Sakum Ishi(Cherry Blossom Stones):Mica Pseudomorphs of ComplexCordierite-lndialite lntergrowthsfrom Kameoka, Kyoto Prefecture, Japan

Figure /. Sakura ishi (cherry blossom stones) colored by minor heniatilc,Kameoka, Japan. Masutomi Museum specimens.

JOHN RAKOVANDepartment of GeologyMiami UniversityOxford, Ohio 45056rakovajf@muohio.edu

MASAO KITAMURADepartment of Geology &Mineralogy Kyoto UniversityKyoto 606-85012, Japankitamura@kueps.kyoto-u.ac.jp

OSAMU TAMADAGraduate School of Human andEnvironmental Sciences KyotoUniversity, Kyoto 606-85021, Japantamada@t02.mbox.media.kyoto-u.ac.jp

Cherry blossoms have been revered for more thana thousand years in Japan and have becomeone of its most recognized icons. In the city of

Kameoka. which lies just over the western mountains ofKyoto City (figs. 2,3), mica pseudomorphs after complexcordierite-indialite intergrowths that resemble cherryblossoms are found (fig. 1) (Rakovan 2005). As with thereal blossoms, these stones, known as sakura ishi (cherryblossom stones), are also revered by mineralogists andmineral collectors in Japan. The cordierite-indialite pre-

Dr. John Rakovan, an executive editor o/Rocks & Minerals,is n professor of mineralogy and geochemistry at MiamiUniversity in Oxford, Ohio.

Dr. Masao Kitamura is a professor of mineralogy in tin-Department of Geology and Mineralogy and is dean ojthe Graduate School of Science, Kyoto University, Japan.

Dr. Osamu Tamada is a professor of mineralogy in theDepartment of Interdisciplinary Environment, GraduateSchool of Human and Environmental Studies, KyotoUniversity, Japan.

284 ROCKS & MrNERAU

Figure 2. Map of Japan, by William Besse.

cursors to these pseudomorphs are found throughout centralJapan, especially in Kyoto Prefecture, where igneous intru-sions have baked clay-rich sedimentary rocks (fig. 3). Kyotohas been the cultural center of Japan for a millennium, andit is fortuitous that sakura ishi are from an area so intimatelyassociated with an admiration for cherry blossoms. If theJapanese were inclined to name prefecture minerals, as stateminerals are named in the United States, then sakura ishiwould be the obvious choice for Kyoto Prefecture.

The first mention of sakura ishi was made by Kikuchi(1889), who described cordierite crystals with a flowerlikecross section from a hornfels (then called a spotted slate) inJapan. Only a few years later a description appeared in Dana(1892). Cordierite with this texture has been called cerasite inJapan, stemming fi-om cera, which means cherry blossom, asdoes sakura (Kitamura and Yamada 1987). The name cerasitehas sometimes been confused in the West because of its simi-larity to sericitc (a term used to describe a fine-grained mica,usually muscovite or paragonite), which often forms pseudo-morphic replacements of cordierite. Yet another term used todescribe these pseudomorphs of mica or other clay mineralsafter cordierite is pinite.

Sakura IshiThe sakura ishi from Kameoka City in Kyoto Prefecture

are the best known because they are so easily removedfrom their host hornfels matrix and because they are easilybroken to expose their flowerlike internal morphology (fig.1). This is the consequence of postmetamorphic alterationof the hornfels in the Kameoka area that has resulted in

Osaka Bay

Mudstones and Chert(Tamba Group)

Urban Area

• | Granite (98 Ma)

A Contact Hornfelst^ (with sakura ishi)

Figure 3. Geological shaded relief map of the Kyoto andKameoka area. Modified from Geologic Map Database ofJapan (map 5235 Kyoto), Geological Survey of Japan.

l-igure 4. Hornfelswith several completecrystals of sakura ishtexposed.

