The Story of Cavansite

Northwest Micro Mineral Study Group MICRO PROBE FALL, 2008 VOLUME X, Number 8 FALL MEETING . .VANCOUVER, WASHINGTON November 8, 2008 9:00 am to 5:00 pm Clark County P. U. D. Building 1200 Fort Vancouver Way Vancouver, Washington Come find out what everyone has been up to this summer. Bring your microscopes and something for the free table to share with others. There will be plenty of room and ample time to check out all the new things that people have to brag about. We will have our usual brief business meeting in the afternoon, a discussion of future articles, and our update session on the status of localities. No guest speaker has been planned, but we will N be showing pictures of the new twins from Lemolo Lake, as well as of other choice pieces from the Northern California meeting in July. If you have digitals or slides of mineral specimens or collecting localities, this would be a perfect time to share them with the group. We will have projectors and a screen waiting. I5 The kitchen area is again available and we will plan on sharing lunch together. We will provide meat, cheese, bread, lettuce, tomatoes, mayo and mustard for sand-wiches as well as coffee, PUD Mill Plain Blvd. tea, cider, and cocoa. Members need to bring some sides, ie, salads, chips, desserts and anything else that they would like to have to Park Ft. Vancouver Way munch on. Washington In the evening, many of us plan to go to a local Interstate buffet restaurant, so please plan to join us if you Bridge Columbia River can. Oregon 2 Garnets at Summit Rock Don Howard “I never heard of that!” you say? That was my first reaction as well. But there it was in a box of stuff from Summit Rock, in little itty bitty cardboard boxes that verified what was printed on the folded label: Dave Yeomans. It was stuff he had collected years ago, moved after his death into the hands of Fred Devito, and now at last appearing on the free table at the Northern California Mineralogical Association meeting at El Dorado, California in July, 2008. And all the little label said was „garnet‟. The rock was clearly the right stuff for Summit Rock: the heavily oxidized material with rounded stubs that had once been enstatite, grayish blades of ilmenite, lots of prisms of plagioclase, a few aegirine needles, and a smattering of while apatite crystals laying like jackstraws in the bottom of the cavities. And among them were very obvious dodecahedral crystals, some with rounded corners and edges, others sharper but with a dusky coating of something siliceous. The color varies from a clear golden brown to much darker, often streaked through a single individual. Their identity as garnet was obvious. Which garnet? The EDAX attachment to the SEM quickly gives an answer: calcium iron silicate with a little manganese and only the barest hint of aluminum. Andradite. And that makes sense, because aluminum minerals, except for the feldspar, are in short supply at Summit Rock. The manganese could be divalent and substitute at the calcium site, or it could be trivalent and substitute for the ferric iron. In neither case would there be enough to make this anything other than a clear example of andradite. We usually think of garnets as occurring in metamorphic rock, in schists and limestones that have been heated and altered from sediments. But garnets can be formed in primary environments too, directly from the vapor phase. I think of the beautiful spessartine from Ruby Mountain, Colorado. So we chalk up another mineral to look for in our Summit Rock specimens. Thank you, Dave Yeomans, for recognizing and noting in passing this interesting material. A dodecahedral crystal of Andradite, showing slightly rounded edges and etched faces. This crystal is nearly black, but others are a golden brown. The surrounding material is mainly plagioclase and the stumps of oxidized enstatite and ilmenite. 3 Pseudobrookite Twins from the Lemolo Lake Quarry Don Howard and Kent England Pseudobrookite was originally named in 1878 for material found at Uroiu, Transylvania, Romania. It is a high temperature mineral, usually formed directly from heated gases in igneous rock. For this reason, there are only a limited number of occurrences worldwide. Fortunately, it is fairly common in the rhyolites of the Thomas Range, Utah, so that it is familiar to collectors in the United States. The crystal structure of pseudobrookite is orthorhombic, the most common form being lathlike needles elongated along the c-axis. Most crystals are black in color, with a submetallic luster. Chemically, the composition is a ferric titanium oxide, Fe2TiO5 , but many analyses show excess titanium oxide. This is thought to be due to inclusions of rutile, but because most crystals are opaque except on the thinnest edges, the inclusions are not