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