the pseudomorphic replacement of the original cordieritecrystals by fine-grained mica. The original crystals wereactually cordierite-indialite intergrowths (discussed in detaillater) that formed with a wagon-wheel or flowerlike patternin cross section (perpendicular to their length). The degreeof pseudomorphic replacement is regionally variable, butwhen it is complete, the sakura ishi crystals are quite fragile.They can easily be snapped in half or crushed betweenone's fingers. Although they are delicate, complete crystals,showing well-preserved external morphology, are commonlyfound weathered out of the hornfels (tigs. 4, 5a, 6). Inareas where the cordierite is completely replaced hy micathe hornfels is also altered such that it is very friable andpoorly consolidated. This leads to the easy release of crystalsthrough physical weathering, as well as the excavation ofcrystals embedded in matrix. A local trick for partiallystabilizing fragile matrix specimens is to paint them with adiluted solution of wood glue in water.

Volume 81, July/August 2006 285

MorphologicLong Axis Apparent

,• Pinacoid

5a

Figure 5. (a) Hornfels witha complete crystal of sakuraishi exposed, (b) Schematicof crystal morphology in a.

ApparentHexagonal

Prism

Figure 6. Complete crystal ofsakura ishi exhibiting reentrantsbetween apparent hexagonal prismfaces and a protruding apparentpinacoid sector.

Complete crystals usually exhibit a simple external mor-phology consisting of an elongated hexagonal prism ter-minated on both ends by the basal pinacoid {fig. 4). Somecrystals show tapering edges between the prism and pinac-oid that appear to be pyramidal faces (fig. 5). Because theprecursors to these pseudomorphs are intergrowths of twominerals with different symmetries, it may lead to confusionif Miller indices are used to describe their faces or if specificcrystallographic directions are referred to. Therefore, themorphology will be described using the terms "apparenthexagonal prism" and "apparent pinacoid," and directionswill be referenced to the morphologic long axis, as indicatedin fig. 5b. Likewise, the internal sectoral morphology, dis-cussed below, will also be described in terms of "apparentsectors." Another common feature is the presence of reen-trants along the edges between apparent prism faces (fig. 6).The depth of such reentrants varies from sample to sampleand is particularly noticeable in cross section.

The pseudomorphs exhibit a three-dimensional internalmorphology that resembles the sectors of a simple hexago-nal crystal bounded by the hexagonal prism and pinacoid(fig. 7). Figure 8 shows the internal morphology of severaldozen sakura ishi crystals in cross section. Their broken sur-

7a

Figure 7. (a) Schematic of sectors in a crystalbound by the hexagonal prism and pinacoid.One of the prism sectors and its associatedprism face are shaded, (b) Cross sectiontbrough a, perpendicular to the morphologiclong axis, at the position indicated by redmarkers.

ApparentPinacoidSector

ApparentPrism Sector

7b

faces are perpendicular to the morphologic long axis of thecrystals (fig. 5b). All sections exhibit six apparent hexagonalprism sectors (fig. 7b). A hexagonal center is evident in mostof the sections; this is an apparent pinacoid sector. The sizeof the central hexagon is dependent on where the sectioncuts the crystal. If the crystal is broken directly through itscenter, the section will not intersect the apparent pinacoidsector, and the hexagonal center will be absent. Sectionscut progressively farther from the crystal center have largerhexagonal centers (fig. 7a). Another feature common to allcrystals is a distinct contrast in color and luster betweenthe apparent sectors and the boundaries between them (fig.8). This is due to the inclusion of fine-grained, brown-graymatrix material along the apparent sector boundaries. In

286 ROCKS & MINERALS

Figure 8. Sections o( sakura ishi, broken perpendicular tothe morphologic long axis (fig. 5) exhibiting a flowerlike (orapparent sectoral) internal morphology. The size of the apparentpinacoid sector, the center hexagon, depends on where along itslong axis the crystal is hroken (fig. 7). Note the varying devel-opment of reentrants between apparent prism sectors and theinclusion of the fine-grained, brown-gray matrix material.

some cases this is hardly noticeable, whereas in othersthere may be a large amount of included material. It isalso apparent that there are varying degrees of reentrantdevelopment between apparent prism sectors. Importantly,where reentrants are deep there is much more includedmatrix material. In extreme cases the internal morphologyresembles the spokes of a wagon wheel.