generally visible. The blades have well-developed {100} faces that are usually striated parallel to the c- axis. Such striations in minerals are commonly caused by twinning. Considerable consideration for this was put forward during the 1920‟s, and a number of twin planes of the form {hk0} were proposed. But a lack of truly twinned crystals to date has left the matter of twinning in pseudobrookite to be considerably in doubt. Pseudobrookite is reported to occur with enstatite and apatite at Red Cone in Crater Lake National Park, and is found very sparingly on oxidized ilmenite in the cavities at Summit Rock just to the north. But the most collectable material has come from Lemolo Lake, where it forms dark blood-red blades along the bottom margin of the flow exposed along the spillway to the dam. Most of these blades are clear enough to see that there are no inclusions of rutile present. The forms are very simple, with large {100} faces edged by {010} or {210} and terminated by either {101} or {301}. The {100} faces are very smooth and shiny and seldom show any striations at all. That is why it came as a surprise to find so many twinned pseudobrookite crystals, and not just one type of twin, but a bewildering variety of twinning angles present. The twins were found along the margins of a boulder in the west end of the quarry that was originally collected in 1982 and 1983. This is the same boulder that has produced interesting „cyclic twins‟ of enstatite (see accompanying article). Though this particular boulder shows the characteristic contact with the scoria-like flow beneath it, there are considerable differences from the flow exposed along the spillway, in which most of the interesting minerals reported so far have occurred. The cavities in this boulder show primarily three minerals: enstatite, pseudobrookite, and tridymite, with ilmenite crystals conspicuously absent. Extensive twinning is present in all three minerals. The color of the enstatite is yellow rather than the more tan color of those found along the spillway. Though it is possible this rock comes from a different flow, it is more likely from a different region of the same flow with slightly different chemistry conditions present. While most of the twins observed have a composition plane of the form {hk0}, one surprising kind of twin was found with the composition plane {101}. Several examples were observed, and photographs taken looking down the b axis to be able to measure the angle (see insert in the diagram below). However, in at least one case, one member of the twin was terminated by the {301} planes, and it was observed that the {301} of one was very nearly parallel to the {100} of the other by observing the simultaneous reflections from the illuminator. Most of the twins are simple contact twins, but one example of a penetration twin of this type has 4 Contact twin on (101), viewed in the a-b plane. The termination faces are also {101} for both crystals. (101) ______________________________________________________________________________ also been observed, though it was in a position difficult to photograph. The rest of the twins observed all involve rotations about the c-axis. Photographs were taken parallel to that axis of a number of examples, and the angles were measured from them. Only those cases are reported where two or more of the same intercrystal angles were found, in order to avoid any accidental parallel growths. One trouble in identifying just which composition LATTICE PARAMETERS planes are involved in the twinning comes from the fact FOR PSEUDOBROOKITE that the a and b cell dimensions are very nearly the same. a = 9.797 A For the most common twin type, that on {110}, there is no b = 9.981 A problem; the crystals are only about 1o from being exactly c = 3.370 A perpendicular. For the other cases, we have calculated the angles for a variety of low-index planes and taken the ones that come closest to the measured values. Generally, only a couple of degrees separate the candidates, so it is possible that other composition planes could be involved beyond the ones we have identified. Many of these pseudobrookites show penetration twinning, so in the diagram on the next page, we have depicted each case as a penetration twin. The insets show a typical example for each composition plane. We summarize in the table below the types of twins observed. Only those cases have been included where two or more examples could be found and photographed of the same intercrystal angles. Very low angle twins, though present, have not been analyzed and included because of difficulty of determining angles with sufficient accuracy to be able to assign a composition plane, which would have had to have some very high indices.

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