, v. 78, no. 3.

Haidinger, W., 1828, On herderite, a new mineral species: Philosophical Magazine, 4, 1-3.

Palache, C. and E.V. Shannon, 1928) Beryllonite and other phosphates from Newry, Maine: American Mineralogist v. 13, 392-396.

Penfield, S. L., 1894, On the crystallization of herderite: American Journal of Science, 3rd Series, 47, 329-339.

Yatsevich, G. M., 1935, The crystallography of herderite from Topsham, Maine: American Mineralogist, v. 20, 426--437.

ADDENDIUM I

Above I stated that I was uncertain about how Penfield knew the Maine mineral this was not herderite and indeed was a new mineral.  I finally located an 1894 copy of the American Journal of Science (wow, James and Edward Dana, editors) and Penfield fully explained: DURING the past summer Mr. L. K. Stone of Paris, Maine, sent to Prof. H. L. Wells of the Sheffield Scientific School several specimens of an unknown mineral for identification. The specimens were collected at Paris, Me., but not at the noted Mt. Mica locality. They presented well defined, transparent and almost colorless monoclinic crystals, measuring up to 2mm in diameter and 6mm in length. The crystals are implanted mostly upon quartz hut some are on feldspar. Their hardness is a little over 5. When tested before the blowpipe they at first sprouted and turned white, but afterwards fused at about 4 to a white, blebby enamel, tinging the flame very pale green, indicating phosphoric acid. In the closed tube at a high temperature the crystals whitened, threw off quite violently a fine scaly powder or dust and gave water which showed only a faint acid reaction. The mineral was slowly but completely soluble in hydrochloric acid. As these characters apparently did not agree with the description of any known species, the mineral was supposed to be new and accordingly the best material available for the chemical analysis was carefully selected and eventually separated from any attached quartz or gangue by means of the heavy eolution. The pure mineral, amounting to about one and a half grams, and varying in specific gravity from 2'936 to 2'968, was analyzed by Professor Wells to whom the author's sincere thanks are due. The analysis revealed the interesting fact that the mineral is herderite and that it contains practically no fluorine, agreeing in this latter respect with a variety described by Professor Wells and the author* from Hebron, Me….The analyses indicate a well defined type of herderite which may well be called hydro-herderite in distinction from the variety containing fluorine.

Now, that is real science!  But I am still uncertain about Professor Wells and his analysis indicating “no fluorine.”  How was that fact determined?  A chemist friend of mine explained that in 1894 elemental fluorine had only been isolated by Henri Moissan 8 years earlier in 1886.  Although Moissan received the Nobel Prize in Chemistry for his work, he had studied results of other chemists who had been trying to isolate the element for about 75 years.  Some of these boys (all were males) had ended up as "fluorine martyrs" while working with highly corrosive hydrofluoric acid (HF).

So, the identification of fluorine in early chemical analyses is one of life’s persistent questions that I will continue to explore.  The beautiful thing about learning is nobody can take it away from you  (B.B. King).

ADDEEDUM II 

R.I.P.  JOHN McCAIN 

I prefer to give thanks for those blessings, and my love to the people who blessed me with theirs.  The bells toll for me.  I knew it would.  So I tried , as best I could, to stay a part of the main.  I hope those who mourn my passing, and even those who don't, will celebrate as I celebrate a happy life lived in imperfect service to a country made of ideals, whose continued service is the hope of the world.  And I wish all of you great adventures, good company, and lives as lucky of mine.