An unusual property of these pseudomorphs is theirtendency to break along the apparent sector boundaries(fig. 9). With skill and the right specimen, one can separatea pseudomorph into its individual and complete "sector"

components (fig. 10). This is not a behavior that one wouldexpect for a single crystal or a pseudomorph of an originallysingle crystal. To understand this unique behavior it is worthexploring the nature of sectors and how they form.

A true sector is a region or volume ofa single crystal thathas formed by growth on a specific crystal face. Figure 7 isa schematic of a simple hexagonal crystal composed of twoforms, a hexagonal prism {100} and a pinacoid {001}. Theinternal, sectoral morphology (which is the shape, size, andarrangement of the sectors within a crystal) is also sbown.Indicated by dotted lines are tbe sectors associated with eachcrystal face, and one of the prism sectors is shaded. Imaginethat this crystal grew from a small nucleus located at itscenter, and tbat the sbape of the crystal did not change overtime—only its size increased. Growth would occur by tbeaddition of atoms on tbe surface of tbe crystal. Tbe shadedsector would then be tbe result of growtb on the shadedface only In tbis simple case the sectors are pyramid-shapedregions that start at a specific crystal face and taper towardthe center of tbe crystal. The entire volume of the crystalin figure 7a is made up of six prism sectors with four-sidedpyramidal shapes and two pinacoid sectors with six-sidedpyramidal shapes (fig. 10). All crystals that are bounded byfaces have sectors associated with tbose faces.

Sector boundaries follow tbe patb of crystal edges astbey advance outward during tbe growtb process. In all butthe rarest of instances, tbe atomic structure of a crystal isuninterrupted and undistorted at sector boundaries. Theremay be a compositional difference on eitber side ofa sectorboundary (this is known as sectoral zoning), but sucb differ-ences are usually seen for trace or minor constituents anddo not result in a pbysical or structural demarcation of tbeboundary (Dowty 1976; Reeder and Grams 1987; Bosze andRakovan 2002). Tbus, sector boundaries do not influencetbe physical behavior of a crystal. For example, crystals donot preferentially cleave or fracture along sector boundarieswben broken. Also, because crystals are usually structurallycontinuous across sector boundaries, there is normally nooptical indication of sectors (unless color zoning is present).

9a 9c

Figure 9. (a) Section of sfllcura I's/it, broken perpendicular to the morphologic long axis (fig. 5) exhibiting a flowerlike(or apparent sectoral) internal morphology, (b) Fragmentation of the crystal along apparent sector boundaries.(c) View of the fragmented crystal with the morphologic long axis vertical. The apparent sectoral morphology(fig. 7) is exhibited.

Volume 81, |uly/August 2006 287

Figure 10. Schematic of the complete fragmentationof a sakura ishi crystal into its apparent prism andpinacoid sectors.

For example, when a thin section of a crystal is observed incrossed polars, optical properties such as extinction angleand birefringence are homogeneous across the crystal sec-tion. There are rare exceptions to the normal behavior ofcrystals discussed above. These are scientifically very inter-esting but are outside the scope of this article (see, e.g., Aki-zuki and Sunagawa 1978; Akizuki, Hampar, and Zussman1979; Rakovan and Reeder 1994).

