Náttúrufræðingurinn - 1990, Blaðsíða 11

Þetta eru stærstu kristallar sem kunn-
ugt er um hér á landi. Fundist hafa
jaspísmolar sem eru enn stærri, en
jaspís er safn smásærra kvarskristalla,
eins og kunnugt er, og getur því ekki
keppt við aragónítið að þessu leyti.
Af öðrum svæðum þar sem fallegt
og stórt aragónít hefur fundist má
helst nefna nágrenni Siglufjarðar,
Norðurárdal í Borgarfirði og fjalllend-
ið á milli Hnappadals og Norðurárdals
á Vesturlandi.
ÞAKKIR
Magnús Sigurgeirsson greindi sex sýni í
röntgentækjum Jarðfræðahúss Háskóla ís-
lands, og Sigurður Sveinn Jónsson tók
myndir þær er fylgja greininni.
HEIMILDIR
Gibson, I.L., D.J.J. Kinsman & G.P.L.
Walker 1966. Geology of the Fáskrúðs-
fjörður area, eastern Iceland. Vísinda-
félag íslendinga, Greinar IV, 2. 1-52.
Guðmundur Jónsson 1945. Skaftfellskar
þjóðsögur og sagnir. Marteinn Skaft-
fells tók saman. Bókaverslun Guð-
mundar Gamalíelssonar, Reykjavík.
324 bls.
Klein, C. & C.S. Hurlbut 1985. Manual of
Mineralogy (after J.D. Dana)../. Wiley
& Sons, New York. 596 bls.
Sveinn P. Jakobsson 1980. Ágrip um berg-
fræði íslands. Jökull 29. 96-99.
Walker, G.P.L. 1960. Zeolite zones and
dike distribution in relation to the
structure of the basalts of eastern Ice-
land. Journal of Geology 68. 515-528.
SUMMARY
Aragonite from Hólsvör,
Stöðvarfjörður, E Iceland
by
Sveinn P. Jakobsson
Icelandic Museum of Natural History
P.O. Box 5320
IS-125 REYKJAVÍK
Iceland
An exceptional collection of large ara-
gonite crystals from Hólsvör, Stöðvar-
fjörður, was presented as a gift to the Mu-
seum. It was discovered by a local ama-
teur mineralogist in a crevasse in a basaltic
lava from the Upper Miocene. The collec-
tion is made up of 53 pieces, the largest of
which is 65 cm across. Single crystals of
aragonite average between 10 and 20 cm
in length, the longest measuring 26 cm.
Measurements of crystal angles perpen-
dicular to the c-axis indicates aragonite;
however, x-ray diffraction determinations
show that the main part of each crystal
group is calcite. The crystals are therefore
in the main calcite pseudomorphs after
aragonite, except the core, which is still
aragonite.
The aragonite crystals are coated with
alternating layers of calcite and opal. An
interesting feature is the presence of a few
aragonite crystals which were broken, pre-
sumably in a tectonic event, and then ce-
mented by Iayers of calcite with dissemi-
nated pyrite.
The main localities for aragonite in Ice-
land are briefly mentioned. The most fa-
mous locality is without doubt Hoffellsfjall
in Hornafjörður, SE Iceland. Both arago-
nite and calcite were mined at this locality
for industrial use, for example between
the two world wars.
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