The surprising behavior of the sakura ishi from Kameokais that they so readily break along what appear to be sectorboundaries (fig. 9). One might conjecture that this unusualbehavior is the result of the mineralogical replacement,but if other pseudomorphs are considered (and there arehundreds of examples in the mineral world), this is stillvery unusual. Why do the sakura ishi pseudomorphs fromKameoka exhibit this behavior? The answer lies in the modeof formation of their precursor minerals; what appear tobe sector boundaries from a single crystal are actuallyboundaries between multiple crystals {grain boundaries). Tounderstand just what the precursors to the pseudomorphs ofKameoka were and how they formed it is necessary to lookat unreptaced sakura ishi from other areas. There have beenseveral studies of cordierites from argillaceous hornfels andother metasediments in central Japan {Kitamura and Hiroi1982; Kitamura and Yamada 1987; Miyake 1990; Miyaza-ki 2001). The work on specimens from Daimonji-yama(Daimonji Mountain), Higaslnyama, Kyoto, by Kitamuraand Yamada is key to understanding sakura ishi.

Geological SettingSakura ishi from Kameoka (pseudomorphs after cordier-

ite-indialite) and from Daimonji-yama in Kyoto (unalteredcordierite-indialite) are found in metamorphosed clay and

Figure 11. Daimonji-yama (Daimonji Mountain), Kyoto,Japan. The Chinese character 'k {dai) on the mountainsidemeans big.

sand-rich sediments ofthe Mesozoic Tamba group (Imoto1984). In both locations contact with intruded granites andgranodiorites (approximately 98 million years ago in theLate Cretaceous) baked the sediments, creating contact-metamorphic rocks known as hornfels (fig. 3). These consistmainly of large crystals of cordierite (up to 2-3 cm) in afine-grained matrix of biotite, feldspar, quartz, monazite,ilmenite, and graphite. Daimonji-yama, however, is the loca-tion for more than just pristine sakura ishi; it is the site ofone of Japan's most important festivals.

Each year on 16 August, to signal the end of the Buddhistfestival of the dead (known as 0-bon), gigantic bonfires inthe shapes of Chinese characters, a ship, and a Torii gate areset ablaze on five mountains surrounding the city of Kyoto.Daimonji-yama, located in the mountains that border theeastern side of Kyoto City {higashiyama), is the most famousof the fire festival sites (fig. 11). The three strokes of the daicharacter on the side of Daimonji-yama, "^^ are 80,160, and120 meters long, respectively, and are visible fi-om most partsof the city.

Pristine or unaltered sakura ishi are found completelyembedded in matrix and cannot be easily removed (fig. 12).For this study, numerous samples of hornfels were cut toexpose their embedded cordierite crystals. Polished thin sec-tions were made for optical study from several samples thathad cordierites oriented with their morphologic long axis(fig. 5) either perpendicular or parallel to the cut surfaces.On first seeing the cut sections of cordierite-bearing horn-fels from Daimonji, a friend, Mieko Ono, commented thatthe grayish-white cordierites set against their charcoal-graymatrix resembled cherry blossoms against a night sky. Poeti-cally, she named them yozakura ishi (in reference to sakuraviewing at night).

288 ROCKS & MINERALS

Figure 12. Cordierite crystals imbedded in a fine-grainedhornfels matrix from Daimotijt-yama, Kyoto, Japan. Thedarker crystals on the right are patially altered to muscovite.Those with the flowerlike internal morphology are also calledsakt4ra ishi and are thought to he the same as the precursors tothe pseudomorphic sahura ishi from Kameoka.

Optical PropertiesThe optical properties described herein are those

measured on a petrographic microscope where the sampleis observed in transmitted light between two polarizingplates {crossed polars). For more information on thistechnique, see a standard mineralogy textbook or opticalmineralogy text (e.g., Bloss 1999; Klein 2001). As discussedabove, the separation ofthe Kameoka pseudomorphs alongapparent sector boundaries when broken is a behavior thatis unexpected for a pseudomorph that has replaced a singlecrystal. Similarly, the optical properties of the unalteredsakimi ishi are different from what is expected for a singlecrystal and indicate that these actually formed as a complexintergrowth of multiple crystals.

Figure 13 is an optical image of cordierite (crossed polars)in section perpendicular to the morphologic long axis ofthe original crystal (fig. 5). For a dynamic view of this thinsection rotating between crossed polars on a petrographicmiscroscope go to the Supplementary Materials page oftheRocks & Minerals Web site: http://www.rocksandminerals.org/. The six triangular regions are sections throughapparent prism sectors (fig. 7b). There are three differentoptical orientations among the six apparent sectors. Thisis indicated by the degree of extinction of each apparentsector (i.e., the amount of light that is blocked as it passesthrough the section, indicated by how dark the regionsare). Sectors opposite one another have tbe same degreeof extinction and the same optical orientation. The upperand lower apparent sectors are completely extinct in the

c /3. Optical photomicrograph of cordierite (crossedpolars) in thin section from Daimoiiji-yama, Kyoto, Japan.The section is perpendicular to the morphologic long axis ofthe original crystal, similar to the schematic section in figure7b. The six triangular regions (in three different degrees ofextinction; degrees of darkness) are apparent prism sectors.Dashed lines indicate the approximate boundaries betweenthe apparent prism sectors. The circular boundary around theimage is from the microscope ohjective lens. For a dynamicview of this thin section rotating between crossed polars on apetrographic miscroscope go to the Supplementary Materialspage of the Rocks & Minerals Web site: http://www.rocksandminerals.org/.

image. As discussed above, this behavior is anomalous for asingle crystal and indicates that the six apparent bexagonalprism sectors are actually six individual cordierite crystals.Other than the presence of inclusions and imperfections, theindividual cordierites are optically homogeneous. In figure13 no central apparent pinacoid sector is visible. Figure 14sbows a second cordierite in section perpendicular to itsmorphologic long axis. In this case, the section cuts througha central apparent pinacoid sector (fig. 7b). The image isat a higher magnification than in figure 13, and the centralapparent pinacoid sector fills most of the image. Altbougbthey are not shown in figure 14, there are six apparent prismsectors, around tbe central bexagonal-shaped core, withthree different optical orientations.

Unlike tbe apparent prism sectors that are optically homo-geneous (i.e., the entire volume of each apparent sectorbecomes extinct at the same time between crossed polars),tbe apparent pinacoid sector is optically heterogeneous.Figure 14 shows a zigzag-shaped extinction texture withsome areas that are ligbt and other areas that are simultane-ously dark. This type of texture is often an indication of apostgrowtb phase transformation (e.g., the tartan texture ofmicrocline twinning in K-feldspar and tetragonal domainsin leucite |Putnis 1992]) and is thought to be due to tbetransformation of indialite to cordierite (Kitamura andHiroi 1982; Kitamura and Yamada 1987).

Volume 81, July/August 2U0fa 2ii9

DiscussionInterpretation ofthe optical properties of cordierites from

Daimonji-yama, described above, was first presented byKitamura and Yamada (1987); this discussion is based pri-marily on tbat work. The optical properties provide a greatdeal of information regarding the mechanism of formationof sakura ishi. The most obvious characteristic is that theyare not single crystals but rather complex intergrowths ofmultiple crystals. In the center of these intergrowths is theapparent pinacoid sector (figs. 7, 14). The zigzag extinctiontexture of these is thought to result from the transformationof an original indialite crystal to cordierite. The structuresof these two minerals are very similar and are related bythe distribution of Si and Al among the tetrahedral sites.Indialite, the high-temperature polymorph of cordierite,is hexagonal. In indialite the Si and Al are randomly dis-tributed among the tetrahedral sites. When indialite coolsbelow tbe transition temperature to cordierite, the Si and Almove into a nonrandom distribution among the tetrahedralsites (this is exactly analogous to tbe transition betweenthe sanidine and microcline structures [Putnis 1992]). Thetransition temperature is dependent on the exact composi-tion of the indialite and is about 700°C for the Daimonjicrystals (Kitamura and Hiroi 1982; Kitamura and Yamada1987). Tbe process of an element going from a random toa nonrandom distribution in a crystal structure is calledordering. The result of ordering in this case is a lowering ofthe symmetry from hexagonal to ortborhombic, and thusthe transition (phase transition) from indialite to cordierite.There are also small structural changes such as the rotationof the tetrabedra around their shared oxygens that occur inresponse to the redistribution of Si and Al. Ordering of Siand Al occurs such tbat the d-axis of the cordierite will beparallel to one of the three equivalent a-axes of the originalindialite (fig. 14b). This ordering can start independently at

14b

Figure 14. (a) Optical photomicrograph of cordierite (crossedpolars) in thin section from Daimonji-yama, Kyoto, Japan.The section is perpendicular to the morphologic long axis ofthe original crystal, similar to the schematic section in figure7b. The central area with the zigzag extinction texture (lightand dark areas) is an apparent pinacoid sector. The dashedlines roughly show the boundaries of the apparent pinacoidsector. The circular boundary around the image is from themicroscope objective tens, (b) Schematic of the three possibleorientations that the orthorhombic cordierite (shades of gray)structure can take when formed by solid-state phase transfor-mation from hexagonal indialite (black).

different points within the indialite, and areas (domains) ofall tbree equivalent orientations will form. Once a domainof a particular orientation starts, it will spread outward untilit hits another domain. The end result is an intergrowth ofcordierite domains, each witb one of three crystallographicorientations related by a 120° rotation to each other, andtbus different extinction orientations in crossed polars,as is observed here (fig. 14). The intergrown domains ofcordierite are in a pseudotwin relationship (Kitamura andHiroi 1982; Kitamura and Yamada 1987). Tbe indialite tocordierite transformation texture is found in the central coreof the sakura ishi as well as tbroughout the entire apparentpinacoid sectors. This geometry is fully discussed below andshown in figure 18.

The transformation texture observed in the apparentpinacoid sectors is not observed in tbe six apparent prismsectors. This indicates tbat tbese grew directly as cordieriteand were not tbe result of a transformation from indialite.The specific orientational relationship among the six cor-dierite crystals was interpreted by Kitamura and Yamada(1987) to result from tbe epitaxic overgrowth (sa' Word totbe Wise, tbis issue) of cordierite on six {100} faces of anoriginal indialite crystal (fig. 15).

Tbe apparent sectoral morphology o^ sakura ishi (fig. 7),and the indialite to cordierite transformation texture foundthroughout the apparent pinacoid sectors, indicate that oncethe cordierites had epitaxicly nucleated on tbe six {100} facesofthe indialite, tbe indialite continued to grow concurrentlywith the cordierite overgrowths. Because the prism faces oftbe original indialite were covered by the growing cordieritecrystals, continued growth of the indialite could only occuron its pinacoid faces (fig. 15). From this scenario an obviousquestion arises: If cordierite epitaxicly overgrew the prism

290 ROCKS & MINERALS

(100)interface

Indialite

Cordierite

Figure 16. Schematic of apossible interface structurebetween epitaxic cordieriteand the hexagonal prism (100)of indialite.

(001)interface

C = cordieritei = indialite

Figure 15. Schematic showing the hypothesizedinitial formation ofthe cordierite (C)-indialite(I) intergrowths. The central crystal (gray) isindialite.

faces of the original indialite crystal, why did it not do thesame on the pinacoid faces of the indialite? As discussed inthe Word to the Wise column, titled "Epitaxy," in this issue,the likelihood of epitaxy occurring is related to the degreeof mismatch between the atomic structures ofthe substrateand the overgrowth. Thus, it is worthwhile to make a com-parison of the cordierite structure to the indialite structureon the prism {100} and pinacoid jOOl} faces. The mismatchbetween the structures at the (100) ,, - (100). ,.,interface (fig. 16) is relatively small; thus, the energy barrierfor cordierite to epitaxicly nucleate and grow on the {100}faces of indialite is also relatively small. In contrast, the(001),,,,,,,,, - (001),,,.,,,^ interface (fig. 17) exhibits a muchgreater degree of structural mismatch between the twophases. Hence, it is energetically less favorable for cordieriteto nucleate on the {001} faces of indialite. It is suggested thatthis difference in the energy necessary for epitaxic nucleationof cordierite on the two different crystal forms of indialite isthe reason that it only nucleated on the {100} faces.

Formation Mechanism and HistoryBased on the types of optical features described above,

as well as other data presented in their paper, Kitamuraand Yamada (1987) proposed the following mechanism forthe formation of sakura ishi found on Daimonji-yama inKyoto, which we extend here to the pseudomorphs found inKameoka (fig. 18):• Contact with igneous intrusions caused the thermalmetamorphism of sedimentary rocks in both locations andthe formation of cordierite-indialite intergrowths. The first

Cordierite

indialite

Figure 17. Schematic of a possible interface structure betweenepitaxic cordierite and the pinacoid (001) of indialite. Thelower set of red arrows point to connective sites (shared oxy-gens) between tetrahedra and octahedra within the indialitestructure. The upper set of red arrows points to the equivalentset of sites at the indialite-cordierite interface. The arrow far-thest to the left points to a spot where there is perfect registrybetween tetrahedra and octahedra across the interface. Note,however, the increasing degree of mismatch as you move tothe right along the interface.

things to form were morphologically simple indialite crystalsbound by hexagonal prism {100} and pinacoid {001} faces.• At some point, probably due to a decrease in temperature,conditions were right for either cordierite or indialite to grow.Cordierite crystals then epitaxicly nucleated on each of thesix {100} faces of the indialite and started to grow outward,away from the indialite center. Because the mismatch betweenthe structures at the (001 )̂ ,,̂ j.̂ .̂̂ .̂ - (OOl),,,̂ ,̂ ,.,̂ . interface wastoo large, indialite rather than cordierite continued to growon the {001} faces. The unique orientational relationshipamong the resulting intergrowth of seven crystals (thecentral indialite and six cordierites) resembles the sectoralinternal morphology of a single crystal (figs. 7, 18). Thus,what appear to be sector boundaries (fig. 7) in the sakuraishi are actually grain boundaries in a complex intergrowthof multiple crystals (fig. 18).

• During the formation of these intergrowths a high densityof inclusions from the surrounding matrix was incorporatedat the grain boundaries between the individual cordieritecrystals. The amount of included matrix at the grain

Volume 81 , July/August 2006 291

gramboundaryInclusions

Figure 18. (a) Schematic of the complexintergrowth of indialite and cordierite that isknown as sakura ishi. The gray core is a hex-agonal crystal of indialite (I); the six apparentprism sectors that extend from the core areactually six individual cordierite (C) crystalsthat epitaxicly grew out from the prism j 100(faces of the indialite. The apparent pinacoidsectors that extend out from the core are a con-tinuation of indialite growth on the pinacoid{001 }faces. As the individual epitaxic crystalsof cordierite and indialite grow outward fromthe core, fine-grained matrix material is incor-porated along the grain boundaries (the appar-ent sector boundaries). After their formationthe indialite components of this intergrowth

l o b undergo a phase transformation and alsobecome cordierite. (b) In Kameoka, these intergrowths are pseudomorphically replaced hy fine-grained muscovite. When broken perpendicular to the long axis of the intergrowth, they exhibitan apparent sectoral morphology that resembles a flower.

boundaries varies from crystal to crystal, as is evident infigure 8.• After formation of the intergrown crystals, again inresponse to a further decrease in temperature, the indialitetransformed to cordierite, resulting in pseudotwinning. Theobserved heterogeneous extinction that results from theindialite to cordierite transformation is found within thecentral core of the sakura ishi (the original indialite hexago-nal prism) and in the apparent pinacoid sectors that extendfrom that core (fig. 18).

• For the sakurn ishi found in Kameoka, the final step intheir formation was the hydrothermal alteration of the horn-fels and replacement of the cordierite by fine-grained mica.This replacement preserved both the external and internalmorphology of the original cordierite-indialite intergrowthsand thus resulted in pseudomorphs of these (fig. 18b). Afterreplacement of the cordierite by mica, it is easy to separatethe pseudomorphs at the relict grain boundaries because ofthe included matrix material (fig. 9).

The complex formational history of sakura ishi requireda pressure-temperature-compositional pathway of very nar-row conditions. This may be the reason for their rarity andoccurrence being restricted to central lapan. Although thesakura are ephemeral in their beauty, lasting only a few weekseach year, their image has been set in stone in the sakura ishiof Kameoka.

A sheet of rain.Only one stone remains among

cherry blossom shadows

Modified haiku by Matsuo Basho

ACKNOWLEDGMENTSThanks are extended to Alfredo Petrov and Ms. Mayumi Sakashi-

ta for organizing and Mr. Akiyoshi Okamoto for guiding one of ourfield trips to Kameoka for the collection of sakura ishi. Thanksalso go to Teruo Makabe for helping to collect yozakura ishi onDaimonji-yama and for the white-knuckle bike ride down the steepmountain trails. This manuscript benefited greatly from the carefulreviews of Pete Richards, Alfredo Petrov, and Bob Feldman.

REFERENCESAkizuki, M., and I. Sunagawa. 1978. Study of the sector structure in

adularia by means of optical microscopy, infra-red absorption,and electron microscopy. Mitieralogical Magazine 42:453-62.

Akizuki, M., M. S. Hampar, and J. Zussman. 1979. An explanationof anomalous optical properties of topaz. Mineralogical Magazine43:237-41.

Bloss, F. D. 1999. Optical crystallography. MSA monograph series.Washington, D.C: Mineralogical Society of America.

Bosze, S., and ). Rakovan. 2002. Surface structure controlled sec-toral zoning of the rare earth elements in fluorite from LongLake, N.Y. and Bingham, N.M. Geochimica et Cosmochiinica Acra66:997-1009.

Dana, E. S. 1892. The system of mineralogy. 6th ed. New York: JohnWiley & Sons.

Dowty, E. 1976. Crystal structure and crystal growth: II. Sector zon-ing in minerais. American Mineralogist 6\:46(i-69.

Imoto, N. 1984. Late Paleozoic and Mesozoic cherts in the Tambabelt, southwest lapan. Bulletin of Kyoto University of Education65:15-71.

Kikuchi, Y. 1889. Cerasite in slate. Chigaku Zasshi 1:36-38. [journalof Geography; in Japanese]

Kitamura, M., and Y. Hiroi. 1982. Indiatite from Unazuki peliticschist, lapan, and its transition texture to cordierite. Contributionsto Mineralogy ami Petrology 8:110-16.

Kitamura, M., and H. Yamada. 1987. Origin of sector trilling incordierite in Daimonji hornfels, Kyoto, Japan. Cotitributions toMineralogy and Petrology 97:1-6.

Klein, C. 2001. Manual of mineral science. 22nd ed. {Manual of min-eralogy). New York: Wiley.

Miyake, A. 1990. Dendritic cordierite in argillaceous hornfels fromthe Toki area, Gifij Prefecture, central Japan. Contributions toMineralogy and Petrology 104:390-96.

Miyazaki, K. 2001. Heterogeneous growth of cordierite in low P/TTsukuba metamorphic rocks from central lapan. Journal ofMeta-morphic Geology 19:155-64.

Putnis, A. 1992. Introduction to mineral sciences. New York: Cam-bridge University Press.

Rakovan, |. 2005. News from lapan—Part 3: Ko mineral collection,Osaka and Tokyo shows, and collecting sakura ishi and rainbowgarnets. Rocks & Minerals 80:440-45.

Rakovan J., and R. I. Reeder. 1994. Differential incorporation of traceelements and dissymmetrization in apatite: The role of surfacestructure during growth. American Mineralogist 79:892-903.

Reeder R. J., and ]. C. Grams. 1987. Sector zoning in calcite cementcrystals: Imphcations for trace element distributions in carbon-ates. Geochimica et Cosmochimica Acta 51:187-94. •

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